Weight / Mass Converter
Free weight and mass converter between metric (mg, g, kg, t) and imperial (oz, lb) units. Uses the exact international avoirdupois definitions. Works in your browser.
Choose source and target units, type a value, and the calculator converts between any pair of metric and avoirdupois mass units.
The conversion factors
All exact since the 1959 international yard-and-pound agreement:
| Unit | Kilograms |
|---|---|
| 1 mg | 0.000001 kg |
| 1 g | 0.001 kg |
| 1 kg | 1 kg |
| 1 t (metric tonne) | 1,000 kg |
| 1 oz (avoirdupois) | 0.028349523125 kg |
| 1 lb (avoirdupois) | 0.45359237 kg |
Everything goes through kilograms internally.
Avoirdupois only
This is the “everyday” mass system — what your kitchen scale and your shipping label use. We do not handle:
- Troy ounces (precious-metal trade; 31.1035 g, different from the ~28.35 g avoirdupois ounce).
- Fluid ounces (a volume unit, not mass; the UK and US fluid ounces also differ).
- Stones (1 stone = 14 lb; common only in British personal-weight contexts).
If you need any of those, conversion by hand against the table above is straightforward.
Useful approximate equivalents
- 1 kg ≈ 2.205 lb (a 70 kg person ≈ 154 lb)
- 1 lb ≈ 0.454 kg (a 200 lb person ≈ 90.7 kg)
- 1 oz ≈ 28.35 g (a 4 oz beef patty ≈ 113 g)
- 1 stone = 14 lb ≈ 6.35 kg (12 stone ≈ 76 kg)
For precision use the calculator; for back-of-the-napkin estimates, the ratios above are tight enough.
Worked examples
-
1 pound in kilograms
1 lb = 0.4535924 kg
-
1 kilogram in pounds
1 kg = 2.204623 lb
Frequently asked questions
What's the difference between weight and mass?
Strictly speaking, mass is how much stuff there is (constant), and weight is the gravitational force on that mass (varies — you'd weigh less on the Moon). Everyday language uses "weight" for both, and so does this calculator: input/output are in mass units (kg, lb, etc.). The conversion factors are the international avoirdupois definitions.
Are these the exact factors?
Yes. Since 1959 the international avoirdupois pound is defined as exactly 0.45359237 kg. The ounce is 1/16 lb (so 0.028349523125 kg, exact). The metric prefixes are SI by definition. Internal math is full double-precision.
Imperial vs. US customary ounces and pounds?
For dry weight, the US and UK use the same avoirdupois pound and ounce. The names that differ in detail are the *fluid* ounce (volume — UK fluid oz ≠ US fluid oz) and the *troy* ounce (used for precious metals; ~31.1035 g, different from the avoirdupois ~28.3495 g). This calculator handles only avoirdupois mass — no fluid or troy units.
What about stones?
Not included in this version. 1 stone = 14 lb = 6.35029318 kg if you need to convert by hand. The British personal-weight unit is the main place stones still appear.
How precise is this?
The conversions are exact to the limit of IEEE-754 double-precision (~15 significant digits). Display rounds trailing zeros and caps at 10 fractional digits — well beyond the precision of any consumer scale.
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