Mass is one of the seven base quantities in the International System of Units (SI), with base unit the kilogram (kg). Since 2019, the kilogram has been redefined in terms of the Planck constant h (6.62607015×10⁻³⁴ J·s) and no longer depends on the physical International Prototype Kilogram artifact. All other units on this page—g, mg, t, lb, oz, jin, liang, troy oz, ct—are defined as precise multiples or agreed values relative to the kilogram.
The metric mass system is the simplest, using factors of 1000 throughout: 1 t (tonne/metric ton)=1000 kg, 1 kg=1000 g, 1 g=1000 mg, 1 mg=1000 μg. Metric units are widely used in science, medicine, engineering, and global trade, and are the legal measurement units in China. Above kg, kt (kiloton) and Mt (megaton) are used for large scales including bulk cargo, minerals, ship displacement, and carbon emissions.
The imperial/US avoirdupois system is the everyday mass system used in English-speaking countries: 1 lb (pound)=16 oz (ounce)=7000 gr (grain)=453.59237 g (exact value, standardized by the 1959 International Yard and Pound Agreement). 1 oz=1/16 lb≈28.3495 g; 1 st (stone)=14 lb≈6.35 kg, used primarily in the UK/Ireland/Australia to describe human body weight. Long ton (UK)=2240 lb≈1016 kg, short ton (US)=2000 lb≈907 kg, metric ton=1000 kg.
Traditional Chinese market units (shizhi) were modernized and anchored to metric standards: 1 jin=500 g (exact)=10 liang=100 qian=1000 fen=10000 li, so 1 liang=50 g, 1 qian=5 g, 1 fen=0.5 g, 1 li=0.05 g. This is the standard progressively unified in mainland China since 1929 (Taiwan retains the Japanese-era 1 tai jin=600 g standard; Hong Kong/Macau have slightly different legal definitions). Market units remain in common use in everyday speech, grocery stores/markets, traditional food service, and Traditional Chinese Medicine / historical TCM prescriptions.
The troy weight system is used specifically for measuring gold, silver, platinum, and other precious metals and gemstones, originating from the medieval market weights of Troyes, France. Core units: 1 troy oz (ozt)=31.1034768 g=20 dwt (pennyweight)=480 gr (grain). Note that a troy pound contains 12 ounces=373.2 g (less than the avoirdupois pound of 453.6 g); the troy pound is now largely obsolete, and modern gold markets use troy oz as the standard. The carat (ct) is defined independently: 1 ct=0.2 g (200 mg), used for diamond and gemstone weight.
All mass conversions on this page use pure proportional linear scaling (mass has no zero offset), so each unit has a fixed factor relative to kg: target value = source value × (source factor / target factor). All factors are pre-calculated precisely, and conversions use only one double-precision multiply/divide operation, with no chained conversion error.
