Stone to Kilograms Converter

The stone to kilograms converter translates any weight between the UK body-weight unit (the stone) and the SI unit of mass (the kilogram). For the Stone to Kilograms Converter, Enter a value in either field and the other updates instantly. The converter is useful for British and Irish readers reading international health guidelines, for non-UK residents interpreting UK medical charts or gym readings, for doctors and nurses working in multi-national clinical settings, for personal trainers switching between metric and imperial clients, for expatriates communicating body weight with family abroad, for journalists reporting on UK health statistics, and for students comparing unit systems across countries. The conversion is exact, the stone and the kilogram are both defined by statute or international agreement and contain no rounding beyond the displayed precision.

How to Use the Stone to Kilograms Converter

1. Type a number into the Stone (st) field to see the equivalent in kilograms and pounds on the right.
2. Type a number into the Pounds (lb) field, this is the residual pounds that ride alongside the stone count (UK convention: 0 to 13 lb).
3. The kilograms field is computed live as the total pounds (stone × 14 + pounds) is multiplied by 0.45359237.
4. Use the quick presets for common body-weight reference points (8, 10, 11, 12, 14, 16, 18 stone) to load an exact pair of values instantly.
5. Read the result box to see the active formula, the unit breakdown (1 st = 14 lb = 6.35029318 kg = 224 oz), and a side-by-side display of the current value in every unit.

The Conversion Formula

The conversion between stones and kilograms uses two exact constants: 1 stone = 14 pounds (UK Weights and Measures Act 1985) and 1 pound = 0.45359237 kilograms (1959 international yard and pound agreement, signed by the US, UK, Canada, Australia, New Zealand, and South Africa). Formally:

Stone to kilograms: kg = stone × 6.35029318 + pounds × 0.45359237

Kilograms to stone: stone = kg ÷ 6.35029318

The full unit breakdown, 1 st = 14 lb = 6.35029318 kg = 224 oz, comes from chaining these definitions: 14 lb per stone, then 0.45359237 kg per pound, then 16 oz per pound for the ounces equivalent.

Because both the stone and the pound are defined in terms of each other with no temperature correction or unit ambiguities, and because the kilogram is the SI base unit of mass defined since the 2019 SI redefinition in terms of the Planck constant, the conversion between stones and kilograms is exact to whatever precision your display can show.

Why the residual pounds? UK body weight is almost always reported as "X stone Y pounds" rather than as a single number of pounds or kilograms. The format survives from medieval English trade where the stone was the standard wool weight and the pound was the smaller subdivision. The "pounds" field above accepts the 0 to 13 lb residual that goes with a whole number of stones; values above 13 lb are usually carried forward (14 lb = 1 stone, so 10 st 14 lb = 11 st 0 lb).

Why 6.35029318? The stone-to-kilogram factor comes from the product 14 × 0.45359237. Both factors are exact by definition, so the product is exact too. The factor 6.35029318 is just the truncated product of 14 × 0.45359237 = 6.35029318 to the precision shown; carrying more digits gives 6.35029318 kg per stone exactly (no further digits needed because the multiplication closes).

Worked Examples

Example 1, A UK doctor's notation. A general practitioner writes "11 st 4 lb" in a patient's notes. What is that in kilograms? Total pounds = 11 × 14 + 4 = 154 + 4 = 158 lb. kg = 158 × 0.45359237 = 71.66759446... ≈ 71.67 kg. So 11 st 4 lb ≈ 71.67 kg, near the Office for National Statistics mean for adult men.

Example 2, A gym instructor reading an American chart. A US-based personal trainer sees "180 lb" on a client's intake form. What is that in stones and pounds? Total kg = 180 × 0.45359237 = 81.6466266 ≈ 81.65 kg. stone = 81.65 ÷ 6.35029318 = 12.857... ≈ 12 st 12 lb. So 180 lb ≈ 12 st 12 lb. (Rounded: 81.65 kg / 6.35029318 kg per stone = 12.857 stone = 12 st + 0.857 × 14 lb = 12 st 12 lb.)

Example 3, A weight-loss goal. A UK reader wants to lose "half a stone". How many kilograms is that? Half a stone = 7 lb. kg = 7 × 0.45359237 = 3.17514659 ≈ 3.18 kg. So half a stone is about 3.18 kg, the canonical "half a stone loss" goal.

Example 4, Round-trip check. Convert 10 stone to kilograms, then back to stones. kg = 10 × 6.35029318 = 63.5029318. stone = 63.5029318 ÷ 6.35029318 = 10.00000000... (within floating-point precision). The round trip is exact, a useful sanity check when implementing the conversion in code.

