Weight Converter: Kilograms, Pounds, Ounces, and Stones

Weight Converter: Kilograms, Pounds, Ounces, and Stones

My British friend Emma told me she weighs "eleven stone." I nodded like I knew what that meant—then googled it later. 154 pounds. That's 70 kg. Three different numbers for the same person. Then she asked how much I weigh. I said "180." "Pounds, right? Not kilograms?" she said, squinting. "Because if that's kilograms, you're a small car." We spent the next ten minutes converting everyone we know. Her dad is 16 stone. My sister is 135 pounds. My cousin in Germany is 65 kilos. Same people, different numbers—and none of us were wrong. Weight and mass measurement has evolved over millennia, from ancient body-part references to the standardized metric system used globally today. Understanding the mathematical relationships between different weight units, their historical origins, and the distinction between mass and weight is essential for accurate measurement and conversion.

Photo by i yunmai on Unsplash

Mass vs Weight: Not the Same Thing

Mass is how much stuff you're made of. Measured in kilograms. It doesn't change no matter where you are.

Weight is the force gravity exerts on that mass. Formula: W = m × g. Measured in Newtons in science class.

On Earth (g = 9.81 m/s²), a 70 kg person weighs about 687 N.
On the Moon (g = 1.62 m/s²), that same person weighs 113 N. But their mass? Still 70 kg.

In real life: Nobody says "I weigh 687 Newtons." We say kilograms or pounds. It's wrong but everyone does it. Including your bathroom scale.

The Pound (lb)

The word "pound" comes from Latin "libra pondo." "lb" comes from "libra." That's why we write "lb" instead of "pd."

Roman libra: About 328.9 grams. Based on a specific volume of water. Romans were pragmatic.

Avoirdupois pound: The one we use today. Exactly 453.59237 grams. Standardized in the UK in 1878. Emma's "eleven stone" = 154 of these.

Troy pound: For gold, silver, gems. 373.242 grams. About 12% lighter than the avoirdupois pound. Because why make it easy?

The Kilogram (kg)

The base unit of the entire metric system.

1795: Defined as one liter of water at 4°C. Simple, elegant.

1889: Replaced by a platinum-iridium cylinder in France. The real kilogram lived in a vault. People visited it.

2019: The kilogram got a major upgrade. Now defined by the Planck constant (6.62607015 × 10⁻³⁴ J⋅s). No more physical reference. Just physics.

The conversion: 1 kg = 1000 grams = 2.20462 pounds.

The Ounce (oz)

From Latin "uncia" meaning one-twelfth. Not to be confused with the other "ounce."

Avoirdupois ounce: 28.3495 grams. 16 ounces make a pound. Standard for groceries, body weight, etc.

Troy ounce: 31.1035 grams. 12 ounces make a troy pound. Used exclusively for precious metals. If you're buying gold, you're using troy ounces.

Fluid ounce: Not weight at all. It's volume. And it's different in the US (29.57 ml) than in the UK (28.41 ml). Classic.

The Stone

This one throws Americans for a loop. It's a British thing.

1 stone = 14 pounds = 6.35 kg

Historically, a stone meant different things for different goods:

• Wool: 14 lbs


• Meat: 8 lbs


• Glass: 5 lbs

Eventually 14 lbs won out, at least for body weight. Emma weighs 11 stone. That's 154 lbs. Or 70 kg. Three numbers, one person.

Conversion Formulas

Pounds ↔ Kilograms:
kg = lbs × 0.453592
lbs = kg × 2.20462

Pounds ↔ Ounces:
oz = lbs × 16
lbs = oz ÷ 16

Stone:
lbs = stone × 14
kg = stone × 6.35
stone = kg ÷ 6.35

Quick Reference

70 kg person:

• 154 lbs


• 2,469 oz


• 11 stone

150 lbs:

• 68 kg


• 10.7 stone


• 2,400 oz

10 stone:

• 140 lbs


• 63.5 kg

Historical Weight Systems

Egyptian: The deben (~91 grams). Used for trade and taxes. Pharaohs wanted their cut.

Chinese: The jin (斤). Thousands of years old. Modern version = 500 grams (half a kilo).

British Imperial: Standardized in 1824. Had the pound, ounce, stone, quarter (28 lbs), hundredweight (112 lbs), and ton (2,240 lbs). Too many units. Very British.

Metric: Born from the French Revolution. Designed to be universal. Based on decimals. Won, eventually.

The Avoirdupois System (Say That Three Times Fast)

The backbone of the US and UK weight system.

The chain:
16 drams = 1 oz
16 oz = 1 lb
14 lbs = 1 stone
2 stones = 1 quarter
4 quarters = 1 hundredweight (112 lbs)
20 hundredweights = 1 ton (2,240 lbs)

The name: From Old French "aveir de pois"—"goods of weight." Wool traders started it.

Who Uses What

Metric (kg): Most of the world. Science, medicine, basically everywhere rational.

US Customary (lbs, oz): The United States. For everything everyday.

UK (stone + lbs + kg): Body weight in stone and pounds. Everything else metric. A glorious mess.

Combination countries: Officially metric. Everyone still uses traditional units at the market. Old habits die hard.

Where You'll Use This

Cooking: Grams, ounces, pounds—recipes come in all of them. Wrong conversion = bad cake.

Fitness: Body weight in kg, lbs, or stone. As long as you're consistent, the trend matters more than the unit. Emma tracks in stone. I track in pounds. My German cousin uses kilos. We're all doing fine.

Shipping: International = kg. US domestic = lbs. Your shipping label doesn't care about your opinion.

Medical: Metric, always. Milligram, gram, kilogram. No exceptions.

The Takeaway

One person can be 11 stone, 154 pounds, or 70 kilograms. All correct. The math is simple once you know the factors: 2.20462 for pounds to kilos, 14 for stone to pounds, 16 for ounces. Keep these in your back pocket, and you'll never get caught off guard by a British friend's weight again. Emma and I now convert everything in seconds. And we finally know who's heavier. (It's me. By a lot.)