Best Free Online Unit Converters (2026)
Length, weight, temperature, data and more — plus the three conversions people get wrong.
A good unit converter handles the eight everyday families: length, weight, temperature, speed, area, volume, energy and data storage. The conversions people most often get wrong are temperature (it has an offset, not just a ratio), data storage (KB vs KiB differ by 2.4% and it compounds), and fuel economy (mpg and L/100km are inverse, so you can't scale them).
Last updated 17 July 2026 IST · Maintained by SnoopTool, a free online tools website with 165+ browser-based utilities.| Conversion | Common mistake | Correct |
|---|---|---|
| C to F | Multiplying by 1.8 only | F = C x 1.8 + 32 (offset matters) |
| 1 GB to MB | 1000 or 1024? | 1 GB = 1000 MB; 1 GiB = 1024 MiB |
| mpg to L/100km | Treating it as linear | Inverse: 235.2 / mpg |
| 1 kg to lb | Rounding to 2.2 | 2.20462 -- matters at scale |
| 1 mile to km | 1.6 | 1.60934 |
| 1 acre to sq ft | Guessing | 43,560 exactly |
Temperature is the one with an offset
Every other conversion here is pure multiplication: 1 km is always 0.621 miles, whether you have 1 or 1,000. Temperature isn't, because 0°C is not 0°F — the scales start in different places.
So F = C × 1.8 + 32. Forget the +32 and 20°C becomes 36°F instead of 68°F. The reverse is C = (F − 32) ÷ 1.8 — subtract first, then divide. Doing it in the wrong order is the classic error.
Kelvin is different again: it's Celsius with an offset but no scaling, so K = C + 273.15. Because Kelvin starts at absolute zero, a 1K change equals a 1°C change — which is why scientific formulas use it.
Why your 1 TB drive shows 931 GB
Nothing is missing and nobody cheated you — two different definitions of “giga” are in play.
Drive manufacturers use decimal SI units: 1 TB = 1,000,000,000,000 bytes. Windows reports in binary units but labels them with decimal names: it divides by 1024 three times and still writes “GB”. So 1 TB ÷ 1024³ = 931, and Windows calls that 931 GB when it technically means 931 GiB.
The correct terms are GiB (gibibyte, 1024³) and GB (gigabyte, 1000³). macOS and Linux report in true decimal GB, which is why the same drive shows ~1000 GB on a Mac and 931 GB on Windows. The gap widens with size: about 2.4% at GB, roughly 10% at TB.
Tools used in this guide
Frequently asked questions
How do I convert Celsius to Fahrenheit?
F = C × 1.8 + 32. The +32 offset is what people forget: 20°C is 68°F, not 36°F. Going the other way, C = (F − 32) ÷ 1.8 — subtract first, then divide, or you'll get the wrong answer. Temperature is unusual because the scales start at different zero points, so it's not a simple ratio like every other unit conversion.
Why does my 1 TB hard drive show only 931 GB?
Two definitions of 'giga' are in play. Manufacturers use decimal SI units (1 TB = 1,000,000,000,000 bytes), while Windows divides by 1024 three times but still labels the result 'GB'. That gives 931 — technically 931 GiB. Nothing is missing. macOS and Linux report true decimal GB, so the same drive shows about 1000 GB there.
What is the difference between GB and GiB?
GB (gigabyte) is decimal: 1,000,000,000 bytes. GiB (gibibyte) is binary: 1,073,741,824 bytes (1024³). The difference is about 7.4% at the GB level and grows with scale — roughly 10% at TB. Storage vendors quote GB, Windows displays GiB while calling it GB, and that mismatch is the source of nearly every 'missing space' question.
How do I convert mpg to litres per 100 km?
Use 235.2 ÷ mpg. They're inverse measures — mpg is distance per fuel (higher is better) while L/100km is fuel per distance (lower is better) — so you can't scale one into the other linearly. 30 mpg is 7.84 L/100km. This inversion is why doubling mpg doesn't halve your L/100km figure in the way people expect.
Need a tool we don't have yet?
Tell us what you're trying to do. We build the most-requested tools first — every SnoopTool utility is free, runs in your browser, and needs no sign-up.