Example 5, The full breakdown at 1 stone. 1 st = 14 lb = 6.35029318 kg = 224 oz. Verify the ounce count: 14 lb × 16 oz/lb = 224 oz. And 224 oz × 28.349523125 g/oz = 6350.29318 g = 6.35029318 kg. The breakdown is consistent.

Where the Conversion Shows Up

• Healthcare and primary care: UK NHS records, fit-notes, and medication charts quote body weight in stones and pounds. International clinical trials and WHO guidelines use kilograms. GPs routinely convert in both directions.
• Diet and weight loss: UK weight-loss programmes, apps, and magazines ("lose half a stone", "drop a stone by summer") use stones as the headline unit. Calorie calculators and BMI tools use kilograms. Cross-unit fluency is essential.
• Fitness and combat sports: Boxing, MMA, wrestling, and strongman competitions in the UK still quote weight classes in stones and pounds; international federations use kilograms. Cut-weight calculations span both.
• Travel and luggage: UK airline passengers often see their hold allowance in kilograms (because airlines are metric) but their own weight on bathroom scales in stones. Converting personal weight for BMI or medical forms is a daily task.
• Insurance and underwriting: UK life-insurance and health-insurance forms often request body weight in either stones or kilograms. Premium calculators sometimes require one or the other, and policy holders may need to convert.
• Education: Teaching the relationship between UK customary, US customary, and SI mass units. The stone is a useful case study in why some units persist in everyday life long after metrication.
• Veterinary and animal care: Pet weights in the UK are often reported in kilograms (vets use metric) while the owner's weight appears in stones. Cross-unit arithmetic is routine in a vet's waiting room.
• Journalism and statistics: UK national statistics (ONS Health Survey for England, NHS Digital) publish body weight in kilograms for international comparability, but British newspapers headline results in stones.

Common Mistakes

• Treating stones as 14 lb in metric countries. Outside the UK and Ireland, stones are unknown. A "stone" is not 14 lb in the US customary system; it is also not a metric unit. If a non-UK form asks for weight in pounds, give pounds, not stones.
• Converting stones to pounds and then to kg with two different factors. Each conversion step introduces rounding error. Convert in one step: kg = stone × 6.35029318. The single-step conversion is exact to the precision of the factor.
• Forgetting the residual pounds. UK body weight is "stone AND pounds", not just stone. A person who is 11 st 7 lb is heavier than 11 st by 7 × 0.45359237 = 3.18 kg. Dropping the residual under-estimates by several kilograms.
• Confusing the stone with the troy pound or the merchant's pound. The "stone" used in UK body weight is the avoirdupois stone of 14 lb. The troy pound (used historically for precious metals) is 0.373 kg, about 12% less; the merchant's pound was 15 oz (not 16). Those legacy definitions are obsolete for body weight.
• Carrying the residual above 13 lb. UK convention says the pounds part runs 0 to 13. If pounds reaches 14, increment stones and reset pounds to 0. Otherwise the representation is non-canonical and may confuse readers used to the standard form.
• Confusing kg with lbs on a scale. Many digital bathroom scales can switch between kg and lb (and sometimes st). If the reading seems "off by a lot," check which unit the scale is displaying before assuming the measurement is wrong.
• Mixing up pounds-mass and pounds-force. The pound (lb) in body weight is a unit of mass. In physics and engineering, "pound-force" (lbf) is a unit of force equal to the weight of a 1-lb mass at standard Earth gravity. Body weight in pounds is lb, never lbf.
• Rounding the factor to 6.35. A common shortcut is "1 stone ≈ 6.35 kg". Over an adult weight range, the missing 0.00029318 kg/stone × 18 stone = 0.005 kg total, small, but not zero. For precise medical or trade work, use the full factor 6.35029318.

Quick Mental Conversion Trick

For an approximate stone → kg conversion in your head: multiply by 6.35. For example, 10 st → 10 × 6.35 = 63.5 kg (actual: 63.50 kg, dead-on). 12 st → 12 × 6.35 = 76.2 kg (actual: 76.20 kg). This trick is accurate to within 0.01 kg across the entire adult body-weight range, because the true factor 6.35029318 rounds so neatly to 6.35.

For an approximate kg → stone conversion: divide by 6.35. 70 kg → 70 ÷ 6.35 = 11.02 stone ≈ 11 st 0 lb. 80 kg → 80 ÷ 6.35 = 12.60 stone ≈ 12 st 8 lb. (0.60 × 14 = 8.4 lb, rounds to 8 lb.) This trick is accurate to within a few hundred grams for normal adult weights.

These tricks come from rounding 6.35029318 to 6.35. The error is small because the factor already truncates to four digits almost exactly. For medical, sports, or trade work where fractions of a kilogram matter, use the calculator, but for everyday conversation, the mental trick is plenty.

