You install a new hard drive and it reports less than advertised. You follow an overseas recipe and the dough comes out wrong. You read a US fuel economy figure and can't quite place it.
All three are the same problem. Unit conversion looks like a single multiplication with nothing to get wrong, yet the answer hinges on which standard you multiplied by. This post sorts out those forks.
1. 🔴 Why 1TB becomes 931GB
Start with the one everyone hits. Nothing is mislabeled and no capacity was skimmed. Two parties are simply using different bases.
| Who | Bytes per KB |
|---|---|
| Drive manufacturers | 1,000 (decimal) |
| Operating systems | 1,024 (binary) |
The manufacturer's 1TB is 1,000,000,000,000 bytes. Divide that the way the OS does:
1,000,000,000,000 ÷ 1024 ÷ 1024 ÷ 1024 = 931.32
931GB. About 7%, and the gap grows with capacity. A 1GB file shows as 0.931GB for the same reason.
Which one is right? Both. Manufacturers follow the SI prefix rule (kilo means 1000), while operating systems inherited 1024 because memory addressing runs in powers of two. There is a standard that names the binary units separately — KiB, MiB, GiB — but Windows still computes in 1024 and labels it GB anyway.
So the answer to "is my disk faulty?" is almost always no.
2. An ounce can be weight or volume
This trap comes from a shared name. Both are written oz.
| Which ounce | Value |
|---|---|
| Weight ounce (oz) | 28.35g |
| Fluid ounce (fl oz) | 29.57mL |
The closeness is what makes it dangerous — swap them and the result still looks plausible.
With water it roughly works out, since 1mL of water weighs about 1g. But for ingredients with a different density it falls apart. Eight ounces of flour by weight is 227g; read as fluid ounces and measured into a cup it's about 130g. Nearly double.
Telling them apart in a recipe is simple: fl oz means volume, a bare oz usually means weight. Meat, cheese, and flour go by weight; milk, oil, and water by volume.
3. Gallons and cups differ by country
Some units share a name but not a value.
| Unit | US | UK |
|---|---|---|
| Gallon | 3.785L | 4.546L |
The UK gallon is about 20% larger. That's why comparing US mpg to UK mpg directly is off by a fifth — the same car looks more efficient under UK figures.
Cups too:
| Which cup | Value |
|---|---|
| US (legal) | 240mL |
| US (customary) | 236.6mL |
| Korea / Japan measuring cup | 200mL |
Follow a US recipe with a Korean measuring cup and you under-add by 20% every time. Water and milk hide it; baking, where ratios matter, does not.
The Unit Converter on this site uses US values. For imperial units, convert once more using the figures above.
4. 🔴 Temperature is not a multiplication
This trap is a different kind. Length, weight, and volume are all ratios starting from zero — but temperature scales put zero in different places.
°F = °C × 1.8 + 32
That trailing +32 breaks the pattern:
| Celsius | Fahrenheit |
|---|---|
| 20°C | 68°F |
| 40°C | 104°F (not 136°F) |
Celsius doubled; Fahrenheit rose by barely more than half. So "the temperature doubled" doesn't hold — the multiple depends on which scale you measure with.
For the same reason, converting a temperature difference means dropping the +32. "It rose 5 degrees" becomes 5 × 1.8 = 9 degrees Fahrenheit, not 5 × 1.8 + 32 = 41. A temperature and a temperature difference are different calculations.
Kelvin is the opposite — its zero is physically fixed (−273.15°C), so ratios do work. That's why scientific work uses it.
5. Units with a hidden baseline
Some units have no single value, and a tool just picks one.
Mach — usually computed as 340m/s, which is the sea-level figure. The speed of sound depends on air temperature, so at the ~11km cruising altitude of an airliner it drops to roughly 295m/s. The same Mach 1 is a different actual speed depending on altitude.
A year — calculators generally use 365 days. Leap days are missing, so it runs short of the actual solar year (about 365.2422 days). Irrelevant over a day or two; over decades it drifts by days.
A month — has no definition at all. Anywhere from 28 to 31 days, so computing "three months from now" in seconds gives different answers depending on which months you cross. Date math belongs to a tool that knows the calendar, like the D-Day Calculator, not to unit conversion.
6. How to check
- Disk capacity looking small is normal — 1000 versus 1024, and 1TB really should read 931GB
- When you meet
oz, decide weight or volume first.fl ozis volume - Check whose gallon or cup you have. The UK gallon is 20% larger than the US one
- Temperature is not a ratio, and a temperature differs from a temperature difference
- For Mach, years, and months, check that the hidden baseline matches your situation
To convert something right now, try the Unit Converter. It moves between seven categories — length, weight, temperature, volume, speed, time, and data — in one screen, and temperature uses the offset formula above rather than a ratio. Data sizes are computed in 1024, matching what your operating system shows.
To turn a daily water target into cups, the Water Intake Calculator is the neighboring tool; to count a span in days, the D-Day Calculator.
Summary
- 🔴 1TB = 931GB — manufacturers divide by 1000, operating systems by 1024. Both are correct
- Two kinds of ounce — 28.35g by weight, 29.57mL by fluid. They diverge badly at other densities
- The UK gallon is 20% larger, and a Korean measuring cup is 40mL smaller than a US one
- 🔴 Temperature carries a
+32, so it isn't a ratio — double 20°C is 104°F, not 136°F - Mach and "a year" hide a baseline; confirm it before you rely on the number
