526: Converting to Metric

Explain xkcd: It's 'cause you're dumb.
Jump to: navigation, search
Converting to Metric
According to River, "adequate" vacuuming systems drain the human body at about half a liter per second.
Title text: According to River, "adequate" vacuuming systems drain the human body at about half a liter per second.


Most people will eventually develop an intuitive feel for how big certain measurements are (e.g., how long an inch or a foot is, how much a pound weighs). This comic points out that people who were brought up using the United States system of customary units probably don't have the same intuitive understanding for metric units and attempts to provide some benchmarks for these people. Most of the benchmarks are common sense, highly-useful ones (e.g., if it's 30 degrees Celsius, you'd be quite comfortable outside dressed for the beach) but some of the benchmarks are humorous and/or completely useless as can be seen below.

Some people argue for switching to metric units in the US, and these people became part of the comic 1982: Evangelism.

In the book Thing Explainer a similar chart for metrics is shown in the explanation for How to count things, with four of the five measures from this comic also explained in simple language. Only volume is left out there. Only thing used in both explanations is the weight of a cat, but in the book it weighs 5 kg rather than 4 kg in this comic.


  • 60⁠ °C - Earth's hottest: The hottest temperature recorded on earth is actually "only" 56.7 °C. There have been reports of temperatures ten-twenty degrees higher (70−80 ⁠°C) but these measurements are not verified or accepted as world records.
  • 45 °C, 40 °C⁠, 35⁠ °C: Various heat waves. Dubai is a city in the United Arab Emirates, and is smack-dab in the middle of an equatorial desert, so their heat waves can get hot!. The southern United States will typically be a few degrees hotter than the northern United States simply because it's closer to the equator, but as mentioned they're both above "Beach Weather".
  • 30 °C: A little too hot so perfect for a trip to the beach.
  • 25 °C: Would as mentioned be too warm for room temperature...
  • 20 °C: Defined as room temperature in many experimental settings. For some this would feel a little cool.
  • 10 °C: Definitely wear a jacket. Especially if there is just a little breeze.
  • 0⁠ °C: The freezing point of water.
  • −5 °C, −10 °C: In Moscow −10 °C is not really that cold - it can go "spit goes clink" cold in Moscow, whereas −5 °C in Boston may be very cold.
  • −20 °C - FuckFuckFuckCold,
  • −30 °C - Fuuuuuuuuuuck!: This is implied to be basically what some people would say when they step outside at this temperature. In reality, it would be best to keep ones's mouth firmly closed. At −30 °C, without taking wind chill into account, exposed skin will feel painful in under a minute and frostbite could begin in as little as ten minutes [1]. The differing statements seem to imply that at −20 °C, the user would be saying "fuck" repeatedly, whereas at −30 °C, the user is incapable of closing their mouth after starting the first "fuck", and so extends it into one long one.
  • −40⁠ °C - Spit goes "clink": As shown in the drawing your spit would freeze before it hits the ground. This is the agreement point of the two temperature scales i.e. −40 °C = −40 °F.

See also 1643: Degrees about not being able to choose between the two temperature scales and 1923: Felsius about a compromise between the two scales. In the comic 1982: Evangelism, some people are stated to argue for the US to convert to the metric system, except for the Fahrenheit scale which they wish to keep.


  • 1 cm - Width of microSD card, 3 cm - Length of SD card: Refers to the memory cards used in cell phones, digital cameras, etc.
  • 12 cm: CD-ROM is a common object so nice to know it is a dozen centimeters.
  • 14 cm: Most males would probably exaggerate the size of their penis, but 14–15 cm is very average.
  • 15 cm: A Bic pen.
  • 80 cm: A typical doorway width is also of standard size. This is barely over the minimum size typically required by codes for buildings (30 inches or 76.2 cm in the US), but more than 50% over the size required for aircraft emergency exits. (It may seem illogical that larger doors are required in buildings than in airplanes, given airplanes are arguably more dangerous. However, there is no real disadvantage to using larger doors in buildings, which are not significantly pressurized, but using larger doors in aircraft would increase the force on the door caused by cabin pressure proportionally.)
  • 1 m - Lightsaber blade: Refers to the weapon used in the Star Wars movie franchise. Canonically, the length of a lightsaber's blade varies greatly depending on the setting of the weapon, but "one meter" is by no means a bad approximation.
  • 170 cm - Summer Glau: Refers to the height of the actress who portrays the character River Tam on the TV show Firefly.
  • 200 cm - Darth Vader: Refers to the height of the main antagonist from Star Wars.
  • 2.5 m: A ceiling - of course very much depending on which type of building you are in!
  • 5 m: A car length - also very much depending on the car...
  • 16 m 4 cm - Human tower of Serenity crew: Again, this refers to the Firefly TV show, which takes place mostly on a space ship called Serenity.
    • Presumably, if all the crew of Serenity were stacked on top of each other, this would be their combined height.
    • The comic depicts four characters from the show standing on top of each other; the bottom figure is the crew's captain, Malcolm Reynolds in his signature coat. Judging from the other drawing of Summer Glau from the volume section, she is standing on top of the captain.
    • The other five members of the crew should also be stacked on top of these four to reach the 16.04 m height - giving them an average height of 1.78 m (8 cm more than Summer Glau's height!)


