Weight Converter

About Weight Converter

How Weight Conversion Works

Every unit on this page can be expressed as a number of grams. The converter multiplies your input by a "to grams" factor, then divides by the target unit's factor. That two-step process means any combination of the seven units works without needing a separate factor for every possible pair.

Worked example - kilograms to pounds:

1. Start with 75 kg
2. Convert to grams: 75 x 1,000 = 75,000 g
3. Convert grams to pounds: 75,000 / 453.59237 = 165.347 lbs

Worked example - ounces to stones:

1. Start with 200 oz
2. Convert to grams: 200 x 28.349523125 = 5,669.905 g
3. Convert grams to stones: 5,669.905 / 6,350.29318 = 0.8929 st

The factors used here are the exact values defined by the 1959 International Yard and Pound Agreement and the SI system. One avoirdupois pound is exactly 0.45359237 kg - not an approximation but a fixed legal definition established when the US, UK, Canada, Australia, New Zealand, and South Africa jointly standardised the pound in terms of the kilogram.

Conversion Factors Reference

From	To	Multiply By	Source
Kilograms	Pounds	2.20462	International avoirdupois pound definition
Pounds	Kilograms	0.453592	Exact: 1 lb = 0.45359237 kg
Ounces	Grams	28.3495	1 oz = 1/16 lb = 28.349523125 g
Grams	Ounces	0.035274	Reciprocal of above
Stones	Kilograms	6.35029	1 stone = 14 lbs = 6.35029318 kg
Kilograms	Stones	0.157473	Reciprocal of above
Metric tons	Kilograms	1,000	SI definition
Kilograms	Metric tons	0.001	SI definition

These factors all trace back to two anchors: the SI definition of the gram (one thousandth of a kilogram, itself defined via the Planck constant since 2019) and the 1959 international agreement that fixed the pound at exactly 0.45359237 kg. Every other factor on this page - ounces, stones, metric tons - derives from these two definitions through simple multiplication.

All Seven Units Compared

Unit	Symbol	Grams	System	Typical Use
Milligram	mg	0.001	Metric	Medication dosages, chemical measurements
Gram	g	1	Metric	Food labels, cooking recipes, postal weight
Kilogram	kg	1,000	Metric (SI base unit)	Body weight, luggage, grocery shopping
Ounce	oz	28.3495	Imperial/US	Food portions, postal weight (US)
Pound	lb	453.592	Imperial/US	Body weight (US), cooking (US), shipping
Stone	st	6,350.29	Imperial (UK)	Body weight (UK and Ireland)
Metric ton	t	1,000,000	Metric	Industrial goods, vehicles, shipping containers

Why Was the Kilogram Redefined in 2019?

Until May 2019, the kilogram was the last SI base unit still defined by a physical object - a platinum-iridium cylinder stored at the Bureau International des Poids et Mesures (BIPM) near Paris. Measurements showed this prototype had drifted by roughly 50 micrograms over a century, which created problems for precision science.

On 20 May 2019 (World Metrology Day), the kilogram was redefined in terms of the Planck constant, fixed at exactly 6.62607015 x 10 to the power of negative 34 joule-seconds. The kilogram, and by extension every weight unit on this page, is now tied to a fundamental constant of nature rather than a single metal cylinder. For everyday weight conversions the practical difference is zero, but the change eliminated a measurable source of drift for laboratory work.

The redefinition also means the avoirdupois pound (0.45359237 kg) is now indirectly defined through the Planck constant, giving it a more rigorous scientific foundation than at any point in its history.

Where Are Pounds, Stones, and Ounces Still Used?

Only three countries have not officially adopted the metric system: the United States, Liberia, and Myanmar. In practice the picture is more complicated.

The United States uses pounds and ounces for nearly everything - grocery shopping, body weight, postal shipping, cooking. Stones are virtually unknown there. The United Kingdom officially uses metric for trade (shops sell produce in kilograms and grams) but informally uses stones and pounds for body weight. Ask someone in London how much they weigh and the answer will almost certainly be in stones. Babies are still commonly weighed in pounds and ounces.

Ireland follows a similar pattern, with stones and pounds used conversationally for body weight. Canada is officially metric but pounds remain common in everyday speech, especially for body weight and cooking. Australia completed its metrication in the 1970s and uses kilograms everywhere.

Most of Asia, Africa, continental Europe, and South America use metric exclusively. Kilograms and grams are the dominant weight units for roughly 95% of the world's population.

System	Countries	Weight Units
Metric only	Most of the world (EU, Asia, Africa, South America, Australia)	mg, g, kg, metric ton
Imperial / US customary	United States	oz, lbs (stones rarely used)
Mixed	United Kingdom (metric official, stones/lbs for body weight)	kg in shops, stones/lbs informally
Mixed	Canada (metric official, pounds still common informally)	kg on labels, lbs in conversation

What Is the Difference Between Avoirdupois and Troy Ounces?

