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Metric to Imperial Conversion: Exact Factors and How to Use Them
Last updated: September 2026 · Independently QA-tested formulas
Most conversion mistakes aren't math mistakes — they're factor mistakes: using a rounded 2.2 for kg→lb on a 90 kg shipment, or confusing US and imperial gallons. The unit converters on CalcProMaster use exact standard factors; this guide gives you the same numbers for hand work.
The one rule: multiply to convert, divide to reverse
metric value = imperial value ÷ factor
Worked example: 72 kg to pounds → 72 × 2.20462 = 158.73 lb. Reverse check: 158.73 ÷ 2.20462 = 72.00 ✓ — if the round trip doesn't return your starting value, you used the wrong factor or divided when you should have multiplied.
Exact conversion factors (standard tables)
Length
| From → To | Factor | Reverse |
|---|---|---|
| cm → in | × 0.393701 | 1 in = 2.54 cm (exact) |
| m → ft | × 3.280840 | 1 ft = 0.3048 m (exact) |
| km → mi | × 0.621371 | 1 mi = 1.609344 km (exact) |
| mm → in | × 0.0393701 | 1 in = 25.4 mm |
Mass & weight
| From → To | Factor | Reverse |
|---|---|---|
| kg → lb | × 2.20462 | 1 lb = 0.45359237 kg (exact) |
| g → oz | × 0.0352740 | 1 oz = 28.3495 g |
| kg → st (stone) | × 0.157473 | 1 st = 6.35029 kg |
Volume (watch the gallon trap)
| From → To | Factor | Note |
|---|---|---|
| l → gal (US) | × 0.264172 | 1 US gal = 3.78541 l |
| l → gal (imp) | × 0.219969 | 1 imp gal = 4.54609 l — 20% bigger! |
| ml → fl oz (US) | × 0.0338140 | 1 fl oz = 29.5735 ml |
Data & speed
| From → To | Factor | Note |
|---|---|---|
| MB → GiB | ÷ 1073.74 | binary GiB = 1024³ bytes, decimal MB = 10⁶ |
| km/h → mph | × 0.621371 | same factor as km→mi |
| m/s → km/h | × 3.6 | exact |
Temperature: a formula, not a factor
Because the scales have different zero points, no single multiplier works. 37 °C → 37 × 1.8 + 32 = 98.6 °F. −40 is the crossover: −40 °F = −40 °C exactly.
Why "close enough" factors cost money
Rounding 2.20462 → 2.2 is a 0.2% error — invisible on a gym scale, meaningful on a 2-ton freight quote (4 kg off) or repeated through a bill of materials. Precision costs nothing when a converter does the work; keep 6 significant figures until the final display, then round once.
Round-trip drift: a 10-second self-check
The fastest way to catch a bad factor is to convert there and back. Convert 10 m to feet: 10 ÷ 0.3048 = 32.8084 ft. Convert back: 32.8084 × 0.3048 = 10.000000 m — exact, because the factor is defined. If your chain returns 9.97 or 10.03, an intermediate rounding (or a US-vs-imperial gallon mix-up) is eating precision. Run the round-trip whenever a spreadsheet someone else built produces numbers you can't reproduce.
Worked example: Length Converter at its default inputs
A worked example is only useful if you can reproduce it. The defaults on Length Converter give exactly the figures in this section, with no rounding done by us.
| Input | Default |
|---|---|
| Value | 1 |
Result: 3.2808 ft
The steps behind that answer
- Step 1: Convert to meters: 1 m = 1.0000 m
- Step 2: Convert to ft: 1.0000 / 0.3048 = 3.2808 ft
Sensitivity: Value from half to one-and-a-half times
Same calculator, same fields, one change: value.
| Value | Result |
|---|---|
| 0.5 | 1.6404 ft |
| 1 | 3.2808 ft |
| 1.5 | 4.9213 ft |
Halve value and the result moves one way; add half again and it moves the other. Watching that gap is quicker than guessing which field matters most.
Related guides
Exact definitions and approximate factors
Some conversions are exact by definition: an inch is 25.4 millimetres, a pound is 0.45359237 kilograms. Others depend on the substance: a cup of flour and a cup of water do not weigh the same, and a US cup is not a metric cup.
Use exact factors for length, mass and volume of liquids. Use a density table for anything dry and bulky, or weigh it.
Temperature is not a simple multiplication
Celsius and Fahrenheit have different zero points as well as different step sizes, so converting requires an offset: F equals C times 9/5 plus 32. Applying a multiplier alone is the most common conversion error there is.
Kelvin shares the Celsius step size but starts at absolute zero, so it needs an offset of 273.15 and no multiplication.
Rounding after converting
Convert first, round second. Rounding an input and then converting compounds two errors, and it is easy to spot in a recipe or a specification where the tolerance is tight.
Carry more digits than you need through the calculation, then round the final value to the precision the original measurement supported.
Frequently Asked Questions
Why are some factors "exact" and others not?
Since 1959 the inch is defined as 2.54 cm and the pound as 0.45359237 kg, so those are exact by treaty. Factors like km→mi derive from the exact inch/metre definitions and are exact too (1 mi = 1609.344 m); approximations only creep in when someone rounds early.
How many feet is 180 cm?
180 × 0.393701 = 70.866 in = 5 ft 10.87 in — call it 5′11″. The length converter shows feet+inches directly.
What's 100 km/h in mph?
100 × 0.621371 = 62.14 mph — the origin of the "62 mph ≈ 100 km/h" highway rule of thumb.
GB vs GiB — why does my 1 TB drive show 931 GB?
Drive makers use decimal TB (10¹² bytes); operating systems display binary TiB (2⁴⁰ bytes). 10¹² ÷ 2⁴⁰ = 0.909 → a "1 TB" drive shows ≈ 909–931 "GB" depending on the OS's labeling. Nothing is missing.
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