History of the Stone Unit
From Roman market weights to the medieval wool trade to your bathroom scales: the 2,000-year story of how 14 pounds became a stone.
Before Standardisation: A Stone Was Any Stone
The earliest "stones" were literal stones. Across Europe, traders kept smoothed rocks as counterweights for balance scales. Because each rock was different, the "stone" varied wildly: records show stones ranging from about 3 kg in some German cities to over 15 kg in parts of Italy. There was no stone unit, only stones.
The Romans used stone and bronze weights (called saxa), but their formal system was built on the libra, a pound of about 328 grams. The abbreviation "lb" for pound survives from libra to this day. Rome never standardised a stone, but its trade networks spread the habit of weighing goods against physical weights across Europe.
The Wool Trade: Why 14 Pounds?
England's answer to the chaos came from its most valuable export: wool. In the 1300s, wool was England's economic engine, and merchants needed a consistent way to weigh fleeces and sacks. Through custom and trade practice, the wool stone settled at 14 pounds.
Why 14? A 14-pound unit divides neatly: half a stone is 7 pounds, a quarter is 3.5. For wool merchants weighing sacks that held multiples of fleeces, 14 was practical arithmetic. In 1389, an Act of Parliament formally recognised the 14-pound stone for the wool trade, giving legal force to what merchants had already agreed.
Note: other trades used different stones for centuries. Butchers used an 8-pound stone for meat; London markets used a 14-pound stone for most goods. The wool stone's 14 pounds eventually won because wool dominated English commerce.
Timeline: From Wool to Bathroom Scales
Roman Britain
Traders use physical stone weights (saxa) alongside the Roman libra. No standardised stone exists.
1300s
English wool merchants settle on a 14-pound stone through trade custom. Wool is England's most valuable export.
1389
Parliament standardises the wool stone at 14 pounds by statute, the first legal definition.
1835
The Weights and Measures Act fixes the stone at exactly 14 pounds for ALL British trade, ending centuries of regional variation.
1878
The Weights and Measures Act confirms the stone as 14 pounds avoirdupois, tying it to the imperial system.
1965-1995
UK metrication makes kilograms compulsory for trade. The stone loses legal status for commerce but is never banned for personal use.
Today
The stone is legally defined as exactly 6.35029318 kg and survives as the UK's customary unit for human body weight.
Why 14 Pounds? (The Short Version)
If you came here just for this question: the stone is 14 pounds because medieval English wool merchants found 14 a practical number for weighing fleeces, Parliament confirmed it in 1389, and the 1835 Weights and Measures Act made it universal. There is no deeper mathematical reason. It is a historical accident that became law.
In metric terms, 14 pounds equals exactly 6.35029318 kilograms. That odd decimal is simply what 14 imperial pounds converts to; the stone was never designed in metric.
Frequently Asked Questions
When was the stone invented?
No single inventor exists. Physical stones were used as weights across Europe for centuries. England standardised the wool stone at 14 pounds by the 1300s, and the 1835 Weights and Measures Act fixed it for all British trade.
Why is a stone 14 pounds?
The 14-pound stone comes from the medieval English wool trade. Wool was sold in standardised sacks, and 14 pounds proved a practical unit for weighing fleeces. Parliament confirmed it in 1389 and again in 1835.
What is a stone in Roman times?
Romans used stone weights (saxa) but had no standardised 'stone' unit. Their system was based on the libra (pound), from which the abbreviation 'lb' survives.
Is the stone still a legal unit?
In the UK, the stone is legally defined as exactly 6.35029318 kg, but it is not permitted for trade. It survives as a customary unit for personal body weight.
What replaced the stone for trade?
The kilogram replaced the stone for all official trade in the UK during metrication (1965 onward). The stone was never banned for informal use, which is why it survives for body weight.