US Customary Units vs. Metric Units: A Weight Conversion Guide

Two systems, one bridge

US customary and metric mass units look unrelated until you find the single number that joins them: the international avoirdupois pound is defined as exactly 0.45359237 kilograms, agreed in 1959. Every customary weight unit is a multiple of the pound, and every metric unit is a multiple of the kilogram, so that one equivalence lets you translate anything in either system into the other with no ambiguity.

Because the kilogram is the common anchor, the cleanest way to compare any two weight units is to express both in kilograms first. That is exactly what the table below does. If you need to run the numbers on your own figures, the ton to kg calculator converts between the systems in either direction.

The three tons are genuinely different weights

The table makes the central trap visible: "ton" names three separate quantities. A US short ton is 907.18474 kg, a UK long ton is 1,016.0469 kg, and a metric ton is 1,000 kg. The long ton is the heaviest, the short ton the lightest, and the metric ton sits between them. The gap from short to long ton is nearly 109 kg — over 12% — which is far too large to treat as interchangeable.

This is why cross-border weights need their ton type stated. A quantity quoted as "10 tons" is 9,071.8474 kg if short, 10,000 kg if metric, and 10,160.469 kg if long — a spread of more than a full metric ton across the same words. The kg to US short tons tool locks in the 907.18474 kg factor so the short-ton case is never mistaken for metric.

Converting pounds and ounces to kilograms

For weights below a ton, the pound is your pivot. Multiply pounds by 0.45359237 to get kilograms: 150 lb is 150 × 0.45359237 = 68.0388555 kg. Ounces are one-sixteenth of a pound, so an ounce is 0.028349523125 kg; a 12-ounce figure is 12 × 0.028349523125 = 0.3401942775 kg. Going the other way, one kilogram is about 2.2046226 pounds.

That kilogram-to-pound figure is worth memorising for quick estimates: a 20 kg suitcase is roughly 44 lb, and a 70 kg person is about 154 lb. For anything invoiced or regulated, though, use the full 0.45359237 factor rather than the rounded 2.2046226, since the rounding compounds as quantities grow.

• 1 ounce = 0.028349523125 kg
• 1 pound = 0.45359237 kg (exact, since 1959)
• 1 kilogram ≈ 2.2046226 pounds
• 1 short ton = 2,000 lb = 907.18474 kg

Which system a number belongs to

Context tells you which system you are reading. US domestic freight, agriculture, and construction typically quote short tons and pounds. International trade, science, and most of the world outside the US quote metric tons and kilograms. UK shipping and older British documents may still use the long ton. When the source is ambiguous, the surrounding units are the giveaway — pounds alongside a "ton" strongly imply a short ton.

Getting this identification right is more than half the battle in any conversion, because the arithmetic is trivial once the unit is fixed. The recurring errors are almost all mis-identification rather than bad multiplication, as the common mistakes guide details. Establish the system, anchor to kilograms, then convert.