Here both the SI unit m/s as well as the more commonly used unit kph (km/h) is given. Note that the SI prefers "km/h" over the non-standard abbreviation "kph".
  • 5 kph - 1.5 m/s: Walking at a normal pace.
  • 13−25 kph - 3.5−7 m/s: Jogging to sprinting.
  • 35 kph - 10 m/s - Fastest human: As of 2009, the fastest a human has been recorded to run in a single sprint is actually 12.4 m/s or 44.7 km/h, a record set by Usain Bolt.
  • 45−55 kph - 13−15 m/s: Both cats and rabbits go much faster than normal people.
  • 75 kph - 20 m/s - Raptor: It's a comic written by Randall; of course a reference to the velociraptors from Jurassic Park was going to be here.
  • 100 kph - 25 m/s: A slow highway. (25 m/s actually exactly equals 90 km/h.)
  • 110 kph - 30 m/s - Interstate (65 mph): Refers to the American highway system. (65 mph would actually be only 104.6 km/h.)
  • 120 kph - 35 m/s - Speed you actually go when it says "65": People routinely break the aforementioned speed limit, and the police typically don't mind as long as it's not posing any danger.
  • 140 kph - 40 m/s - Raptor on hoverboard: The hoverboard and its speed (~88 mph) is probably a reference to the Back to the Future Part II, though hoverboards are a fairly common trope in older science fiction stories. Randall obviously did a lot of google searching on this subject the week before - see 522: Google Trends.


  • 3 mL: The amount of blood in a fieldmouse. A similar amount is used in comic 434: xkcd Goes to the Airport.
  • 5 mL: A teaspoon - a very common measure.
  • 30 mL - Nasal passages, 40 mL - Shot glass: The comic points out that you could just about fill a shot glass using the mucus from your nose. Since shot glasses are usually used for mixed drinks, the comic jokes that this mucus could constitute a new, disgusting drink - and this is depicted in the drawing. Much later, in 2673: Cursed mRNA Cocktail, a similar disgusting drink is suggested to be served in shot glasses. Actually specifically in relation to trying not to get blocked nasal passages as it is in relation to the COVID-19 vaccine.
  • 350 mL: Soda can (this is roughly correct for the cans used in the U.S., which hold 12 fluid ounces or 355 mL; in Europe, soda cans commonly hold 330 mL or 500 mL).
  • 500 mL: Water bottle (this is also the volume of a European water bottle).
  • 3 L - Two-liter bottle: Refers to a bottle which contains 2 L (in the US usually soda). There is debate as to the reason for the discrepancy in volume. It may be a reference to stereotypical Americans consuming a lot of high-calorie foods and drinks. The simpler explanation would be that it is a joke. The two-liter bottle is named using its volume. Labeling it with a volume of three liters is the joke.
  • 5 L: An adult male has about 5 L of blood in his body (An adequate vacuuming system could drain this blood out in 10 s - as per the title text!)
  • 30 L - Milk crate: Refers to a type of small box originally used to transport milk but now often in demand to be used as a bicycle basket, storage spaces, etc.
  • 55 L - Summer Glau: Again, this refers to the actress from Firefly.
  • 65 L - Dennis Kucinich: An American politician belonging to the Democratic party, noted for his relatively strong (for the US) leftist views.
  • 75 L - Ron Paul: An American politician, member of the Republican Party at the time of this comic's release who has since switched back to the Libertarian Party.
  • 200 L: Volume of a refrigerator.
    • As shown in the drawing of this part of the comic, the three persons mentioned above - Glau, Kucinich and Paul (summing up to 195 L) - could in principle all fit inside a standard refrigerator. Cueball thus attempts to push them all inside of one - though human bodies are not likely to be sufficiently malleable for this to succeed.[citation needed]


