The UK’s continued use of stones is largely cultural and historical. While the UK officially adopted the metric system through a gradual metrication process beginning in the 1960s, body weight is one area where traditional units have persisted strongly. Many Britons find stones more intuitive for conceptualizing human weight since they provide a scale with smaller, more manageable numbers than pounds (an average adult might be 11-12 stone rather than 154-168 pounds). This cultural preference has remained despite official changes in other areas of measurement. It’s also a matter of generational habits, with older generations particularly attached to stones and younger people increasingly familiar with both systems. This dual-system approach is common in the UK, where people often use a mix of imperial and metric units in daily life.

Simplified conversion formulas provide good approximations but do introduce small errors. For example, the simplified conversion of 1 stone ≈ 6.35 kg is actually rounded from 6.35029318 kg. For a single stone, this difference is negligible (less than a gram), but when converting larger weights, these small differences can accumulate. For a weight of 20 stone, the difference between the exact conversion (127.01 kg) and the simplified formula (127.0 kg) is still minimal. For everyday purposes like tracking body weight or casual conversations, simplified formulas are more than adequate. However, for medical dosing, scientific research, or any application requiring high precision, the exact conversion factors should be used. Our calculator uses the exact conversion factors to ensure maximum accuracy, but the simplified formulas (st × 6.35 for stone to kg, and kg × 0.157 for kg to stone) work well for mental calculations and estimates.

The stone unit has become specialized for body weight due to a combination of historical factors and practical considerations. Historically, stones were used for weighing many commodities, with the exact weight of a stone varying by item (wool stone, meat stone, etc.). As trade standardized and modernized, these varied uses declined, and the metric system became dominant for commerce and industry. However, body weight remained an area where the stone provided a practical advantage – it offers a more manageable scale than pounds for adult weights while being more precise than using kilograms alone. For example, a 1-stone difference (about 6.35 kg) represents a significant but not extreme weight change that’s easy to conceptualize. Additionally, body weight is often a matter of personal reference rather than precise scientific or commercial measurement, allowing cultural preferences to persist. Finally, the private nature of body weight measurement meant there was less regulatory pressure to standardize compared to commercial contexts.

For quick mental conversions without a calculator, you can use these simplified methods:

1. Stone to Kilograms: Multiply the number of stones by 6.5 (slightly rounding up from the actual 6.35). For greater accuracy, you can use 6.3 plus half of 0.1 (6.3 + 0.05 = 6.35).
2. Pounds to Kilograms: Divide pounds by 2, then subtract 10% of that result. Example: 100 pounds ÷ 2 = 50, minus 10% (5) = 45 kg (actual: 45.36 kg).
3. Kilograms to Stones: Divide kilograms by 6.5, or multiply by 0.16 (rounding up from 0.157). Example: 64 kg ÷ 6.5 ≈ 9.85 st, or approximately 9 st 12 lb.
4. Kilograms to Pounds: Multiply kilograms by 2, then add 10% of that result. Example: 50 kg × 2 = 100, plus 10% (10) = 110 lb (actual: 110.23 lb).

The stone unit is primarily used in the United Kingdom and Ireland, with very limited usage elsewhere. Some former British colonies retained certain imperial measurements after independence, but most have transitioned fully to the metric system for official and everyday use. Australia and New Zealand occasionally used stones historically but have largely abandoned the unit even for body weight. In North America, despite sharing British imperial origins, the United States and Canada never widely adopted the stone as a weight unit, preferring pounds instead. Even within the UK and Ireland, younger generations are increasingly familiar with kilograms due to education in the metric system, though stones remain common in conversational contexts about body weight. The stone unit may occasionally appear in specialized contexts in other countries with historical British connections, such as horse racing in Australia, but such instances are increasingly rare. This limited geographical usage makes the stone one of the most regionally specific units of measurement still in common use.