Convert the same number on two different websites and you'll occasionally get two slightly different answers — 2.2046 lbs on one site, 2.205 lbs on another, for the same 1 kg. Neither tool is broken. This comes down to two things that every conversion tool has to decide on: how many decimal places to display, and how many decimal places to carry in the constant it's converting with. Here's what's actually going on, and when the difference is big enough to matter.
Two separate sources of "different" answers
When two converters disagree, it's almost always one of these two causes, and they're worth telling apart:
- Display rounding — the underlying calculation is identical, but one tool shows 2 decimal places and another shows 4. This is the far more common cause and doesn't reflect any real disagreement about the answer.
- Constant precision — the conversion factor itself is carried to a different number of decimals internally. 1 pound = 0.45359237 kg exactly (this one is an exact, defined constant); a tool that stores it as 0.4536 will drift very slightly from one storing the full value, and that drift compounds on large numbers.
For length, weight, and volume, most modern definitions (meter, kilogram, liter) are either exact by definition or defined to enough decimal places that this second cause is rarely significant for everyday numbers — the first cause, display rounding, explains almost every "why don't these match" case people run into.
Currency conversion is a different problem entirely
Unit conversion factors are fixed, agreed-upon numbers. Currency exchange rates are not — they move continuously during trading hours, and different converters pull from different data providers with different update frequencies. A currency conversion tool showing a rate that's a few minutes old — or, for free feeds that refresh once a day like the one this site uses, up to a day old — is not "wrong," it's just not the rate you'd get executing a trade at this exact second. This is the single biggest thing to understand about any free currency converter, including the Currency Converter on this site: it's built for quick reference and estimates, not for time-sensitive financial decisions. For anything involving an actual money transfer, check the rate your bank or payment provider will actually apply, since they often add a margin on top of the market rate anyway. How currency conversion actually works walks through where those margins come from.
When rounding actually matters
For most everyday use, a couple of decimal places of drift is irrelevant — nobody needs a recipe's cup-to-milliliter conversion accurate to six decimal places. But precision matters more in specific situations:
- Cooking and baking — usually forgiving, except for precise baking chemistry (bread hydration percentages, for example) where rounding a gram figure can shift a ratio noticeably at small batch sizes.
- Medication dosing — never rely on a general-purpose converter for this. Use the dosing figures provided by a pharmacist or the medication's actual labeling, which account for concentration and rounding rules specific to that drug.
- Engineering and construction tolerances — a small conversion error compounds across a large structure. Use the precision your specific field's standards require, and verify against an authoritative reference for anything load-bearing or safety-critical.
- Large financial or currency conversions — even a fraction of a percent of drift matters at scale; use your financial institution's actual quoted rate, not a reference converter, for anything beyond a rough estimate.
The practical takeaway
If two converters disagree by a small amount, it's almost certainly display rounding — check how many decimal places each one shows before assuming either is wrong. If you need more precision than the default display gives you, look for a tool that lets you see more decimal places rather than assuming the number itself is imprecise. And for currency specifically, treat any free converter as a fast estimate, not a live trading rate.
Length, weight, temperature, and volume — calculated instantly in your browser.
Open Unit Converter →Constants that are exact by definition
Many everyday conversion factors aren't measurements at all. They're exact definitions. Since the 1959 international yard and pound agreement, the inch and the pound have been defined in metric terms. That means these factors have no "true value" beyond what's written here:
| Unit | Definition |
|---|---|
| 1 inch | 2.54 cm (exact) |
| 1 foot | 0.3048 m (exact) |
| 1 mile | 1.609344 km (exact) |
| 1 pound (avoirdupois) | 0.45359237 kg (exact) |
| 1 US liquid gallon | 231 cubic inches = 3.785411784 L (exact) |
| 1 US fluid ounce | 1/128 US gallon = 29.5735295625 mL (exact) |
| °F from °C | °F = °C × 9/5 + 32 (exact formula) |
If a converter disagrees with one of these beyond its displayed decimal places, it's using a rounded constant. The U.S. National Institute of Standards and Technology (NIST) publishes these definitions in its guides to SI and U.S. customary units.
Same name, different unit
A bigger source of "wrong" answers is a unit name that means different things in different places:
- Gallon: the US liquid gallon is about 3.785 L, and the UK (imperial) gallon is exactly 4.54609 L, roughly 20% larger. Fuel economy in miles per gallon isn't comparable between US and UK figures without adjusting for this.
- Cup: a US customary cup is 8 US fl oz, about 236.6 mL. US nutrition labels use a rounded 240 mL cup. Many recipes from Australia and elsewhere use a 250 mL metric cup.
- Ton: a US short ton is 2,000 lb (907.18474 kg), a UK long ton is 2,240 lb (about 1,016.05 kg), and a metric tonne is 1,000 kg.
- Foot (surveying): the old US survey foot (1200/3937 m) differs from the international foot by about 2 parts per million. NIST and NOAA retired it for new work at the end of 2022, but older land records may still use it.
Round-trip rounding
Rounding in the middle of a calculation causes drift you can see. Convert 100 kg to pounds and you get 220.462262... lb. Round that to 220.46, convert back, and you get 99.9990 kg instead of 100. The fix is simple: keep full precision until the final step, and round only the number you display.
Floating-point quirks
Computers store most decimals in binary, which can't represent values like 0.1 or 2.55 exactly. In JavaScript, 0.1 + 0.2 gives 0.30000000000000004, and (1.005).toFixed(2) returns "1.00", not "1.01". That's because the stored value is actually 1.00499999... These errors are tiny, far below anything that matters for cooking or travel. But they explain why a tool can occasionally round a "5" in the last place in an unexpected direction.
Choosing the right precision
- Match the precision of your input. A body weight measured to the nearest pound doesn't need a kilogram figure to four decimals.
- Check which variant of a unit your source means (US or imperial, short or long ton) before converting.
- For single conversions, dedicated pages like KG to LBS, CM to Inches, and Celsius to Fahrenheit show the exact factor they use.
Frequently asked questions
Why do two unit converters give slightly different answers for the same conversion?
Usually because they round to a different number of decimal places, or use a slightly different constant for the conversion factor (for example, some tools round the pound-to-kilogram factor to 0.4536, others carry more decimal places). The underlying math is the same; the display precision differs.
Why do currency converters show different exchange rates?
Currency converters pull rates from different data providers, and exchange rates fluctuate continuously during trading hours. A rate that's 30 seconds old can already differ slightly from live market rates, and most free converters are not meant for time-sensitive financial transactions.
Should I trust a converted number to the last decimal place?
It depends on the use case. For everyday use (cooking, travel, casual reference) rounding to 1-2 decimal places is fine. For anything with real consequences — medication dosing, engineering tolerances, financial transactions — use a purpose-built, authoritative source rather than a general free converter.
Is 1 inch exactly 2.54 centimeters?
Yes. Since the 1959 international yard and pound agreement, the inch has been defined as exactly 2.54 cm, and the pound as exactly 0.45359237 kg. Any difference between converters comes from rounding, not from the definition.
Why is a UK gallon different from a US gallon?
They are separate units that share a name. The US liquid gallon is defined as 231 cubic inches (about 3.785 liters), while the imperial gallon used in the UK is exactly 4.54609 liters, about 20% larger.