Most unit conversions are one multiplication — as long as you use the right factor and know which of the three traps you're standing in: an offset unit (temperature), an ambiguous unit (cups, tablespoons, gigabytes), or a rounded factor quietly presented as exact. This guide gives you the exact factors for the conversions people actually do, worked examples you can check by hand, and the gotchas behind most wrong answers. When you just need the number, a free converter does all of this instantly — but two minutes here means you'll never be fooled by a bad one.
The one idea behind every unit conversion
A conversion factor is just a fraction equal to 1. Because 1 inch and 2.54 cm are the same length, multiplying by (2.54 cm / 1 in) changes the label without changing the quantity:
6 ft = 72 in, and 72 × 2.54 = 182.88 cm (exact)
That's the whole trick. The skill is in knowing three things about each factor:
- Is it exact or rounded? Some factors are exact by definition. Since the 1959 international yard and pound agreement, 1 inch = 2.54 cm, 1 pound = 0.45359237 kg, and 1 mile = 1.609344 km — exactly, not approximately. Other values you'll meet (1 kg ≈ 2.2046 lb) are rounded forms of an exact factor's reciprocal.
- Is it a pure multiplication or is there an offset? Length, weight, and volume scale from zero. Temperature doesn't — more on that below.
- Which version of the unit is it? A "cup", a "tablespoon", a "gallon", and a "gigabyte" each have more than one legal meaning.
Length and distance: the safe ones
Length conversions are the friendliest: exact factors, no offsets.
| From | To | Multiply by | Status |
|---|---|---|---|
| inches | centimetres | 2.54 | exact |
| feet | metres | 0.3048 | exact |
| yards | metres | 0.9144 | exact |
| miles | kilometres | 1.609344 | exact |
| kilometres | miles | 0.621371 | rounded (1 ÷ 1.609344) |
Worked examples:
- 5 miles to km: 5 × 1.609344 = 8.04672 km (exact).
- 10 km to miles: 10 ÷ 1.609344 = 6.21 miles (rounded to 2 dp; the unrounded value is 6.213711…).
The only real gotcha in length is going the "wrong way" with a rounded reciprocal. If you multiply by 0.62 instead of dividing by 1.609344, a marathon (42.195 km) comes out at 26.16 miles instead of the true 26.22 — close, but wrong in the second decimal. Rule of thumb: divide by the exact factor rather than multiplying by its rounded reciprocal whenever precision matters.
Weight and mass: exact factor, rounded habits
The pound is defined as exactly 0.45359237 kg. Everything else follows:
- 150 lb to kg: 150 × 0.45359237 = 68.0388555 → 68.04 kg (rounded to 2 dp).
- 80 kg to lb: 80 ÷ 0.45359237 = 176.3698… → 176.37 lb (rounded to 2 dp). The familiar "1 kg ≈ 2.2 lb" shortcut gives 176 — fine for luggage, not for a dosage.
- Ounces to grams: 1 oz = 28.349523125 g exactly (one sixteenth of a pound). So 8 oz = 226.796… ≈ 227 g — which is why recipe sites call a US stick-and-a-stick of butter (1 cup, 8 oz) "227 g".
Gotcha: ounces are overloaded. An ounce of weight (28.35 g) is not a fluid ounce of volume (29.5735295625 mL, exact for the US version — and the UK imperial fluid ounce is a different 28.41 mL again). If a recipe says "8 oz of cream", you need to know whether it's weighing or pouring.
Temperature: the one you can't just multiply
Temperature scales don't share a zero, so Celsius↔Fahrenheit needs a multiply and a shift:
°C = (°F − 32) × 5/9 °F = °C × 9/5 + 32
Worked examples (both exact):
- 68 °F to °C: (68 − 32) × 5/9 = 36 × 5/9 = 20 °C.
- 25 °C to °F: 25 × 9/5 + 32 = 45 + 32 = 77 °F.
The popular mental shortcut "double it and add 30" gives 25 °C → 80 °F — 3 degrees hot. It's a fine estimate for weather; label it as one and never use it for an oven or a fever.
Two more temperature facts worth keeping:
- −40 is the crossover point: −40 °C = −40 °F exactly (check it: −40 × 9/5 + 32 = −72 + 32 = −40).
- Kelvin is Celsius shifted, not scaled: K = °C + 273.15. A 1-degree step is the same size in both.
And the gotcha that ruins bakes: you cannot convert a temperature difference with the offset formula. "Reduce the oven by 20 °C" means reduce by 36 °F (20 × 9/5), not by (20 × 9/5 + 32) = 68 °F.
