Everyday Life · Unit Conversions
Unit Converter (Weight)
Convert weight and mass between kilograms, pounds, ounces, grams, stones, and more with instant, accurate results.
Calculator
Formula
Where m_source is the input mass value, f_source is the conversion factor of the source unit relative to kilograms, and f_target is the conversion factor of the target unit relative to kilograms. All conversions pass through kilograms as the SI base unit of mass.
Source: NIST Special Publication 811 (2008 Edition): Guide for the Use of the International System of Units (SI). Bureau International des Poids et Mesures (BIPM), SI Brochure 9th Edition, 2019.
How it works
Every weight conversion works by first expressing the input quantity in kilograms — the SI base unit of mass — and then converting that intermediate kilogram value into the desired output unit. Each unit has a precisely defined relationship to the kilogram. For example, one pound is defined as exactly 0.45359237 kilograms under the international avoirdupois agreement, and one ounce is exactly 1/16 of a pound, or approximately 0.028349523125 kilograms. These definitions are fixed by international treaty and do not change with measurement conditions.
The distinction between weight and mass is scientifically important but often ignored in everyday use. Mass is the intrinsic amount of matter in an object, measured in kilograms (SI). Weight is the gravitational force on that mass, measured in newtons. In common parlance — and in this calculator — "weight" refers to mass in everyday units like kilograms and pounds, since bathroom scales, food labels, and shipping invoices all express mass using weight-labeled units under standard Earth gravity. This calculator operates entirely in the mass domain, consistent with everyday and commercial usage worldwide.
The stone unit (st) is widely used in the United Kingdom and Ireland for expressing body weight. One stone is exactly 14 pounds, or approximately 6.35029318 kilograms. The metric ton (also spelled tonne) equals exactly 1,000 kilograms, while the US short ton equals 2,000 pounds (~907.18 kg) and the UK long ton equals 2,240 pounds (~1,016.05 kg). Understanding these distinctions is critical in shipping, commodities trading, and international commerce where unit ambiguity can cause costly errors.
Worked example
Example: Converting 165 pounds to kilograms, grams, and stone.
Start with the input: 165 lb. The exact definition of one pound is 0.45359237 kg.
Step 1 — Convert to kilograms:
165 × 0.45359237 = 74.843 kg (rounded to 3 decimal places)
Step 2 — Convert to grams:
74.843 kg × 1,000 = 74,843.24 g
Step 3 — Convert to stone:
165 lb ÷ 14 lb/st = 11.786 st, or equivalently 11 stone 11 pounds.
This type of multi-unit conversion is common when a person measured in pounds needs to report their weight in the UK (stone) or for a medical record using SI units (kilograms). The calculator performs all of these simultaneously so you can verify each output at a glance.
Limitations & notes
This calculator converts units of mass using fixed, internationally defined conversion factors. It does not account for gravitational variation — if you were to take your bathroom scale to the Moon, the reading would change, but the actual mass of the object would not. Additionally, troy ounces (used for precious metals, where 1 troy oz = 31.1035 g) differ from avoirdupois ounces (used in everyday commerce, where 1 oz = 28.3495 g); this calculator uses avoirdupois ounces throughout. For precious metals calculations, always confirm which ounce system your source data uses. Very large or very small values may exhibit minor floating-point rounding from JavaScript's 64-bit arithmetic, but this is negligible for any practical application.
Frequently asked questions
How many kilograms are in a pound?
One pound is defined as exactly 0.45359237 kilograms under the international avoirdupois agreement, adopted in 1959 by the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa. This is an exact definition — there is no rounding involved. Conversely, one kilogram equals approximately 2.20462 pounds.
What is the difference between a metric ton, a short ton, and a long ton?
A metric ton (or tonne) equals exactly 1,000 kilograms (approximately 2,204.62 lb) and is the standard large-mass unit in the SI system used globally. A short ton, used in the United States, equals 2,000 pounds (approximately 907.18 kg). A long ton, used historically in the United Kingdom, equals 2,240 pounds (approximately 1,016.05 kg). Always clarify which 'ton' is meant in shipping contracts or commodity pricing to avoid significant errors.
How is stone used as a unit of weight?
The stone (abbreviated 'st') is a unit of mass equal to exactly 14 pounds or approximately 6.35029318 kilograms. It remains in common everyday use for expressing body weight in the United Kingdom and Ireland, though it is not an SI unit and is not recognized for most other scientific or commercial purposes. A person weighing 75 kg would be approximately 11 stone 11 pounds (11.81 st).
What is the difference between a regular ounce and a troy ounce?
The avoirdupois ounce (the everyday ounce used for food, body weight, and general commerce) is defined as exactly 28.349523125 grams, while the troy ounce (used for precious metals like gold, silver, and platinum) is defined as exactly 31.1034768 grams. This means a troy ounce is about 9.7% heavier than an avoirdupois ounce. This calculator uses avoirdupois ounces exclusively, so if you are converting precious metal weights, be sure to use the correct troy factor.
Is weight the same as mass?
Scientifically, no — mass is the intrinsic amount of matter in an object (measured in kilograms in SI), while weight is the gravitational force acting on that mass (measured in newtons). However, in everyday life, cooking, commerce, and body weight measurement, the word 'weight' is universally used to mean mass under standard Earth gravity (9.80665 m/s²). This calculator treats 'weight' and 'mass' as interchangeable in the common sense, consistent with how scales, food labels, and shipping documents work in practice.
Last updated: 2025-01-15 · Formula verified against primary sources.