The problem: not every ounce is the same ounce
You bought a gold ring listed as 1 ounce. You weigh it on a kitchen scale and it reads 28.3 grams. The jeweler says it should weigh 31.1 grams. Nobody is lying. You are using two different ounces.
The avoirdupois ounce — the one on kitchen scales, postage scales, and grocery packaging — is 28.349 grams. The troy ounce — used for precious metals — is 31.103 grams. The difference is about 9.7 percent. A "1 ounce" gold coin weighs more than a "1 ounce" package of cheese, because the two ounces are units from different systems that happen to share a name.
This is not the only trap. A pound of gold is 12 troy ounces (373.24 grams). A pound of feathers is 16 avoirdupois ounces (453.59 grams). The feathers weigh more — which is why the old riddle "which weighs more, a pound of feathers or a pound of gold" has the counterintuitive answer "feathers." The riddle only works because it relies on you assuming both pounds are the same unit. They are not.
The Weight Converter handles 27 units across five categories — metric, imperial, troy/precious, scientific, and cooking — and keeps the systems separate so you do not accidentally convert a troy ounce as an avoirdupois ounce. It converts everything through kilograms as an intermediary, which avoids the N-squared conversion table problem and guarantees consistency.
Fastest path
Open the Weight Converter, enter your value, pick the source unit and target unit from the dropdowns (or use the category filters to narrow the list), and read the result. The formula 1 {from} = {factor} {to} is shown below the result. The All Units table shows your input value converted to all 27 units simultaneously. Star units to add them to your favorites panel for quick access. Use the Swap button to reverse direction. Adjust precision (2 to 10 decimal places) or toggle scientific notation for very large or very small values.
Mass vs weight: the physics distinction
The tool is called a "weight converter" but the conversions are mass conversions, and mass and weight are not the same thing.
Mass is the amount of matter in an object. It is intrinsic — a 1-kilogram block of steel has a mass of 1 kilogram on Earth, on the Moon, and in deep space. Mass does not change with gravity. The SI unit of mass is the kilogram.
Weight is the force exerted on that mass by gravity. Weight = mass × gravitational acceleration. On Earth, g is about 9.81 m/s², so a 1-kilogram mass weighs about 9.81 newtons. On the Moon, g is 1.62 m/s², so the same mass weighs about 1.62 newtons. The SI unit of weight (force) is the newton, not the kilogram.
In everyday usage, "weight" and "mass" are used interchangeably because we measure both in the same gravitational field. When you stand on a bathroom scale and it says 70 kilograms, it is actually measuring the force you exert on the scale (weight) and dividing by g to display your mass. If you took the same scale to the Moon, it would read about 11.7 kilograms — not because you lost mass, but because the scale's force measurement changed.
The tool converts mass. There are no force units (newtons, pound-force, dynes). If you need force, use a different tool. The slug unit included in the imperial category is a mass unit (about 14.59 kilograms) used in engineering contexts where the English system's force-based units (pound-force) require a consistent mass unit.
The metric system: powers of 1,000
The metric system is the cleanest part of weight conversion. Every step is a factor of 1,000, and the names follow the SI prefix system:
| Unit | Symbol | Value (in grams) | Factor |
|---|---|---|---|
| Microgram | mcg (µg) | 0.000001 g | 10⁻⁶ |
| Milligram | mg | 0.001 g | 10⁻³ |
| Gram | g | 1 g | 1 |
| Kilogram | kg | 1,000 g | 10³ |
| Metric ton (tonne) | t | 1,000,000 g | 10⁶ |
To convert within the metric system, multiply or divide by 1,000 for each step. Milligrams to kilograms: divide by 1,000,000. Kilograms to metric tons: divide by 1,000. The prefixes are the same across all metric quantities — milli is always 10⁻³, kilo is always 10³, whether you are measuring grams, meters, or liters.
The base unit of mass in the SI system is the kilogram, not the gram. This is a historical accident: the original French metric system defined the gram as the mass of one cubic centimeter of water, but the kilogram (1,000 grams) became the practical standard because a gram was too small for most measurements. The kilogram prototype — a platinum-iridium cylinder kept at the International Bureau of Weights and Measures in Sèvres, France — was the definition of the kilogram until 2019, when it was redefined in terms of the Planck constant. The gram is a derived unit (1/1,000 of a kilogram), which is why the prefix appears on the base unit name.
