Weight & Mass Converter

How to Use This Converter

This converter offers four modes for different conversion needs:

• Single Conversion: Enter a value, pick your From and To units, and click Convert. The swap button exchanges the two units, and quick conversion buttons set common pairs with one click.
• Convert to All: See your value in every available unit at once, grouped by category. Useful when sharing information with people who use different measurement systems.
• Body Weight: Handles the stones-and-pounds format common in the UK, along with pounds and kilograms. Includes a reference table for quick lookups.
• Cooking: Focuses on kitchen units (ounces, grams, pounds, kilograms) and includes reference tables for butter and common ingredient weights.

In the options panel you can control decimal precision, enable scientific notation for very large or small numbers, and show or hide formulas and reverse conversions. Results include copy buttons so you can transfer values to other applications.

The Metric System for Mass

Metric came out of the French Revolution in the 1790s when France decided to replace its confusing mix of regional weights with a single standard. For mass, the gram was originally defined as the mass of one cubic centimeter of pure water at its maximum density, and the kilogram (1,000 grams) became the base unit for everyday use.

For over a century, the kilogram was defined by a physical artifact: the International Prototype of the Kilogram, a platinum-iridium cylinder kept in a vault near Paris. Every other kilogram in the world was based on this single object. Physical artifacts can drift over time, though, even if only by micrograms, which made international standardization difficult.

On May 20, 2019, the kilogram was redefined using the Planck constant. By fixing this constant at exactly 6.62607015 × 10⁻³⁴ joule-seconds, scientists created a definition based on atomic measurements rather than a physical object.

Metric uses a decimal structure, where each prefix represents a power of ten: kilo- means thousand, milli- means thousandth, micro- means millionth. Converting between metric units means moving decimal points rather than memorizing factors like 16 ounces per pound or 14 pounds per stone. That's why metric is standard in science, medicine, and international trade.

The Imperial and Avoirdupois System

Imperial weights go back to medieval England, where different trades had their own standards. The avoirdupois system (Old French for "goods of weight") became the standard for general commerce. The units developed over centuries with no master plan, which is how we ended up with 16 ounces in a pound, 14 pounds in a stone, and 2,240 pounds in a long ton.

In 1959, English-speaking nations agreed on fixed definitions. The pound became exactly 0.45359237 kilograms—ending decades of slightly different definitions that had caused headaches in trade and science.

Over time, the US and UK went different directions. Americans dropped the stone (Brits still use it for body weight) and use the short ton of 2,000 pounds instead of the long ton of 2,240 pounds. British shops now sell goods in grams and kilograms, though pounds and ounces remain common in conversation. Imperial units stick around because of tradition and the cost of switching.

Converting Between Metric and Imperial

Pounds to kilograms is the conversion you'll use most. One kilogram equals about 2.2 pounds, and one pound equals about 0.45 kg. For mental math, double the kilograms and add 10 percent to get pounds. Going the other way, halve the pounds and subtract about 10 percent.

One ounce is roughly 28 grams. That means 100 grams is about 3.5 ounces, and a pound (16 ounces) is about 454 grams. You'll run into these conversions in cooking, shipping, and nutrition labels.

Precision needs vary by task. Body weight tracking only needs one decimal place in kilograms or whole pounds, since hydration alone causes bigger swings than that. Scientific work might need eight decimal places. Shipping usually needs tenths of a kilogram. You can adjust decimal precision in the options panel.

Weight vs Mass: The Scientific Difference

Your bathroom scale measures the force pressing down on it, which is technically weight. But it shows the result in kilograms or pounds, which are units of mass. The scale assumes you're on Earth with standard gravity, and since most of us aren't weighing ourselves on other planets, it works fine.

Scientists care about the difference. Mass is how much matter an object contains—a 70 kg person has 70 kg of mass on Earth, on the Moon, or floating in space. Weight is the force that gravity exerts on that mass. The same person would weigh 686 newtons on Earth but only 113 newtons on the Moon, where gravity is about 1/6 as strong. In orbit, they'd weigh nearly zero. The distinction matters for physics and aerospace, but for daily life you can ignore it.

