Measurement Guide

Metric and Imperial Conversions: the Factors, the Traps and the Rounding Rules

By ToolFlic Expert Team Updated 8 October 2026 798 words

Unit conversion is exact when you use the fixed factor and only round at the end: 1 inch = 2.54 cm by definition, 1 pound = 0.45359237 kg by agreement, and 1 foot = 0.3048 m. The errors people hit are not the maths - they are the ambiguous units (ton vs tonne vs stone) and rounding too early, which compounds.

Do the arithmetic with the browser length, weight and temperature converters (or the general unit converter), which apply the standard factors, and use the tables and rounding rules below for the cases where the unit itself is the trap.

The factors that are exact by definition

Memorise the anchors and derive the rest, because these are not rounded approximations:

Quantity	Exact relationship	Handy approximation
Length	1 in = 2.54 cm; 1 ft = 12 in; 1 mi = 1.609344 km	1 m ~ 39.37 in
Weight	1 lb = 0.45359237 kg; 1 oz = 28.349523 g	1 kg ~ 2.205 lb
Temperature	°F = °C x 9/5 + 32	°C = (°F - 32) x 5/9
Volume	1 US gal = 3.785412 L; 1 imp gal = 4.54609 L	US vs Imperial differ!
Area	1 acre = 4046.8564224 m²; 1 ft² = 0.09290304 m²	1 ha = 10,000 m²

Note the volume row: a US gallon and an Imperial gallon are different sizes, so "gallon" is ambiguous the same way "ton" is. Always ask which system a number came from before converting.

The ton family: the most expensive ambiguity

Three unrelated units share the word "ton", and mixing them has broken shipping manifests and material orders:

Metric tonne (t) = exactly 1,000 kg.

US short ton = 2,000 lb = about 907.18 kg.

UK long ton (imperial ton) = 2,240 lb = about 1,016.05 kg.

They sit within roughly 10% of each other, which is enough to be catastrophic at scale. Also know stone (1 stone = 14 lb = about 6.35 kg), used for body weight in the UK and absent from every metric system.

Feet-and-inches and the mixed-unit trap

Human heights and many dimensions are written as mixed units, like 5'7" or 6 ft 2 in, and the failure mode is converting the feet and the inches separately then adding them wrong. Convert the whole thing: 5'7" = (5x12 + 7) = 67 inches, x 2.54 = 170.18 cm.

In construction and DIY the same care applies to area and volume: you cannot convert a square footage to square metres by the length factor alone without squaring it (multiply ft² by 0.09290304), and a concrete order in cubic yards is not the same as cubic metres. Get the units dimensionally right before you round.

Rounding rules for cooking, travel and building

Round to the precision the task actually needs, and only at the end. Cooking tolerates 1 kg = 2.2 lb and an oven at 180°C vs 350°F, but a recipe's small measures (a tablespoon) should be converted with a fixed factor, not eyeballed. Travel is forgiving; body-weight tracking to 0.1 kg is not.

Building and shipping are the opposite: keep full precision through the calculation and round the final figure up for materials where under-ordering costs more than a small surplus. Chaining rounded intermediate values is how a small conversion becomes a 10% error.

Converting cleanly with the tools

The converters apply the exact factors and run entirely in your browser with no upload, so there is no reason to hand-multiply and risk a slip - enter the value once and read the result. The general unit converter covers length, weight, temperature and more across categories when you are unsure which specific tool fits.

What the tools cannot decide for you is which unit was meant: they convert the number you give, so you still have to know whether a source "ton" is a tonne or a short ton. The honest habit is to confirm the unit system first, convert second, and round third.

Frequently Asked Questions

Is a tonne the same as a ton?

No. A metric tonne is exactly 1,000 kg, a US short ton is about 907 kg and a UK long ton is about 1,016 kg. They differ by up to roughly 10%, so at commercial scale confusing them is a real error, not a rounding nit.

How do I convert 5 foot 7 to centimetres?

Turn it all into inches first: 5x12 + 7 = 67 in, then multiply by 2.54 = 170.18 cm. Converting feet and inches separately and then adding is the usual mistake; collapse to one unit before you multiply.

What is a stone in kilograms?

One stone is 14 pounds, about 6.35 kg. It is a UK body-weight unit with no metric counterpart, so convert stone to kg by multiplying by 6.35029318 and round to whatever precision you need.

Why is my oven temperature conversion slightly off?

Because °F = °C x 9/5 + 32 is exact and ovens are named in rounded steps. 180°C is 356°F, but most dials jump by 10 or 25, so you round to the nearest mark - the conversion is right, the dial is coarse.

Should I round during a multi-step conversion?

No - keep full precision through every step and round only the final answer. Rounding intermediates compounds the error, which is how a small length conversion becomes a wrong material quantity in a build or shipment.

Are these unit converters accurate and private?

Yes to both - they apply the standard exact factors and run entirely in your browser, uploading and storing nothing. They cannot tell you which unit a source meant (ton vs tonne), so confirm the system first, then convert.