Weight Converter

Free Online Weight Converter — Kilograms, Pounds, Ounces, Grams, Stones and More

Weight and mass conversion is one of the most frequently performed unit conversions in daily life, touching everything from interpreting a foreign recipe to understanding your body weight in a different country's units, from calculating shipping costs to verifying whether a luggage bag is within the airline's allowance. The global split between the metric and imperial weight systems means that kilograms, grams, pounds, ounces, and stones coexist in everyday use, and converting accurately between them is a routine practical need. Our free online weight converter handles all of these unit families instantly — milligrams, grams, kilograms, metric tons in the metric system, and ounces, pounds, and stones in the imperial system — converting any input value across all units simultaneously.

The converter runs entirely in your browser. Enter a value in any unit, and all equivalents update in real time. No account required, no data sent anywhere.

Kilograms and Grams — The Metric Weight Standard

The kilogram (kg) is the SI base unit of mass, and it is the standard unit for expressing weight in everyday and scientific contexts throughout most of the world. Since the 2019 redefinition of SI base units, the kilogram is defined in terms of the Planck constant — a fixed physical constant — eliminating the dependence on the International Prototype of the Kilogram, a physical platinum-iridium cylinder that had served as the mass reference since 1889 and was subject to tiny changes over time due to surface contamination.

In everyday metric contexts, the kilogram is the appropriate unit for human body weight (typically 50-100 kg for adults), food quantities sold at supermarkets (a kilogram of flour, half a kilogram of cheese), luggage weight limits (typically 20-23 kg for checked baggage), and vehicle payload capacities. The gram (g, one-thousandth of a kilogram) is the standard for smaller food and ingredient quantities: a slice of bread is approximately 25-30 grams, a typical portion of pasta is 80-100 grams dry, and nutritional information on food packaging consistently uses grams for serving sizes and macronutrient content.

For very small quantities — pharmacological doses, laboratory samples, precious metal weights — the milligram (mg, one-thousandth of a gram, or one-millionth of a kilogram) is the working unit. Medication dosing is almost universally expressed in milligrams: a standard aspirin tablet is 325 mg, a common blood pressure medication dose might be 5-10 mg, and vitamin supplement doses range from 0.5 mg (folate) to 1,000 mg (vitamin C). The precision required in pharmacology makes milligram-level weight conversion accuracy important — an error in converting mg to g or g to kg in a medical context can have serious consequences.

Pounds and Ounces — The US and Imperial Weight System

The pound (lb) is the primary weight unit in the United States and retains significant use in the United Kingdom alongside the metric system. One pound is defined as exactly 453.59237 grams (approximately 0.4536 kg). The avoirdupois pound, used for everyday weight measurement, is divided into 16 ounces — 1 ounce equals approximately 28.35 grams. This 16-ounce division contrasts with the troy pound (used for precious metals) which contains 12 troy ounces, a potential source of confusion when weighing gold or silver versus everyday goods.

In the United States, body weight is expressed in pounds, body weight change is tracked in pounds, and food items are sold in pounds and ounces. A person weighing 160 pounds weighs approximately 72.6 kilograms. A steak portion of 8 ounces is approximately 227 grams. Meat at the grocery store is priced per pound (or per 100 grams in metric countries), requiring price-per-unit conversion when comparing costs across markets. Luggage allowances on US domestic flights are typically expressed in pounds (50 lb for checked baggage), requiring conversion from the kilogram allowances posted by international airlines.

The ounce is the dominant unit for small food and liquid quantities in US recipes and product labeling. A can of soup is typically 10.75 oz (305 g). A standard stick of butter is 4 oz (113 g). A cup of all-purpose flour weighs approximately 4.5 oz (128 g) when properly measured. These are weight ounces — not to be confused with fluid ounces, which measure volume. The distinction matters when following recipes: a recipe calling for "8 oz of cream cheese" means weight ounces (a standard block of cream cheese), while "8 fl oz of milk" means fluid volume (one US cup).

The Stone — British Body Weight Unit

The stone (st) is a unit of weight used almost exclusively in the United Kingdom and Ireland for expressing human body weight. One stone equals exactly 14 pounds, or approximately 6.35 kilograms. The typical range for adult human weight in stones is roughly 7 stone (44 kg / 98 lb) at the low end to 20+ stone (127+ kg / 280+ lb) at the high end, with most adults falling between 9 and 15 stone.

The stone-pounds combination (expressed as "11 stone 3 pounds" or "11 st 3 lb") is the conventional way to state body weight in UK context, analogous to the way feet and inches are used for height. Converting stone-pounds to kilograms requires first converting to total pounds (multiply stones by 14 and add the remaining pounds), then dividing by 2.20462 to get kilograms. Our converter accepts the stone as a single input and outputs its kilogram and pound equivalents directly, simplifying the step from UK weight expressions to metric equivalents for anyone managing health metrics, fitness goals, or medical records across the UK-metric divide.

