Weight Converter
Convert between weight units like kilograms, pounds, ounces.
Mass conversion, and why "weight" is the wrong word
Kilograms measure mass, which does not change with location. Weight is a force and does change - you weigh less at the equator and considerably less on the Moon. Everyday usage conflates them, but the distinction becomes practical as soon as physics or aviation is involved.
Formula
target = source x (source factor to kilograms / target factor to kilograms)
All conversions pivot through the kilogram, which since 2019 is defined by a fixed value of the Planck constant.
Mass, weight, and the units that confuse them
Mass is the amount of matter, measured in kilograms. Weight is the gravitational force on that mass, measured in newtons. A 70 kg person has a weight of about 687 N on Earth and 114 N on the Moon, while their mass is 70 kg in both places.
Even on Earth, gravitational acceleration varies from about 9.764 m/s^2 near the equator to 9.834 m/s^2 at the poles, a difference of roughly 0.7%. Precision scales are calibrated for local gravity for this reason.
The pound is ambiguous in exactly this way. The pound-mass (lb) is 0.45359237 kg exactly, while the pound-force (lbf) is a unit of force. Most everyday use means pound-mass, but engineering documents must specify.
Tonnes, tons, and a 10% trap
Three different "tons" are in active use and they differ enough to matter commercially.
- Metric tonne: 1,000 kg exactly. Used internationally and in all SI contexts.
- US short ton: 2,000 lb, or 907.185 kg. Standard in the United States.
- UK long ton: 2,240 lb, or 1,016.047 kg. Still used in some shipping and commodity contexts.
- The spread between short and long ton is about 12%. On a 500-tonne shipment that is a 60-tonne discrepancy, so contracts specify which ton applies.
Troy weight is a separate system
Precious metals are traded in troy ounces, which are heavier than the avoirdupois ounces used for everything else. A troy ounce is 31.1035 g against 28.3495 g for the standard ounce.
The troy pound compounds the confusion by containing 12 troy ounces rather than 16, so it is 373.24 g while the ordinary pound is 453.59 g. A troy ounce is heavier, but a troy pound is lighter.
Gemstones use a third system entirely: the metric carat, fixed at exactly 200 mg since 1907.
Worked example
Convert 11 stone 4 pounds to kilograms. A stone is 14 pounds, so this is (11 x 14) + 4 = 158 lb. Then 158 x 0.45359237 = 71.67 kg.
Going the other way, 80 kg to stones and pounds: 80 / 0.45359237 = 176.37 lb. Divide by 14 to get 12 stone with 8.37 pounds remaining, so 12 stone 8 pounds.
A gold bar of 1 kg contains 1,000 / 31.1035 = 32.15 troy ounces, not the 35.27 avoirdupois ounces the same mass would be in ordinary units. Quoting the wrong ounce overstates the metal by about 10%.
Mass units in kilograms
| Unit | In kilograms | Note |
|---|---|---|
| gram | 0.001 | Exact |
| ounce (avoirdupois) | 0.0283495 | Exact by definition |
| troy ounce | 0.0311035 | Precious metals |
| pound | 0.45359237 | Exact by 1959 agreement |
| stone | 6.35029318 | 14 pounds, UK body weight |
| short ton (US) | 907.18474 | 2,000 pounds |
| tonne (metric) | 1,000 | Exact |
| long ton (UK) | 1,016.0469 | 2,240 pounds |
Frequently Asked Questions
Related calculators
References
- BIPM - The International System of Units (SI Brochure, 9th edition)
- NIST - redefinition of the SI kilogram
Last reviewed: 2026-08-07. This page is informational. For legal, medical, tax, or financial decisions, confirm the result with a qualified professional.