When to Use a Different Scale

The kilogram is the SI base unit of mass and is used in science, medicine, and most of the world's everyday life. The stone is used in the UK and Ireland primarily for personal body weight. The pound (lb) is used in the United States for body weight, groceries, and shipping. The troy pound (5760 grains ≈ 0.373 kg) is obsolete but the troy ounce (≈ 31.10 g) survives in precious-metal pricing. The slug (≈ 14.5939 kg) is the US customary mass unit in mechanics and is occasionally seen in physics textbooks. If you need to convert to or from kilograms, pounds, or ounces, use the additional conversion factors: 1 kg = 1000 g, 1 lb = 16 oz, 1 kg ≈ 35.274 oz, 1 kg ≈ 2.2046 lb.

Frequently Asked Questions

Q: What is the formula to convert stones to kilograms? A: Use kg = stones × 6.35029318 + pounds × 0.45359237. Multiply the stone count by 6.35029318 and add the residual pounds × 0.45359237. For example, 10 stone × 6.35029318 = 63.5029318 kg; adding 7 lb × 0.45359237 = 3.17514659 kg gives 66.67807839 kg total. The factor 6.35029318 comes from the exact definition 1 st = 14 lb × 0.45359237 kg/lb.

Q: What is the formula to convert kilograms to stones? A: Use stone = kg ÷ 6.35029318. Divide the kilogram value by 6.35029318 to get the stone equivalent. For example, 70 kg ÷ 6.35029318 = 11.023... stone ≈ 11 st 0 lb (since 0.023 × 14 = 0.32 lb, which rounds to 0 lb). For the full kg → stones and pounds conversion, take the integer part of the stone quotient and compute pounds = (kg − stone_int × 6.35029318) ÷ 0.45359237.

Q: How many pounds are in a stone? A: Exactly 14 pounds in a stone, by the UK Weights and Measures Act 1985. The factor is exact because both units are defined in terms of each other: 1 stone = 14 lb by statute, and 1 lb = 0.45359237 kg by the 1959 international yard and pound agreement.

Q: How many kilograms are in a stone? A: Exactly 6.35029318 kilograms in a stone, to the precision shown. The factor is the product 14 × 0.45359237. There is no rounding in the conversion; both the stone and the pound are defined in terms of each other with no measurement uncertainty.

Q: Why do British people measure body weight in stones? A: The stone was the standard unit for trade in medieval England (a "stone" of wool was 14 lb, the weight that was convenient for a merchant to carry or for a trading scale to weigh). English law formalised the stone over centuries and the UK kept it for personal body weight even after general metrication in the 1960s-70s. Most other Commonwealth countries switched to kilograms. Ireland kept the stone from British rule. The result is that the UK and Ireland are the only countries in the world where the stone is still in everyday use for body weight, and the convention survives in health, fitness, and insurance contexts.

Q: What is the difference between the stone and the US pound? A: The stone (st) equals 14 lb. The international avoirdupois pound (lb) is a unit of mass defined as exactly 0.45359237 kg by the 1959 international yard and pound agreement. The stone is a multiple of the pound used in the UK and Ireland; outside those countries the stone is rarely used and weights are quoted in lb or kg directly.

q: can the Stone to Kilograms Converter be used for professional or commercial purposes? A: Yes, the Stone to Kilograms Converter provides mathematically correct results that are suitable for professional, commercial, and educational use. For high-stakes applications (medical dosing, pharmaceutical manufacture, engineering safety cases, legal metrology, jewellery), verify results with a domain expert and use a properly calibrated instrument traceable to national standards. The factors used are exact by international definition.

Q: How often are the underlying conversion factors updated? A: The conversion factors used here (1 st = 14 lb, 1 lb = 0.45359237 kg) have been fixed since the UK Weights and Measures Act 1985 and the 1959 international yard and pound agreement, and are exact. The kilogram itself was redefined in 2019 via the Planck constant (rather than via a physical prototype), but this change is invisible to everyday conversions, the new kilogram has the same mass as the old one to well beyond the precision of any scale you own.

References

• UK Weights and Measures Act 1985, defines the stone as 14 pounds for trade and personal use in the United Kingdom.
• NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices (annual edition), defines the legal units of mass used in commerce in the United States and discusses the international pound.
• NIST Special Publication 811, Guide for the Use of the SI Units, defines the kilogram as the SI base unit of mass and discusses its relationship with customary units.
• 1959 international yard and pound agreement, defines the international avoirdupois pound as exactly 0.45359237 kilograms, signed by the US, UK, Canada, Australia, New Zealand, and South Africa.
• Bureau International des Poids et Mesures (BIPM), the international authority on SI units; maintains the formal definition of the kilogram since the 2019 SI redefinition.
• Office for National Statistics (ONS), Health Survey for England and UK national body-weight statistics; reference population means and distribution by age and sex.