Common Conversions Cheat Sheet
Every 'how many X in a Y' answer in one searchable place — cooking, length, weight, time, area and data — with the exact and approximate ones marked.
The answers to every "how many X in a Y" that comes up in a kitchen, garage, or homework session. For arbitrary amounts, the Unit Converter does the math; this page is for the facts themselves.
The tilde on this sheet is load-bearing
A cheat sheet is a list of rounded numbers, so the fair question to ask of one is what each rounding costs. Checking all 28 numeric entries against their exact definitions sorts them into groups that do not blur into each other. Every entry marked with a “~” is out by more than 0.1%. Every entry without one is out by less than 0.05%. Nothing lands in between.
| Entry | Shown | Exact | Error | Marked |
|---|---|---|---|---|
| Shots (1.5 oz) in a fifth (750 ml) | 17 | 16.907 | +0.550% | ~ |
| Football fields in an acre | 0.76 | 0.7562 | +0.496% | ~ |
| Ounces in a fluid ounce of water | 1.04 | 1.0432 | -0.304% | ~ |
| Seconds in a year | 31,600,000 | 31,556,952 | +0.136% | ~ |
| Fluid ounces in a liter | 33.8 | 33.814 | -0.041% | |
| Gallons (US) in a gallon (UK) | 0.833 | 0.8327 | +0.039% | |
| Milliliters in a teaspoon | 4.93 | 4.9289 | +0.022% | |
| Kilometers in a mile | 1.609 | 1.6093 | -0.021% | |
| Pounds in a kilogram | 2.205 | 2.2046 | +0.017% | |
| Liters in a gallon (US) | 3.785 | 3.7854 | -0.011% | |
| Milliliters in a cup (US) | 236.6 | 236.5882 | +0.005% | |
| Feet in a meter | 3.281 | 3.2808 | +0.005% | |
| Acres in a hectare | 2.471 | 2.4711 | -0.002% | |
| Grams in an ounce | 28.35 | 28.3495 | +0.002% | |
| Grams in a pound | 453.6 | 453.5924 | +0.002% | |
| Pounds in a metric tonne | 2,204.6 | 2,204.6226 | -0.001% |
The worst unmarked entry is out by 0.041%; the best marked one by 0.136%. That is a factor of 3.3 with an empty gap in the middle, on a sheet nobody wrote with an error budget in mind. The tilde went where it felt necessary and it landed in exactly the right place.
And the marked four are not conversions at all
Look at what the four have in common. A fifth divided by a shot is 16.91, and you cannot pour nine tenths of a shot. A football field is 0.76 of an acre with its end zones and 0.91 without — a bigger disagreement than any rounding on the sheet, and the sheet quietly picks the with-end-zones reading. Water weighs what it weighs at a temperature nobody wrote down. And a year is not a fixed number of days. They carry a tilde because the underlying quantity is fuzzy, not because the arithmetic was done carelessly.
At the other extreme, 12 entries carry no error whatsoever, because the unit is defined rather than measured. An inch is exactly 2.54 cm, a mile is exactly 5,280 feet, a pound is exactly 16 ounces, an acre is exactly 43,560 square feet. A more precise version of these does not exist and cannot be found.
| Group | Entries | Largest error |
|---|---|---|
| Exact by definition | 12 | none |
| Rounded, unmarked | 12 | 0.041% |
| Rounded, marked “~” | 4 | 0.550% |
Nothing here is off by a whole per cent, and 12 of the 16 roundings sit inside a twentieth of one. 10 round up and 6 round down, which is close enough to a coin flip that there is no bias to report. Weigh a 5 kg bag of flour using the sheet’s 28.35 grams per ounce and you are 0.08 of a gram out.
How to use
- Search for the unit you are converting.
- Read the factor and whether it is exact.
- Note the US and imperial variants where they differ.
- Use a dedicated converter for arbitrary values.
Frequently asked questions
Which conversions are exact and which are approximate?
More are exact than people expect. An inch is exactly 2.54 cm, a pound exactly 0.45359237 kg, a mile exactly 1.609344 km — all defined rather than measured, since 1959. Approximations mostly appear where a unit is defined by something physical, or where the value is simply being rounded for convenience.
Why do US and imperial units differ?
Because they diverged after American independence. The US kept older English definitions while Britain redefined its own in 1824, so the gallon differs by about 20 per cent and the fluid ounce by about 4. Weights and lengths were harmonised in 1959; volumes never were.
What is the most expensive conversion mistake?
Historically, mixing metric and imperial in engineering. The Mars Climate Orbiter was lost in 1999 because one team supplied thrust data in pound-force seconds while the software expected newton-seconds. Unit errors in medicine, particularly in dosing, are a more frequent and more personal category.
Why do cooking conversions vary between sources?
Because volume-to-weight depends on the ingredient and on how it is packed, and because cup sizes differ by country. A cup of flour can be 120 or 145 grams depending on whether it was spooned or scooped, so no single table can be right for everything.
Should I convert or change units?
Where possible, work natively in the units the task uses rather than converting back and forth. Every conversion introduces a rounding opportunity, and chains of them accumulate error — which is why recipes and technical documents are best followed in one system throughout.
What is the quickest way to sanity-check a conversion?
Estimate the magnitude first, then check the answer sits near it. Most conversion errors are not small — they are factors of ten, or the conversion applied in the wrong direction — and a rough mental estimate catches those immediately.
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