  • 3 g - Peanut M&M: A small chocolate candy with a peanut inside.
  • 100 g - Cell phone: The weight of a cell phone very much depends on the age, type etc.
  • 500 g: A bottle of water contains 500 mL according to the volume section and thus has a mass of 500 g.
  • 1−3 kg: Different types of laptops.
  • 5 kg - LCD monitor: A modern flat-screen-style monitor.
  • 15 kg - CRT monitor: An older-style, cathode ray tube-based monitor.
    • This ends the section on computer screens, which overrode the normal sequence by weight as the next two feline inspired entries are lighter than the two before. This was presumably done so that the reader's eye will be confused or amused at seeing (in the comic's caseless captioning font) CRT immediately followed by CAT in the vertical text column.
  • 4 kg - Cat, 4.1 kg - Cat (with caption): Refers to the internet's love of putting captions on cats. Usually, this is done in a graphics program, but here the cat is actually physically carrying around his caption. The "with caption" part is most likely a reference to 262: IN UR REALITY, where Black Hat glues captions to cats, after running out of staples.
  • 60 kg - Lady: For instance if she is Summer Glau - could be her again depicted in the comic - the average weight of an adult woman.
  • 70 kg - Dude: Here depicted as Cueball who is the average guy, and 70 kg is average weight for an adult man.
  • 150 kg - Shaq: Shaquille O'Neal, a famously tall basketball player.
  • 200 kg - Your mom,
  • 220 kg - Your mom (incl. cheap jewelry),
  • 223 kg - Your mom (also incl. makeup)
    • The last three refer to a common type of Your mom joking insult whereby someone insults someone else's mother in a (theoretically) creative way. Here, the comic slyly calls your mom fat, then implies she wears way too much jewelry and finally also 3 kg of makeup. This is a common theme in xkcd. (20 kg of "cheap" jewelry has several times the volume than 20 kg of gold jewelry, because of the difference in density.)

Title text[edit]

The title text refers once again to Summer Glau's Firefly character, River Tam, who (after being subjected to a long series of medical experiments) is severely mentally ill and often comes out with macabre — though scientifically accurate — pronouncements. In Firefly episode "Safe" (season 1, episode 7), she says: "The human body can be drained of blood in 8.6 seconds given adequate vacuuming systems."

Conversion table[edit]

The idea of the comic is to establish new metric reference points and not to resort to unit conversions. Nevertheless, the following table lists all units from the comic with their US customary equivalents:

Temp. Length Speed Volume Mass
60 °C 140 °F 1 cm 0.4 in km/h mph m/s 3 mL 0.10 fl oz 3 g 0.11 oz
45 °C 113 °F 3 cm 1.2 in 5 3 1.5 5 mL 0.17 fl oz 100 g 3.5 oz
40 °C 104 F 12 cm 4.7 in 13 8 3.5 30 mL 1.0 fl oz 500 g 1.1 lb
35 °C 95 °F 14 cm 5.5 in 25 16 7 40 mL 1.4 fl oz 1 kg 2.2 lb
30 °C 86 °F 15 cm 5.9 in 35 22 10 350 mL 12 fl oz 2 kg 4.4 lb
25 °C 77 °F 80 cm 31 in 45 28 13 500 mL 17 fl oz 3 kg 6.6 lb
20 °C 68 °F 1 m 3 ft 3 in 55 34 15 3 L 0.8 gal 5 kg 11 lb
10 °C 50 °F 170 cm 5 ft 7 in 75 47 20 5 L 1.3 gal 15 kg 33 lb
0 °C 32 °F 200 cm 6 ft 7 in 100 62 25 30 L 7.9 gal 4 kg 8.8 lb
-5 °C 23 °F 2.5 m 8 ft 2 in 110 68 30 55 L 15 gal 4.1 kg 9.0 lb
-10 °C 14 °F 5 m 16 ft 120 75 35 65 L 17 gal 60 kg 130 lb
-20 °C -4 °F 16.04 m 52 ft 7 in 140 87 40 75 L 20 gal 70 kg 150 lb
-30 °C -22 °F 200 L 53 gal 150 kg 330 lb
-40 °C -40 °F 200 kg 440 lb
220 kg 485 lb
223 kg 492 lb


