Mass Converter for SI Units (kg, g, mg, µg, t)

A mass converter built around SI units

This mass converter is for the metric side of measurement: micrograms, milligrams, grams, kilograms and tonnes, the units used in laboratories, science classes, technical data sheets and dosage calculations. It starts from grams, lists the value in every supported unit at once, and lets you set how many significant digits are shown.

The same unit list is available in the weight converter, which is the better page for pounds, ounces, stones and everyday uses such as body weight, cooking and shipping. Use this page when you work in SI units and care about exact powers of ten, notation and precision.

SI prefixes and the gram

The kilogram is the SI base unit of mass. It is the only base unit whose name already carries a prefix, which is why the other prefixes are attached to the gram: milligram, microgram and so on. Since 2019 the kilogram has been defined by fixing the numerical value of the Planck constant at 6.626 070 15 × 10⁻³⁴ J s.

Each step in this ladder is a factor of 1000. The tonne is not itself an SI unit but is accepted for use with the SI; in prefix terms it is a megagram. In the unit list the microgram appears as ug, since the micro sign is awkward on most keyboards.

Mass is not weight

In physics, mass measures how much matter an object contains, while weight is the force that gravity exerts on it. Weight is measured in newtons and equals mass times the local gravitational acceleration. Using the conventional standard gravity of 9.80665 m/s², a 1 kg mass has a weight of 9.80665 N. The same object would weigh less on the Moon while its mass stayed 1 kg.

This tool converts mass only. If a problem asks for a force, convert the mass to kilograms here, multiply by the acceleration, and then use the force converter to move between newtons, kilogram-force and other force units.

Worked examples for lab and science work

0.5 mg    = 500 ug
250 ug    = 0.25 mg
0.0123 g  = 12.3 mg
2.5 g     = 0.0025 kg
1500 kg   = 1.5 t
3 t       = 3000000 g

For concentrations and solutions, mass alone is rarely enough. Combine a result from this page with the density converter, which works in units such as g/cm³, g/L and mg/L, or with the volume converter when a recipe or protocol mixes mass and volume.

Precision, significant digits and notation

Conversions between SI mass units are exact powers of ten, so the digits you see depend only on your input and on the Significant digits setting. You can choose 3, 4, 5, 6, 8, 10, 12 or 15 digits; 10 is the default. Precision matters more when you cross into non-metric units in the list: 1 g is 0.0353 oz at 3 significant digits and 0.0352739619495804 oz at 15.

A good rule is to show no more digits than the measurement supports. Note that trailing zeros are not displayed, so a reading of 0.2500 g converts to 250 mg, not 250.0 mg; record the precision in your notes if it matters.

Very small and very large results switch to scientific notation. Values below 0.000001 or at 10¹⁵ and above appear as, for example, 1e-9 (1 µg in kilograms) or 1e+18 (a million tonnes in micrograms). A Thousands separators option groups long numbers for easier reading.

Entering values correctly

• Type a plain number; the unit comes from the list, so 2.5 kg is rejected while 2.5 with kilograms selected works.
• Scientific notation such as 1.5e-3 is accepted.
• Use a point for decimals. Commas are treated as thousands separators and removed, so 1,5 is read as 15, and a hint in the status line says so.
• You can type into the result box to convert backwards, use Swap to reverse the units, or click any unit in the table to make it the target.
• The smallest unit is the microgram. For nanograms, divide by 1000 and enter the value in micrograms.

All calculations run in your browser, and nothing you enter is sent to a server.