BIPM-ratified constants · v1.0

Converter

Stones, to kilograms converter calculator.

Convert stones to kilograms instantly using the formula kg = st × 6.35029318. Supports whole stones plus additional pounds for precise metric conversion.

Stones (st)

Kilograms

63.5029

The conversion

How the value
is computed.

Stones to Kilograms Conversion: Formula and Methodology

The stone is a traditional unit of mass still widely used in the United Kingdom and Ireland, particularly for expressing human body weight. Converting stones to kilograms is essential for medical consultations, international fitness tracking, and travel documentation wherever the metric system serves as the global standard.

The Core Conversion Formula

The exact formula for converting stones to kilograms is:

kg = st × 6.35029318

Where st represents the number of whole stones. This conversion factor is internationally defined: one stone equals exactly 14 avoirdupois pounds, and one pound equals exactly 0.45359237 kilograms. The derivation is straightforward:

1 stone = 14 × 0.45359237 = 6.35029318 kg

This value is grounded in the 1959 International Yard and Pound Agreement, which fixed the avoirdupois pound at exactly 0.45359237 kg. The NIST SI Units and Conversion Factors confirms this exact pound-to-kilogram ratio, making the stone-to-kilogram factor a mathematically precise constant rather than an approximation.

Variables Explained

Stones (st): The whole number of stones in the weight measurement. One stone equals 14 pounds or 6.35029318 kg. This unit is most commonly used in the UK and Ireland for expressing body weight.
Additional Pounds (lb): An optional component representing pounds beyond the whole stones value, ranging from 0 to 13.99. A weight expressed as 11 stone 7 pounds uses 11 as the stone value and 7 as the additional pounds value.

Full Formula Including Additional Pounds

When a weight includes both whole stones and extra pounds, the complete conversion formula is:

kg = (st × 6.35029318) + (lb × 0.45359237)

Consider a practical example — a person weighing 11 stone 7 pounds:

11 × 6.35029318 = 69.8532 kg
7 × 0.45359237 = 3.1751 kg
Total: 69.8532 + 3.1751 = 73.0284 kg

Historical Context and Standardization

The stone has been used as a unit of measurement for centuries across England and much of northern Europe, with its value historically varying between 8 and 24 pounds depending on the commodity. Parliament standardized it at 14 pounds in 1835. Today, while the SI metric system is the UK official standard, the stone remains culturally embedded for personal weight. According to the BIPM SI Brochure, the International System of Units defines the kilogram as the base unit of mass. Non-SI units such as the stone are permitted in specific national contexts but must be defined by an exact conversion to SI — hence the fixed value of 6.35029318 kg per stone.

Practical Use Cases

Medical and Clinical Settings: Healthcare professionals in metric-system countries require kilogram values for drug dosing, BMI calculations, and nutritional assessments. Patients from the UK often report weight in stones, making accurate conversion critical.
Fitness and Weight Management: International gym equipment, nutrition apps, and sports science standards use kilograms. Converting from stones enables accurate calorie tracking and body composition analysis.
Sports and Athletic Competition: Boxing weight classes, Olympic weightlifting, and international sports classifications all use kilograms. A boxer competing at 11 stone needs to know that this equals approximately 69.85 kg.
Travel and Official Documentation: Passports, visa applications, and international health records frequently require weight in kilograms.

Quick Reference Conversion Table

8 st = 50.80 kg
10 st = 63.50 kg
11 st = 69.85 kg
12 st = 76.20 kg
13 st = 82.55 kg
14 st = 88.90 kg
15 st = 95.25 kg

Accuracy and Precision

The conversion factor 6.35029318 is exact to eight decimal places, derived from the internationally agreed pound value of 0.45359237 kg. For everyday use, rounding to two decimal places (6.35 kg per stone) introduces negligible error. Clinical or scientific applications should use the full-precision factor. The NIST Units Outside the SI reference provides authoritative guidance on exact conversion values for all imperial and customary units, confirming that this factor is a defined constant fixed by international agreement rather than a measured approximation.

Reference

Frequently asked questions

How many kilograms is 1 stone?

One stone equals exactly 6.35029318 kilograms. This figure derives from the definition of one stone as 14 avoirdupois pounds, multiplied by the exact pound-to-kilogram conversion of 0.45359237 kg per pound (14 × 0.45359237 = 6.35029318). For practical everyday purposes, rounding to 6.35 kg per stone is accurate enough for most uses.

How do you convert stones and pounds to kilograms?

To convert a weight in stones and pounds to kilograms, multiply the stones by 6.35029318 and the additional pounds by 0.45359237, then add both results together. For example, 12 stone 4 pounds converts as: (12 × 6.35029318) + (4 × 0.45359237) = 76.2035 + 1.8144 = 78.0179 kg.

Why do people in the UK still use stones instead of kilograms?

The stone became standardized at 14 pounds under British law in 1835 and has remained embedded in British and Irish culture for expressing body weight ever since. Despite the UK officially adopting the metric system in the 1970s, the stone persists in everyday conversation, NHS weight communications, and media reporting — a cultural tradition that has outlasted official metrication efforts.

What is 10 stone in kg?

Ten stone equals exactly 63.5029318 kilograms, which rounds to 63.50 kg. This is calculated by multiplying 10 by the exact conversion factor: 10 × 6.35029318 = 63.5029318 kg. At 63.5 kg, this weight falls within the healthy BMI range for adults standing approximately 5 feet 6 inches to 5 feet 9 inches tall.

Is the stone-to-kilogram conversion exact or approximate?

The conversion is exact, not an approximation. One stone is defined as exactly 14 pounds, and one avoirdupois pound is defined as exactly 0.45359237 kilograms under the 1959 International Yard and Pound Agreement. Therefore 1 stone = 14 × 0.45359237 = 6.35029318 kg exactly — a defined constant confirmed by both NIST and BIPM international standards.

How do I convert my body weight from stones to kilograms for a doctor abroad?

Multiply the stone value by 6.35029318, then add any extra pounds multiplied by 0.45359237. For example, 13 stone 5 pounds converts as: (13 × 6.35029318) + (5 × 0.45359237) = 82.5538 + 2.268 = 84.822 kg. This stones to kilos converter performs this calculation instantly, giving a result ready to share with any healthcare provider.