Guide to Converting to Metric
[There are five frames with tables for different units. Between the two upper frames is the following text:]
The key to converting to metric is establishing new reference points. When you hear "26°C", instead of thinking "that's 79°F" you should think, "that's warmer than a house but cool for swimming."
Here are some helpful tables of reference points:
[The frame in the top left lists the following temperatures on the left, with the corresponding descriptions on the right. Next to the last three entries we see Cueball spitting on the ground. The spit freezes.]
60°C    Earth's hottest
45°C    Dubai heat wave
40°C    Southern US heat wave
35°C    Northern US heat wave
30°C    Beach weather
25°C    Warm room
20°C    Room temperature
10°C    Jacket weather
0°C      Snow!
-5°C     Cold day (Boston)
-10°C   Cold day (Moscow)
-20°C   Fuckfuckfuckcold
-30°C   Fuuuuuuuuuuck!
-40°C   Spit goes "clink"
Cueball: Ptoo
Spit: Clink!
[The frame in the top right lists the following lengths on the left, with their corresponding descriptions on the right. To the right of the table is a human tower of four of the people from the Serenity crew. The head of the upper person is right below the first entry.]
1 cm         Width of microSD card
3 cm         Length of SD card
12 cm       CD diameter
14 cm       Penis
15 cm       BIC pen
80 cm       Doorway width
1 m           Lightsaber blade
170 cm     Summer Glau
200 cm     Darth Vader
2.5 m        Ceiling
5 m           Car-length
16 m 4 cm Human tower of Serenity crew.
[The frame in the bottom left has three columns]
kph   m/s
5      1.5    Walking
13    3.5    Jogging
25    7       Sprinting
35    10     Fastest human
45    13     Housecat
55    15     Rabbit
75    20     Raptor
100   25    Slow highway
110   30    Interstate (65 mph)
120   35    Speed you actually go when it says "65"
140   40    Raptor on hoverboard
[The frame in the bottom middle lists the following volumes on the left, with their corresponding descriptions on the right.
3 mL     Blood in a fieldmouse
5 mL     Teaspoon
30 mL    Nasal passages
40 mL    Shot glass
350 mL   Soda can
500 mL   Water bottle
3 L         Two-liter bottle
5 L          Blood in a human male
30 L        Milk crate
55 L        Summer Glau
65 L        Dennis Kucinich
75 L        Ron Paul
200 L      Fridge
[Next to the entry on nasal passages and shot glass (starting one entry higher and finishing one entry lower) is the following text:]
So, when it's blocked, the mucus in your nose could about fill a shot glass.
[Below this text is a drawing of a mucus filled shot glass.]
Related: I've invented the worst mixed drink ever.
[Below this next to the four last entries we see Cueball shoving Summer Glau, Dennis Kucinich, and Ron Paul into an open fridge. Above the fridge in a loosely drawn ellipse is the following text:]
55+65+75 < 200
[The frame in the bottom right lists the following masses on the left, with their corresponding descriptions on the right
3 g       Peanut M&M
100 g    Cell phone
500 g    Bottled water
1 kg      Ultraportable laptop
2 kg      Light-medium laptop
3 kg      Heavy laptop
5 kg      LCD monitor
15 kg     CRT monitor
4 kg       Cat
4.1 kg    Cat (with caption)
60 kg     Lady
70 kg     Dude
150 kg   Shaq
200 kg   Your mom
220 kg   Your mom (incl. cheap jewelry)
223 kg   Your mom (also incl. makeup)
[Next to the entries of cat and cat (with caption) are two drawings of cats. The second one has a caption across its chest.]
Cat (with caption): Mrowl?
[Below this and next to the lady and dude entries are drawings of Megan and Cueball.]

comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!


Why is 3L a two-liter bottle? 21:16, 28 April 2013 (UTC)

Because this is America and we supersize our sodas! (talk) (please sign your comments with ~~~~)
It is the volume of the bottle itself. I have added this explanation. Sten (talk) 22:39, 29 August 2013 (UTC)
A 2L bottle doesn't take 3L of space, not even close. I also think it's a reference to overly large drinks in the US. But even if it isn't, the current explanation is wrong. 22:28, 12 December 2013 (UTC)
I suspect he's just messing with us, because the approximate volume of a two-liter bottle should be obvious. Many beverages sold in the US are already labeled in metric. Soda is routinely sold in one and two liter bottles, with three-liter bottles common in some markets. Bottled water is often sold in liters and half-liters. Liquor and wine are sold in 375 and 750 mL bottles. Also, since 1 quart = 946 mL, an approximate (+/- 5%) mental conversion from quarts to liters is already quite easy. Fryhole (talk) 01:35, 25 January 2014 (UTC)
Yes. He is just joking. The name of the bottle contains its volume. The 3 liter measurement is a joke. It would be like saying a cup has 2 cups of volume. flewk (talk) 01:58, 4 January 2016 (UTC)
My coffee cup is 12 ounces. Given a standard 8 ounce cup, my coffee cup is 1.5 cups.Seebert (talk) 15:24, 16 February 2016 (UTC)