The imperial system: non-decimal ratios
The imperial system does not use powers of 10. The ratios are historical, derived from agricultural and trade measurements, and they are not consistent:
| Unit | Symbol | Value | Ratio to previous |
|---|---|---|---|
| Ounce | oz | 28.349 g | — |
| Pound | lb | 453.592 g | 16 oz per lb |
| Stone | st | 6,350.29 g | 14 lb per st |
| US (short) ton | ton | 907,185 g | 2,000 lb per ton |
| UK (long) ton | long ton | 1,016,047 g | 2,240 lb per long ton |
The ratios are 16, 14, 2,000, and 2,240 — no decimal pattern. This is why imperial conversion requires a lookup table rather than moving a decimal point. The tool stores each unit's toBase factor (multiplier to kilograms) and converts by going through kilograms as an intermediary, so the inconsistent ratios are handled once at the factor-definition level and never again.
The US ton (2,000 lb) and UK long ton (2,240 lb) differ by 12 percent. A "ton" of steel in the US is 907 metric tons; in the UK, the same word means 1,016 metric tons. The tool labels them explicitly as "Short Tons/US" and "Long Tons/UK" to avoid the ambiguity. If you see "ton" in a trade document, check which country wrote it.
Troy weight: precious metals use a different system
Precious metals, gemstones, and some pharmaceuticals use the troy system, which has different units from the avoirdupois system:
| Unit | Symbol | Value | Notes |
|---|---|---|---|
| Grain | gr | 64.799 mg | Same in both systems |
| Pennyweight | dwt | 1.555 g | 24 grains |
| Troy ounce | oz t | 31.103 g | 20 pennyweights |
| Troy pound | lb t | 373.242 g | 12 troy ounces |
| Carat | ct | 200 mg | For gemstones |
The troy ounce is about 9.7 percent heavier than the avoirdupois ounce. The troy pound is lighter than the avoirdupois pound (373 g vs 454 g) because a troy pound has only 12 troy ounces, while an avoirdupois pound has 16 avoirdupois ounces. The grain is the one unit shared between both systems — 64.799 milligrams in troy and avoirdupois, which is why it survives as a unit in bullet weights and some pharmaceutical doses.
The spot price of gold and silver is quoted per troy ounce. A standard gold bar (the "good delivery" bar held by central banks) is 400 troy ounces, about 12.4 kilograms. If you convert 400 "ounces" using the avoirdupois factor, you get 11.34 kilograms — about 9 percent underweight. The tool keeps troy and avoirdupois in separate categories so you have to explicitly pick which ounce you mean.
Carats measure gemstone mass (200 milligrams, so a 1-carat diamond is 0.2 grams) but the word also describes gold purity (24-carat gold is pure, 18-carat is 75 percent). These are unrelated meanings of the same word. The tool converts carats as mass, not purity.
Cooking units: density matters
The cooking units in the tool are the most approximate, because they convert volume to mass using ingredient density — and density varies:
| Unit | Mass | Notes |
|---|---|---|
| Teaspoon of water | 4.93 g | Water, exact |
| Tablespoon of water | 14.79 g | 3 teaspoons |
| Cup of water | 236.59 g | 8 fluid ounces |
| Cup of flour | 120 g | Depends on how it is packed |
| Cup of sugar | 200 g | Granulated white sugar |
The water-based conversions are exact (at 4°C, where water has its maximum density of 1 g/mL). The flour and sugar conversions are approximations — a cup of flour can weigh 110 to 130 grams depending on whether it is spooned, scooped, or packed, and whether it is all-purpose, bread, or cake flour. A cup of brown sugar packed firmly weighs about 220 grams; unpacked, about 170 grams. The tool uses a single representative value; for baking where precision matters, use a kitchen scale instead of volume measurements.
Scientific units: 57 orders of magnitude
The scientific units span an enormous range:
| Unit | Value (kilograms) | Use |
|---|---|---|
| Atomic mass unit (amu) | 1.6605 × 10⁻²⁷ | Particle physics, chemistry |
| Electron rest mass | 9.109 × 10⁻³¹ | Particle physics |
| Planck mass | 2.176 × 10⁻⁸ | Theoretical physics |
| Solar mass | 1.989 × 10³⁰ | Astronomy |
The atomic mass unit is the mass of one twelfth of a carbon-12 atom — the unit used for atomic and molecular weights. An electron is about 1/1,836 of a proton, which is why its mass is in the 10⁻³¹ range. The solar mass is the mass of the Sun, used in astronomy because kilograms are impractically small for stellar-scale objects. The Milky Way galaxy is about 1.5 trillion solar masses.
