Weight Converter

What this tool does

The Weight Converter takes a number in one unit of mass and re-expresses it in another. It handles seven units: kilograms (kg), pounds (lb), ounces (oz), grams (g), stone (st), milligrams (mg), and metric tons (t). Any unit can be either the source or the target, so there are 49 possible conversion pairs (including the trivial ones like kg to kg).

Everything runs in your browser. There's no calculate button, no sign-up, no upload, and no rate limit. As soon as you've typed a number and chosen both units, the result appears.

Who actually needs a weight converter

Most of the people I see reaching for one are doing very normal things:

• Reading a recipe written in grams when you cook in ounces (or the other way around)
• Tracking body weight in pounds at the doctor and stone at home, or kg at the gym
• Reading a shipping label that lists weight in pounds when your carrier wants kilos
• Translating a nutrition figure from milligrams of sodium into grams
• Sanity-checking a freight quote priced per metric ton
• Loading luggage to a 23 kg airline limit when your scale only shows pounds

You almost never need 6 decimal places of precision for any of these. The tool gives them to you anyway, which is fine — you can just read the first one or two.

How to use it

Three inputs, in this order:

1. Value — type the number you want to convert. The field is a numeric input, so decimals like 2.5 work. Negative numbers are accepted by the field even though mass is normally positive; the math will run on them anyway.
2. From — pick the unit your number is currently in.
3. To — pick the unit you want the answer in.

The Result card only appears once you have a value and have actively selected both dropdowns. There's no submit step — change any input and the answer recalculates immediately. The result is shown rounded to 6 decimal places.

How the math actually works

Every unit is stored as the number of kilograms it equals. The kilogram is the pivot. To convert, the tool does two multiplications:

1. Multiply your value by the kilograms-per-unit factor of the From unit. That gives you kilograms.
2. Divide that kilogram value by the kilograms-per-unit factor of the To unit. That gives you the answer.

The factors built into the tool are these exact values:

• kg = 1
• lb = 0.45359237
• oz = 0.028349523125
• g = 0.001
• st = 6.35029318
• mg = 0.000001
• t = 1000

These aren't approximations. The pound here is the international avoirdupois pound, which has been defined since 1959 as exactly 0.45359237 kg. The ounce is exactly 1/16 of that pound, and the stone is exactly 14 pounds, so all the imperial units stay perfectly consistent with each other. Gram, milligram, and metric ton are exact decimal fractions or multiples of the kilogram by SI definition.

A worked example

Say you want to know what 70 kilograms is in stone.

• Step 1: 70 × 1 = 70 kg
• Step 2: 70 ÷ 6.35029318 ≈ 11.023113 st

So the result shows 11.023113. In everyday terms that's about 11 stone 0.3 pounds, since a stone is 14 pounds and 0.023113 × 14 ≈ 0.32 lb. The tool itself only gives you the decimal answer in the target unit — converting that decimal stone into the traditional stone and pounds format is something you do in your head.

Quick mental sense-checks

Before you trust any conversion, it's worth having a rough number in your head so you'd notice if something looked very wrong. The handy ones:

• 1 kg ≈ 2.2 lb (more precisely 2.20462)
• 1 lb ≈ 454 g
• 1 oz ≈ 28.35 g
• 1 stone = 14 lb ≈ 6.35 kg
• 1 metric ton = 1000 kg ≈ 2204.6 lb

If the tool tells you 1 kg is anywhere near 22 pounds or 0.22 pounds, you've got the From and To swapped.

Common pitfalls

Ounces are not all the same ounce

This tool uses the avoirdupois ounce, the everyday one used for groceries, body weight, and shipping. It's about 28.35 g. The troy ounce, used for gold, silver, and other precious metals, is about 31.1035 g — heavier by roughly 10%. If you're pricing precious metals or reading a bullion spec, do not use this converter; you'll be off by about 10%.

"Ton" means three different things

The t option here is the metric ton (also called tonne), defined as exactly 1000 kg. It is not the same as:

• US short ton = 2000 lb ≈ 907.18 kg
• UK long ton = 2240 lb ≈ 1016.05 kg

If your source says "ton" and you're in the US (especially in shipping, mining, or agriculture), it's probably the short ton. If you're reading a British source, especially an older one, it could be the long ton. For either of those, convert via pounds instead — type the value in lb and you'll get the right kilogram number out.

Mass vs. weight

Strictly speaking, kilograms, pounds, and the others on this tool are units of mass. Weight is a force and is measured in newtons or pounds-force. On Earth's surface we treat the two as interchangeable, which is what this tool assumes and what almost everyone means in everyday speech. If you actually need force — for an engineering calculation, say — use a Force Converter instead.

Rounding to 6 places

Every result is rounded to 6 decimal places. For anything you can step on a scale and feel, that's far more precision than you need. For very small mass conversions (milligrams to grams, say), the rounding can show 0 or chop a few digits you cared about. If that matters, scale your input up first — convert 1000 mg instead of 0.001 mg and divide later.

When not to use this tool

• Precious metals (gold, silver, platinum) — those are quoted in troy ounces, not avoirdupois ounces.
• US or UK tons in shipping or industry — convert via pounds, not via the t option.
• Force, thrust, or load — you want newtons or pounds-force, not mass units.
• Volume or capacity — fluid ounces are a different beast from weight ounces, and the tool doesn't handle them. Cups, pints, liters, and millilitres are volume units; weight depends on what's in the cup. A cup of flour and a cup of honey weigh very different amounts.
• Crystals, gemstones, and pearls are sold in carats (1 ct = 200 mg). You'd have to multiply through manually.

What to do if the result looks wrong

In order of likelihood:

1. From and To are reversed. Easy to do when you're in a hurry. 70 lb to kg is 31.75; 70 kg to lb is 154.32. If the answer is much bigger or smaller than you expected, swap the dropdowns.
2. Wrong "ton". If you read "ton" off a US shipping doc, you probably want short tons, not metric. Go through pounds.
3. Wrong "ounce". If you're working with gold, you want troy, not the one here.
4. Decimal separator. The input expects a period for decimals (2.5), not a comma. If you type 2,5 the input may reject it or parse only the leading digit.

Related conversions

The page links to a couple of physically adjacent tools: Force Converter for newtons, pounds-force, and similar (when you actually mean weight as a force), and Density Converter for kg/m³ and friends (when you're connecting a mass to a volume). For length, volume, temperature, or speed conversions, those are separate tools.