Metric and imperial conversions people actually need
A short reference for the four everyday conversions on financecalc — with the exact factors and when to round.
Direct answer
Use the exact published factor for the pair you need, convert with full precision, and round once at the end. Length, mass, and distance are simple multiplications; temperature needs the 32°F offset as well.
Travel, recipes, fitness trackers, and school problems still mix metric and imperial units. Using a consistent factor — and rounding only at the end — avoids compounding error. Below are the definitions our converters use, with practical notes unique to each pair — including UK/EU contexts where metric meets imperial road signs, stone weight, or US product labels.
Quick factor table
| Pair | Exact factor / formula | Open tool |
|---|---|---|
| in ↔ cm | 1 in = 2.54 cm | Converter |
| lb ↔ kg | 1 lb = 0.45359237 kg | Converter |
| mi ↔ km | 1 mi = 1.609344 km | Converter |
| °F ↔ °C | °C = (°F − 32) × 5/9 | Converter |
Length: inches and centimeters
By international agreement, 1 inch = 2.54 centimeters exactly. Convert with multiply or divide; there is no offset. Height on forms is often given in feet and inches — convert total inches to centimeters in one step (for example, 70 inches × 2.54 = 177.8 cm).
UK height sketch: 5′9″ → 69 × 2.54 = 175.26 cm. Round once for the form. Use the inches ↔ centimeters calculator when you need a quick bidirectional check. Round to one decimal for clothing and many product specs; keep more digits for engineering drawings until the final dimension callout.
Mass: pounds and kilograms
The international avoirdupois pound is 1 lb = 0.45359237 kg exactly. Bathroom scales and shipping labels often show one decimal place; airline baggage rules may state limits in either unit (for example 23 kg ≈ 50.7 lb).
Note: UK “stone” (14 lb) and troy pounds for precious metals are different systems. Convert stone → pounds (×14) before using our pounds ↔ kilograms converter. Example: 11 st 0 lb = 154 lb ≈ 69.85 kg.
Distance: miles and kilometers
The international mile is 1 mile = 1.609344 km exactly. UK road signs still use miles; continental Europe uses kilometres. A nautical mile (~1.852 km) is different — do not use the road-mile factor for nautical charts.
Speed conversion uses the same factor when time is fixed (70 mph ≈ 112.65 km/h). Try the miles ↔ kilometers calculator for race and trip planning.
Temperature: Fahrenheit and Celsius
Temperature needs an offset as well as a scale factor:
- °C = (°F − 32) × 5/9
- °F = °C × 9/5 + 32
Water freezes at 0°C / 32°F and boils at 100°C / 212°F at standard pressure. Oven recipes and weather apps are the usual mix-ups; converting before you set a thermostat or bake temperature prevents large errors. Use the Fahrenheit ↔ Celsius converter for linked fields.
Rounding without lying to yourself
- Convert with full precision first.
- Round once for display or for a form field.
- Do not convert a rounded result back and forth repeatedly — error grows.
Bottom line
Exact definitions make length, mass, and distance conversions simple multiplications. Temperature needs the 32°F offset. Know which mile you mean, and round only when you present the answer. These notes are educational; for legal metrology or lab work, follow the standard your jurisdiction requires.