Troy Weight for Precious Metals

When you see gold quoted at "$1,900 per ounce," that's per troy ounce, not regular ounce. A troy ounce is 31.1 grams, about 10 percent heavier than the regular 28.35-gram ounce. So a "one-ounce gold coin" actually contains 31.1 grams of gold. This catches a lot of first-time gold buyers off guard.

Troy gets its name from Troyes, a French city with major medieval trade fairs. Troy and regular (avoirdupois) ounces are different, and so are troy and regular pounds. A troy pound contains only 12 troy ounces (373 grams) versus the regular pound's 16 ounces (454 grams). Both systems use the same grain at 64.8 milligrams.

Precious metals markets have stuck with troy weights for quotation. Tradition is part of it, and the troy ounce is a convenient size for standard coins and small bars.

Carats and Gemstones

The carat is an old unit named after carob seeds, which gem traders used as counterweights because the seeds are fairly consistent in size. In 1907, the metric carat was set at exactly 200 milligrams (0.2 grams). A 5-carat diamond weighs exactly 1 gram. The Hope Diamond weighs 45.52 carats (about 9.1 grams), and the Great Star of Africa, the largest cut diamond, weighs 530.2 carats (106 grams).

Carat (ct) and karat (K) are different things. Carat measures gemstone weight. Karat measures gold purity, where 24 karats means pure gold—so 18-karat gold is 75% gold by weight. The words come from the same root but mean completely different things now.

Body Weight Conversions

Americans, Brits, and Australians all measure body weight differently. Americans use pounds exclusively ("I weigh 165"). The British and Irish use stones and pounds ("eleven stone seven" instead of "161 pounds"). Australians, New Zealanders, and most Canadians use kilograms.

Hospitals worldwide now use kilograms. Drug dosages are calculated per kilogram of body weight, and BMI requires kilograms and meters. Even in the US, your doctor might record your weight in kg because drug doses are calculated per kilogram.

A weight loss of "half a stone" equals 7 pounds or 3.2 kg—more than it might sound. Daily fluctuations of 1-2 pounds from hydration are normal and don't reflect actual fat changes. Pick whatever unit you like and stick with it—consistency matters more than the unit itself.

Kitchen and Cooking Weights

Bakers weigh ingredients in grams because cup measurements are inconsistent. A cup of flour can be 120 to 150 grams depending on how you scoop it. Brown sugar depends on how hard you pack. Grams don't have this problem.

American recipes use volume (cups and tablespoons), European recipes use grams. Converting means knowing what each ingredient weighs per cup, and these vary a lot: a cup of flour is about 125 grams, a cup of honey is 340 grams, a cup of butter is 227 grams (two US sticks). When you need to scale a recipe up or down, grams make the math much easier. Doubling 200 grams of sugar is simple; doubling "1¾ cups minus 2 tablespoons" requires a calculator.

A lot of home bakers have switched to scales for the same reason the pros use them. A basic digital scale costs little and the results are more reliable.

Different Types of Tons

Three different "tons" often confuse people in shipping, industry, and everyday conversation:

A metric ton falls between the other two: about 10 percent larger than a short ton and 2 percent smaller than a long ton. When precision matters, documents should specify which ton is meant. Scientific and international contexts almost always use metric tons. American domestic shipping typically uses short tons. Historical British records use long tons. Spelling it "tonne" (with -ne) always means the metric version.

Other specialized "tons" measure volume rather than weight. A register ton equals 100 cubic feet of cargo space on ships. A displacement ton measures the weight of water displaced by a ship's hull. A refrigeration ton measures cooling capacity.

Practical Conversion Tips

A few key relationships make most everyday conversions quick:

• Kg to pounds: Double the kilograms and add 10 percent. For 30 kg: double to 60, add 6, get 66 lb. (Actual: 66.14 lb)
• Pounds to kg: Halve the pounds and subtract 10 percent. For 150 lb: halve to 75, minus 7.5, get 67.5 kg. (Actual: 68.04 kg)
• Key numbers: 1 kg ≈ 2.2 lb, 1 oz ≈ 28 g, 1 stone = exactly 14 lb, 4 oz ≈ 113 g (quarter pound)

These numbers let you estimate most conversions in your head.

For cooking, US butter comes in 113-gram (4-ounce) sticks, 8 tablespoons per stick. European butter usually comes in 250-gram blocks.