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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.
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.
| 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 |
| 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 |
| 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 |
| 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 |
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.
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.
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.
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.
180 × 0.393701 = 70.866 in = 5 ft 10.87 in — call it 5′11″. The length converter shows feet+inches directly.
100 × 0.621371 = 62.14 mph — the origin of the "62 mph ≈ 100 km/h" highway rule of thumb.
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.