Converting between these requires scientific notation. The tool's scientific notation toggle switches to exponential display (e.g., 1.6605e-27) for values outside the practical fixed-point range. With 10 decimal places and fixed notation, the atomic mass unit displays as 0.0000000000000000000000000017 kg — readable only with difficulty. Scientific notation collapses it to 1.6605e-27.
The intermediary-base-unit approach
The tool does not store a 27-by-27 conversion table (which would require 729 factors). It stores each unit's factor to kilograms — one number per unit — and converts in two steps:
- Multiply the input value by the source unit's
toBasefactor → kilograms - Divide the kilograms by the target unit's
toBasefactor → target value
This is O(N) in units rather than O(N²). It also guarantees consistency: if 1 ounce is 0.0283495 kg and 1 pound is 0.4535924 kg, then 1 pound is automatically 16 ounces (0.4535924 / 0.0283495 = 16.00) without defining that ratio explicitly. If one factor is wrong, every conversion involving that unit is wrong in the same way, which makes errors easier to find than in a full table where the same unit appears in many ratios.
Gotchas
- Mass and weight are not the same. The tool converts mass. If you need force (newtons, pound-force, dynes), this is the wrong tool. A 70-kilogram person weighs 686 newtons on Earth and 112 newtons on the Moon, but their mass is 70 kilograms in both places. The kilogram, pound, and ounce are mass units in this tool — the "weight" in the name is colloquial usage, not a physics claim.
- Troy and avoirdupois units share names but not values. A troy ounce is 31.1 grams; an avoirdupois ounce is 28.3 grams. A troy pound is 373 grams; an avoirdupois pound is 454 grams. If you are converting precious metals, make sure you pick the troy unit — the category filter "Troy/Precious" isolates them. The default favorites (kg, lb, g, oz, st) include the avoirdupois ounce, not the troy ounce.
- Cooking conversions are approximate. A cup of flour in the tool is 120 grams, but your cup of flour might be 110 or 130 grams depending on how it is packed and what type of flour. The water-based conversions (teaspoon, tablespoon, cup) are exact for water at 4°C but only approximate for other liquids — a cup of milk is about 240 grams, a cup of oil is about 218 grams, because their densities differ from water.
- US and UK tons differ by 12 percent. The US short ton is 2,000 pounds (907 kg). The UK long ton is 2,240 pounds (1,016 kg). A "ton" in a shipping or industrial document could be either. The metric ton (tonne, 1,000 kg) is a third unit that is neither the US nor the UK ton. The tool labels all three explicitly.
- Floating-point precision limits very large or very small conversions. Converting solar masses to atomic mass units involves a factor of about 1.2 × 10⁵⁷ — within the range of JavaScript's number type (which can represent up to about 1.8 × 10³⁰⁸), but with 15-17 significant digits of precision. For most conversions, this is irrelevant. For scientific work requiring more than 15 significant digits, use a tool with arbitrary-precision arithmetic.
Summary
- Weight conversion is multiplication by a factor, but the factors are not decimal. The metric system uses powers of 1,000 (microgram to metric ton, five steps). The imperial system uses non-decimal ratios (16 oz/lb, 14 st/lb, 2,000 lb/US ton, 2,240 lb/UK ton). The tool converts through kilograms as an intermediary, which is O(N) in units and guarantees consistency.
- Mass and weight are different. Mass is intrinsic (kilograms); weight is the force of gravity on that mass (newtons). The tool converts mass. The slug (14.59 kg) is a mass unit included in imperial, but there are no force units.
- Troy and avoirdupois units share names but not values. A troy ounce (31.1 g) is 9.7 percent heavier than an avoirdupois ounce (28.3 g). A troy pound (12 troy ounces, 373 g) is lighter than an avoirdupois pound (16 avoirdupois ounces, 454 g). Precious metals use troy; everything else uses avoirdupois. The tool keeps them in separate categories.
- Cooking conversions depend on ingredient density. A cup of water is 236.6 grams; a cup of flour is 120 grams; a cup of sugar is 200 grams. The water conversions are exact (at 4°C); the others are approximations that vary with packing and ingredient type.
- Use the Weight Converter for mass conversions across 27 units, the Length Converter for distance, the Temperature Converter for temperature (which has offset, not just scaling), and the BMI Calculator for weight-to-height health screening.