This converter uses avoirdupois ounces (28.349523125 g), which is what people mean by "ounce" in everyday use. Troy ounces (31.1035 g) are used exclusively for precious metals like gold, silver, and platinum. When financial news reports gold priced "per ounce," that is a troy ounce - about 10% heavier than a regular ounce.

The distinction matters when buying or selling precious metals. One troy ounce of gold weighs 31.1035 g, while one avoirdupois ounce of flour weighs 28.3495 g. A troy pound (12 troy ounces, 373.242 g) is actually lighter than an avoirdupois pound (16 avoirdupois ounces, 453.592 g), which catches many people off guard.

Type	Grams	Used For
Avoirdupois ounce (oz)	28.3495 g	Everything except precious metals
Troy ounce (oz t)	31.1035 g	Gold, silver, platinum pricing
Fluid ounce (fl oz)	N/A (volume, not weight)	Liquid measurements

How Did the Stone Get Its Value?

The stone dates back to ancient marketplaces where actual stones served as counterweights on balance scales. By the 13th century, England had multiple "stones" for different goods: 5 pounds for glass, 8 pounds for beeswax, 12 pounds for lead, and 12.5 pounds for wool (per the Assize of Weights and Measures, c. 1300).

The modern 14-pound stone was fixed by a royal statute in 1389 during the reign of Richard II. The wool trade was the backbone of the English economy, and standardising the stone at 14 pounds (making a sack exactly 26 stones, or 364 pounds) reduced fraud and simplified commerce with buyers in Flanders and Florence. Wool merchants continued using the same balancing method - literal stones on one side of a scale - well into the 20th century. The Weights and Measures Act of 1985 made the stone unofficial for UK trade, but it lives on in everyday conversation. British gyms and doctors' surgeries still show both kilograms and stones on their scales.

Common Conversion Scenarios

Scenario	From	To	Example
Following a US recipe in Europe	Ounces	Grams	8 oz butter = 227 g
Weighing yourself at a UK gym	Kilograms	Stones + lbs	75 kg = 11 st 11.4 lbs
Shipping a package internationally	Pounds	Kilograms	15 lbs = 6.80 kg
Comparing airline luggage limits	Kilograms	Pounds	23 kg limit = 50.7 lbs
Reading medication labels	Milligrams	Grams	500 mg = 0.5 g
Industrial freight weight	Metric tons	Pounds	2 t = 4,409 lbs

Airline luggage limits are one of the most common reasons to convert between kilograms and pounds. Most international airlines set the standard checked bag limit at 23 kg (about 50.7 lbs) for economy class, with carry-on limits ranging from 7 kg to 10 kg depending on the carrier. Business and first class passengers often get 32 kg (70.5 lbs) per bag.

Cooking is another frequent conversion situation. American recipes use ounces and pounds while European and Australian recipes use grams. A quick reference: 1 cup of all-purpose flour weighs roughly 125 g (4.4 oz), 1 cup of granulated sugar roughly 200 g (7 oz), and a stick of butter is 113 g (4 oz, or one quarter of a pound). The Cooking Unit Converter handles cups, tablespoons, and millilitres alongside weight for recipe conversions.

Tips for Accurate Weight Conversion

When converting for cooking, remember that weight and volume are not the same thing. A cup of feathers weighs far less than a cup of sugar. Recipes that list ingredients by weight (grams or ounces) are more precise than those using volume measurements, because density varies between ingredients.

For postal and shipping conversions, round up rather than down. If your package weighs 6.8 kg and the shipping limit is 15 lbs, converting gives 14.99 lbs - technically under the limit, but any postal clerk who rounds differently will charge for the next weight bracket. Build in a small margin when close to a limit. Royal Mail uses grams and kilograms for domestic post (standard letter up to 100 g, large letter up to 750 g), while USPS uses ounces and pounds (first-class letter up to 3.5 oz, first-class package up to 13 oz). Knowing these thresholds saves money when shipping internationally.

When converting stones to kilograms for medical records, use the full decimal value rather than rounding to whole numbers. A difference of 0.5 kg can affect medication dosing, BMI classifications, and other health assessments. The BMI Calculator accepts both metric and imperial input if you want to check your classification.

For a focused kg-to-lbs lookup with gym plate references, the dedicated KG to LBS Converter covers that single conversion. The Length Converter handles distance and dimension conversions in the same real-time format.

Sources

• BIPM - SI Base Units (Kilogram Definition)
• NIST - Kilogram Redefinition (2019)
• NIST - SI Units of Mass
• UK Legislation - Weights and Measures Act 1985
• Royal Mint - What is a Troy Ounce?
• UK OPSS - Office for Product Safety and Standards (Metrology)