Cooking measures: same word, different sizes
Kitchen units are where conversions get genuinely ambiguous, because the same name means different volumes in different countries:
| Unit | US customary | Metric (AU/NZ, most of EU) | UK note |
|---|---|---|---|
| cup | 236.588 mL (exact: 236.5882365) | 250 mL | UK recipes rarely use cups |
| tablespoon | 14.79 mL (3 tsp) | 15 mL | Australia: 20 mL (4 tsp) |
| teaspoon | 4.93 mL | 5 mL | — |
So "1 cup" can legitimately mean anything from ~237 to 250 mL — about a 6% spread, enough to matter in baking. US nutrition labelling additionally rounds the cup to a "legal" 240 mL, which is why bottles and blogs disagree.
The bigger kitchen gotcha is volume vs weight. "1 cup of flour ≈ 120 g" is an approximation that depends on how the flour is packed — scooped vs spooned can differ by 20% or more. Water is the friendly exception: 1 mL of water weighs almost exactly 1 g at room temperature, so 1 cup (US) of water ≈ 237 g. For everything else, a conversion can only be as precise as the ingredient's density; good recipes (and good converters) say so.
Data sizes: GB vs GiB, the 7% that goes "missing"
Data units have two rival systems, and the confusion is institutionalized:
- Decimal (SI): 1 kB = 1,000 bytes; 1 GB = 1,000,000,000 bytes. Used by drive manufacturers and networking.
- Binary (IEC): 1 KiB = 1,024 bytes; 1 GiB = 2³⁰ = 1,073,741,824 bytes. Used by most operating systems' file managers (sometimes while displaying "GB").
That's why a "500 GB" drive shows up smaller on your computer:
500 GB = 500,000,000,000 bytes ÷ 1,073,741,824 ≈ 465.66 GiB
Nothing is missing — the same bytes are being described in two different units. The gap widens with each prefix step: a "1 TB" drive is 1,000,000,000,000 ÷ 2⁴⁰ ≈ 0.91 TiB, about 9% "shrinkage" on paper.
Gotcha inside the gotcha: bits vs bytes. Internet speeds are sold in megabits per second (Mb/s), files are measured in megabytes (MB), and 1 byte = 8 bits. A 100 Mb/s connection moves at most 12.5 MB/s — so a 1 GB (decimal) download takes at least 1,000 ÷ 12.5 = 80 seconds, not 10.
A five-line sanity checklist
Before trusting any conversion — yours or a website's:
- Direction check: did you multiply when you should divide? (Converting to a bigger unit should make the number smaller.)
- Offset check: is this temperature? Then a bare factor is wrong.
- Ambiguity check: which cup / tablespoon / ounce / GB is this?
- Rounding check: is the factor exact or rounded, and did you round only at the end?
- Ballpark check: does the answer pass a rough estimate? (5 miles should be "about 8 km", not 3 or 80.)
Once a conversion involves time — timezones, timestamps, durations — the rules change completely; see our guide to date and time tools. And if the thing you're converting is a colour code rather than a quantity, that's its own system too: hex, RGB, and HSL explained.
FAQ
Is 1 inch exactly 2.54 cm, or is that rounded?
Exactly. Since the 1959 international yard and pound agreement, the inch is defined as 2.54 cm, the pound as 0.45359237 kg, and the mile as 1.609344 km. Any extra decimals you see on those particular factors are noise; any fewer are rounding.
Why does my 500 GB drive show only about 465 GB?
The manufacturer counts in decimal gigabytes (10⁹ bytes) while your operating system likely counts in binary gibibytes (2³⁰ bytes) — and may still label them "GB". 500,000,000,000 ÷ 1,073,741,824 ≈ 465.66, so both numbers describe the same drive.
How do I convert Celsius to Fahrenheit in my head?
Exact: multiply by 9/5 and add 32 (25 °C → 77 °F). Estimate: double it and add 30 (25 °C → 80 °F) — good enough for weather, but treat it as ±4 °F and never use it for cooking or medical readings.
How many millilitres are in a cup?
It depends on whose cup: a US customary cup is 236.588 mL (rounded to 240 mL on US nutrition labels), a metric cup is 250 mL. For baking, the safest move is to use weight instead — or a converter that lets you pick which cup you mean.
Need the number right now? Medley Web is building a free hub of instant, in-browser converters — length, weight, temperature, cooking measures, data sizes, and more — one input in, many formats out. Convert any unit free at medley-web.com.
