Weight Converter

Using the Mass / Weight Converter

1. Choose the source unit in the "From" dropdown. You will find milligram, gram, kilogram, metric ton, ounce (avoirdupois), pound, stone and US short ton - covering laboratory weighing, cooking, body weight and shipping manifests.
2. Enter the quantity into the input field. The output refreshes with every keystroke because the handler is wired to onInput, not to a submit button.
3. Choose the destination unit in the "To" dropdown. The tool computes both directions from the same kilogram-based factors, so the answer is the same whichever side you type into.
4. Click swap to mirror the conversion. Handy when you want a round trip (say, convert 2.5 kg to lb, then discover what 5.51 lb is in kg).
5. Hit Copy to send the output to your clipboard via the Clipboard API.

Mass vs Weight, and What the Tool Actually Computes

Strictly speaking, mass is the amount of matter (measured in kilograms) and weight is the force gravity exerts on that mass (measured in newtons). In everyday language "weight" means mass, and that is what this converter handles. Every unit in the picker is stored as a ratio to the kilogram - the SI base unit of mass - and conversions reduce to result = value * factorFrom / factorTo in IEEE 754 double precision. The pound is defined as exactly 0.45359237 kg by the 1959 international yard and pound agreement; the avoirdupois ounce is 1/16 of a pound; stone is 14 pounds; the US short ton is 2000 pounds. All those ratios are rational, so the chain has no definition-level rounding.

When to Use This Tool

• Translating a US recipe giving ingredients in ounces into grams for a precise kitchen scale.
• Converting package weight from kilograms to pounds to fill a carrier label or customs form.
• Checking a gym weight plate stamped in pounds against a metric program written in kilograms.
• Reading a UK medical note that gives body weight in stones and pounds, while your chart is in kilograms.
• Spec-ing cargo for freight: short tons, metric tons (tonnes) and pounds get mixed constantly in logistics paperwork.
• Laboratory notebooks where a milligram entry needs to match a paper that reported micrograms or grams.

Edge Cases and Pitfalls

The biggest trap is confusing the avoirdupois ounce (the everyday ounce, 28.349523125 g) with the troy ounce (used for precious metals, 31.1034768 g). This tool only offers avoirdupois. If you are weighing gold or silver, multiply by 1.09714 to get troy. A second trap is the "ton": in the US, "ton" usually means short ton (2000 lb, 907.18474 kg); in the UK it historically meant long ton (2240 lb, 1016.047 kg); and in the metric world "tonne" means 1000 kg. Shipping carriers are often ambiguous - always verify with the carrier which one the rate card uses. A third trap: gravity. On the Moon your mass is unchanged but your weight-as-force drops to about one sixth. If you need Newtons, multiply kilograms by the local gravitational acceleration (about 9.80665 m/s^2 on Earth per CGPM 1901, exactly defining standard gravity).

A Short History of the Kilogram

The kilogram was originally defined in 1795 as the mass of one liter of water at the temperature of melting ice. From 1889 until 2019 it was defined as the mass of the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder held at the BIPM outside Paris. Comparison over a century showed that the IPK and its sister copies drifted against each other by tens of micrograms - embarrassing for a base unit - which triggered the May 2019 SI redefinition. The kilogram is now fixed by assigning an exact value to the Planck constant (h = 6.62607015 x 10^-34 J s) and realizing mass through a Kibble (watt) balance. See the SI Brochure (BIPM, 9th edition) and NIST SP 811 Appendix B.8. That makes the kilogram, finally, a unit of nature rather than a lump of metal in a vault.

Comparing the Alternatives

Google's inline converter is quick for a single question but sends your query to their servers and does not preserve state. The GNU units command-line tool supports troy ounces, carats, grains and hundreds of other units this tool does not expose - better for apothecary and historical work. Scientific software (MATLAB, SciPy's scipy.constants) is the right choice when you need to propagate uncertainty or chain mass into force, pressure or energy equations. For commercial legal-for-trade weighing (a butcher's scale, a truck scale), you need a NTEP or OIML certified device, not a web converter - the tool is a reference, not a regulated instrument.