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Weight Converter

Convert kilograms, grams, pounds, ounces, stone, tonnes, and other verified mass units locally.

Privacy: your files never leave your device. All processing happens locally in your browser.

How to use

  1. 1.Enter a non-negative mass using decimal or scientific notation without commas or unit text.
  2. 2.Choose the source and destination mass units; use the swap button to reverse them if needed.
  3. 3.Check the labeled result and kilogram basis, then copy the equation if you want to reuse it.

About Weight Converter

Weight Converter translates numerical mass values among kilogram, gram, milligram, microgram, metric tonne, avoirdupois pound, avoirdupois ounce, stone, grain, and metric carat. Enter a non-negative decimal value, select the source and destination units, and read the labeled equation immediately. Everything runs in the browser. No measurement is uploaded, no account is required, and the converter does not call a remote calculation service. You can reverse the selected units with the swap control and copy a successful result for a worksheet, shipping note, product specification, recipe, or engineering draft.

The implementation uses one transparent reference path. Every source value is first multiplied by its kilograms-per-unit factor, then the kilogram value is divided by the destination factor. Using a single base prevents a separate pairwise formula from being maintained for every combination. It also makes the reference table easy to audit: kilogram is the only row with factor 1, every other factor is finite and positive, and unit identifiers and symbols are unique. Tests lock the table at ten entries and exercise independently sourced conversions rather than relying only on round trips that could repeat the same mistake in both directions.

The SI relationships follow the BIPM prefix system. One kilogram is 1,000 grams, one gram is 1,000 milligrams, and one milligram is 1,000 micrograms. The prefix is attached to gram for these mass units; the interface therefore uses mg and µg rather than informal constructions such as microkilogram. One metric tonne, symbol t, is exactly 1,000 kilograms. This tool deliberately labels that unit “Metric tonne.” It does not offer an unlabeled “ton,” because a US short ton, UK long ton, and metric tonne are different quantities and an ambiguous selector could create a consequential conversion error.

The customary units in this scope use the avoirdupois system. The international avoirdupois pound is exactly 0.45359237 kilogram. One avoirdupois ounce is one sixteenth of that pound, so it equals 0.028349523125 kilogram. One stone is 14 avoirdupois pounds, or 6.35029318 kilograms. One grain is one seven-thousandth of an avoirdupois pound, or 0.00006479891 kilogram. The ounce selector explicitly says “avoirdupois” because a troy ounce is different and is not supported. The metric carat used for gemstone mass is 0.2 gram, or 0.0002 kilogram; it is not a purity percentage.

Although everyday English often uses “weight” for the amount shown on a scale, the supported units in this converter describe mass. Force is a different physical quantity. Newton, pound-force, and kilogram-force are intentionally absent because converting mass to force requires an acceleration, such as a specified local gravitational acceleration. Adding those labels to the same direct-factor table would hide that assumption. Use a force converter when the desired quantity is force, and preserve the relevant acceleration and uncertainty rather than treating kilograms and newtons as interchangeable.

Input parsing is intentionally strict. Ordinary unsigned decimal notation and scientific notation are accepted, including values such as 2.5, .5, 1., and 1e3. Empty input, negative values, plus-prefixed values, commas, unit text, hexadecimal notation, Infinity, NaN, and malformed exponents are rejected with an explicit message. Zero is valid and converts to zero in every supported unit. Negative mass is outside this general-purpose product scope even though signed changes or mathematical models may use negative numerical differences; this interface converts absolute mass quantities rather than changes in mass.

The calculations use JavaScript IEEE 754 double-precision numbers. Results are displayed with up to twelve significant digits, unnecessary trailing zeros are removed, and very large or very small nonzero results use scientific notation. This keeps the output readable without claiming arbitrary decimal precision. The converter rejects an extreme input when multiplication or division would overflow to Infinity, when a positive value would underflow to zero, or when the intermediate kilogram magnitude exceeds the software guard of 1e300. That limit is a numerical safety boundary, not a physical statement about the largest possible mass.

Conversion does not improve the quality of the original measurement. A scale reading recorded to three significant figures does not become more accurate when the result displays additional digits. For trade measurement, medication, laboratory work, structural calculations, or regulated labeling, retain the instrument calibration, measurement uncertainty, applicable rounding rule, and legally required unit convention. The linked BIPM, NIST, and legislation sources document the reference relationships used here, but this browser tool is not a substitute for a calibrated instrument or a jurisdiction-specific compliance review.

Editing the value or changing either unit immediately replaces the prior equation with the new valid result or the current error state, so a successful result is never left visible after invalid input. The copy confirmation is also invalidated whenever the inputs change. An in-flight clipboard operation is tracked, preventing a late response from marking a newer value as copied. These details make repeated conversions safer while keeping the interaction simple: enter a mass, choose two clearly named units, inspect the equation and kilogram basis, then copy only if the displayed labels match the intended measurement system.

Methodology & sources

Each supported mass unit has a positive kilograms-per-unit factor verified against BIPM SI material, NIST SP 811 Appendix B.9, and independent UK statutory definitions. Conversion uses kilograms = input × source factor, followed by result = kilograms ÷ destination factor. The ten-row table is tested for exact size, unique identifiers, unique symbols, positive finite factors, and a single factor-1 kilogram base. Twelve external golden cases cover every row plus the exact 16 oz = 1 lb and 14 lb = 1 st relationships. Parsing accepts unsigned decimal and scientific notation, accepts zero, and rejects negative, non-finite, loosely formatted, overflowing, or underflowing input. Output is limited to twelve significant digits and does not claim arbitrary precision. Force units, troy mass, and ambiguous short or long tons are outside scope.

Frequently asked questions

Does this converter calculate mass or force?
It converts mass. Kilograms, grams, avoirdupois pounds, ounces, stone, grains, metric tonnes, and metric carats are routed through kilograms. Newtons and kilogram-force are excluded because force conversion requires an acceleration assumption.
Which ton and ounce does the converter use?
The tonne is the metric tonne of exactly 1,000 kg, and the ounce is the avoirdupois ounce of exactly one sixteenth of an international avoirdupois pound. US short tons, UK long tons, and troy ounces are not included.
Why are negative values and commas rejected?
This tool converts absolute non-negative mass values and uses strict machine-readable decimal input. Rejecting signed negatives, grouping commas, and appended unit text avoids silently interpreting an unintended format. Scientific notation such as 1e3 is accepted.
How precise are the displayed results?
The defining factors are applied directly, but calculations use IEEE 754 double precision and display at most twelve significant digits. Preserve the source measurement's significant figures and uncertainty for scientific, trade, medical, or regulated work.

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