The stone's persistence in the UK reflects the same cultural inertia that keeps miles on British road signs and pints in British pubs — metric education has coexisted with deeply embedded everyday imperial conventions for decades, and body weight is one of the areas where the traditional unit remains strongest in popular usage even as medical contexts increasingly use kilograms.

The Metric Ton — Large-Scale Weight Measurement

The metric ton (also called tonne, abbreviated t) equals exactly 1,000 kilograms — or approximately 2,204.62 pounds or 1.1023 short tons (the US short ton equals 2,000 pounds). The metric ton is the standard unit for large-scale weight measurement in industrial, agricultural, shipping, and commodity trading contexts worldwide.

Agricultural commodity production and trade are expressed in metric tons: global wheat production is approximately 750-800 million metric tons per year, global rice production around 500 million metric tons, and global iron ore mining approximately 2.5 billion metric tons annually. Steel production, cement production, and bulk chemical manufacturing are all tracked in metric tons at the national and global level. Shipping container capacity is often expressed in metric tons of cargo weight (a standard 20-foot container can carry up to approximately 21-28 metric tons depending on the container specification).

In the United States, large-scale weight is often expressed in short tons (2,000 lb) rather than metric tons. The difference of approximately 10% between a short ton and a metric ton is significant in commodity trading: a contract for 1,000 short tons involves approximately 907 metric tons, while a contract for 1,000 metric tons involves approximately 1,102 short tons. Ensuring clarity about which ton is being referenced in international contracts and specifications is essential for accurate trade and logistics calculations.

Weight Conversion in Fitness and Health Contexts

Body weight is the most personal and frequently monitored weight measurement for most individuals, and it is expressed in different units depending on country of residence, medical context, and personal preference. In the US, body weight is tracked in pounds. In UK and Ireland, in stone and pounds. Everywhere else, in kilograms. Fitness apps, medical records, and health devices must accommodate all three systems, and individuals moving between countries or using apps with different default units constantly encounter the need for body weight conversion.

The Body Mass Index (BMI) calculation requires weight in kilograms and height in meters — regardless of what units the person knows their measurements in. An American who knows their weight in pounds and height in feet and inches must convert both before calculating BMI. Converting 175 pounds to kilograms (79.4 kg) and 5 feet 9 inches to meters (1.75 m), then applying the formula (weight in kg ÷ height in m²) yields BMI = 79.4 ÷ (1.75²) = 79.4 ÷ 3.0625 = 25.9. Our weight converter handles the pounds-to-kilograms step in this calculation instantly.

Nutritional tracking and dietary management also generate frequent weight conversion needs. Recipes from different countries use different weight units for ingredients. A British recipe calling for 500 grams of chicken breast, an American recipe calling for a pound of ground beef, and a Japanese recipe calling for 200 grams of tofu all need to be understood in the cook's native weight system for practical kitchen use. Our converter makes these recipe adaptations straightforward regardless of which weight system the original recipe uses.

Shipping, Postal, and Luggage Weight

Commercial shipping and postal services create practical weight conversion needs at both consumer and business scales. International parcel services publish rate tables in kilograms, while US domestic services often use pounds and ounces. The maximum weight for a standard USPS First Class letter is 3.5 ounces (99 grams). UPS and FedEx international service weight limits are expressed in kilograms. Airlines typically specify baggage allowances in kilograms for international flights but may use pounds on US domestic routes.

E-commerce businesses shipping internationally must accurately convert product weights between unit systems for shipping cost calculation, customs declaration forms, and marketplace listing requirements. Amazon requires product weights in pounds and ounces for US fulfillment center entries but accepts kilograms for European fulfillment centers — the same product listing needs accurate weight entries in both unit systems. Our weight converter ensures these figures are consistent and accurate across all platforms and markets.

Weight vs. Mass — A Technical Distinction

In everyday usage, "weight" and "mass" are treated as interchangeable, and our converter works with this everyday understanding. Technically, however, mass and weight are distinct physical quantities. Mass is an intrinsic property of matter — a measure of the amount of material in an object, independent of gravity. Weight is a force — the gravitational pull exerted on a mass, equal to mass multiplied by gravitational acceleration.

On Earth's surface, where gravitational acceleration is approximately 9.81 m/s², an object with a mass of 1 kilogram has a weight of 9.81 newtons. On the Moon, where gravity is about 1/6 of Earth's, the same 1 kg mass would weigh only 1.62 newtons. The mass is unchanged; only the weight varies. This distinction matters in engineering and physics calculations, but for everyday purposes — stepping on a scale, reading a postal label, following a recipe — "weight" and "mass" refer to the same measurement, and our converter serves both uses accurately.