My names River, that is all -- 02:52, 29 August 2013 (UTC)

Tick tock, goes the clock, 'till River kills the Doctor

Earth's hottest is 70,7 °C... 03:06, 24 February 2014 (UTC)

The world record as per wikipedia (and Guiness) is "only" 56.7. See corrected explanation above. Kynde (talk) 17:03, 6 March 2014 (UTC)

There was an incomplete asking for checking all measures and also for making sure that the references to serenity and velociraptors was mentioned. I did this, the last two by assigning the categories (firefly), and then also creating a new category:Your Mom. I now consider this explanation complete. Although if someone will speculate two whom the remaining two from the Serenity crew tower then please do so ;-) Kynde (talk) 17:03, 6 March 2014 (UTC)

Simon and Kaylee is my guess. 22:17, 30 June 2014 (UTC)

The speed of rapors given here is very different from 135: Substitute. B jonas (talk) 14:58, 25 March 2014 (UTC)

This sentence in the 'Mass' section has a [small] error: 'This is a common theme in XKCD.'-- it should be 'xkcd', not 'XKCD'. See the website for Randall's personal opinion on this. Anyways, it's small, but kinda stands out if your a reeallyy hardcore fan. 05:39, 9 April 2014 (UTC)

Thanks for your hint, an update is done. But please add your comments here at the bottom. --Dgbrt (talk) 21:26, 9 April 2014 (UTC)

-40 degrees centigrade is also -40 degrees Fahrenheit! The only such temperature.--DrMath 07:51, 7 July 2014 (UTC)

This is tagged with "Featuring real people" -- I don't see any real people in here, should we removethat tag? Spongebog (talk) 02:21, 25 July 2014 (UTC)

It refers to Shaq, Ron Paul, Dennis Kucinich, Summer Glau, your mom... --Pudder (talk) 08:18, 29 September 2014 (UTC)

Ron Paul is not a republican though. Yourlifeisalie (talk) 16:33, 5 May 2015 (UTC)

kph got me confused, because "km/h" is the usual way of displaying kilometers per hour.

As a Maine resident I concur with this sentiment: "at -30°, the user is incapable of closing their mouth after starting the first "fuck", and so extends it into one long one." However, try uttering the word "fuck" without closing your mouth... uck-uck-uck... Npsych (talk) (please sign your comments with ~~~~)

13kph is not a typical jogging pace. At least I hope not. That would make me depressingly slow. (talk) (please sign your comments with ~~~~)

The whole point of this comic

I thought the whole point of this comic was NOT to think in terms of non-SI units. The only reason (we) Amercans cling to customary/imperial units is because when some quantity of something is expressed in SI units you may as well tell them it's however many quatloos, because the average American has no idea what the units are like...nothing to which to compare that item. Approach learning SI units EXACTLY how you learned customary units: pick up a kilo (pound) and feel how much Earth's gravity tugs on it, stick your hand out in the outdoor air and feel what the NWS or a thermometer tells you what the C (F) temp is, eyeball a meter (yard) stick and try to remember how long that is, and so on. You're only hamstringing yourself by constant numeric conversions to some other system. For example, just accept a cm is a cm, and DON'T WORRY about how big that is in any other system. IMHO it is counterproductive to have the customary units (the conversions) in this explanation, and ALL of them should be removed. -- RChandra (talk) (please sign your comments with ~~~~)

Objection: In Australia are bottled drinks are 600 mL. -- 00:45, 6 March 2016 (UTC)

Was it really necessary to say 'Season 1' when talking about Firefly?  ;_; 04:10, 4 May 2016 (UTC)

Anyone know if there's something like this comic for learning Customary when you're used to Metric? I have a European friend with an American copy of D&D 5th edition that's tasked with DM'ing on short notice. 21:43, 26 January 2017 (UTC)

25 m/s is 90 km/h, not 100 km/h as stated in the comic. Is this also supposed to be a joke? (talk) (please sign your comments with ~~~~)

The inaccuracy is mentioned in the explanation. And please sign you comments.--Dgbrt (talk) 20:48, 25 September 2018 (UTC)

23 degrees is a more comfortable value for room temperature. It also gets used in school physics questions, where we were allowed to convert it to 300 kelvin. (Sorry, but 26.85 degrees is too warm for a room.) 16:58, 14 October 2018 (UTC)

It does not get to -40 in Moscow, however. It did once, like in the 1940s, but even then it was a temperature record. Now, with all the global warming business, -10 is indeed a cold day. In the more northen cities of Russia, like Murmansk, the "spit goes clink" temperature is possible.