Free Weight Converter Online

A weight converter turns a value in one mass unit — kilograms, pounds, ounces, whatever a label or scale gave you — into its equivalent in another, instantly and without you needing to remember whether a stone is 14 pounds or 16. This tool covers seven units: milligrams, grams, kilograms, tonnes, ounces, pounds, and stone. Enter a number, pick the source unit and the target unit from the two dropdown menus, and watch the result recalculate live — no separate button to press and no round trip to a server.

How the Weight Converter Works

Type your value and pick a source and target unit from the dropdowns. Thanks to the tool's real-time calculation and live preview, the converted figure appears the moment you finish typing — there's nothing to submit and nothing to wait for. Internally, every unit is stored as a factor relative to kilograms, the base unit: converting 5 pounds to ounces multiplies 5 by the pound-to-kilogram factor and then divides by the ounce-to-kilogram factor, arriving at the answer without needing a dedicated pound-to-ounce formula. That base-unit design is what lets all seven units convert cleanly into each other from just seven stored numbers.

Mass Versus Weight (and Why "Weight Converter" Is a Bit of a Misnomer)

Strictly speaking, physics draws a sharp line between mass and weight: mass is the amount of matter an object contains, measured in kilograms or pounds, while weight is the force gravity exerts on that mass, measured in newtons, and it changes depending on where you are — an astronaut's mass stays constant on the Moon while their weight drops to roughly a sixth of what it was on Earth. Every unit in this tool — milligram through stone — is actually a mass unit, not a force unit, which puts it in good company: bathroom scales, shipping labels, and recipe books all use "weight" to mean mass in everyday speech, and this converter follows that same widespread convention rather than the stricter physics definition.

Where this distinction becomes more than academic is in the units themselves. The kilogram is defined today by fixing the value of the Planck constant, a change adopted in 2019 that replaced the physical platinum-iridium cylinder previously kept in France as the literal reference kilogram. The pound, by contrast, is defined relative to the kilogram: the international avoirdupois pound was fixed at exactly 0.45359237 kilograms by the same 1959 agreement that standardized inches and miles, which is why the pound-to-kilogram conversion here is an exact ratio rather than a rounded approximation.

Avoirdupois and Troy: Two Different Ounces

One detail trips up more people than it should: there are two completely different "ounce" measurements in common use, and they are not interchangeable. The ounce in this converter is the avoirdupois ounce — the one used for body weight, food packaging, and general commerce — equal to about 28.35 grams, or exactly one-sixteenth of an avoirdupois pound. Precious metals like gold and silver, however, are priced and weighed using the troy ounce, an older system that predates avoirdupois measures and is still used exclusively in bullion, jewelry, and coin markets. A troy ounce is heavier, at roughly 31.10 grams — about 10% more than the avoirdupois ounce this tool converts. This converter does not include a troy ounce unit, precisely because mixing the two systems silently is how people misjudge the weight, and therefore the value, of precious metal. If you're weighing gold, always confirm which ounce a quoted price refers to before converting anything by hand.

The stone, meanwhile, is a unit largely unique to the United Kingdom and Ireland, where body weight is still commonly reported in stone and pounds together (as in "12 stone 4") rather than in a single unit. One stone equals exactly 14 avoirdupois pounds, which works out to 6.35029318 kilograms — a clean, exact figure inherited from the pound's own exact metric definition.

Tonne vs. Ton: One Word, Three Different Weights

The tonne in this converter is the metric tonne, defined as exactly 1,000 kilograms — a clean figure with no historical baggage attached. But "ton," spelled without the extra "ne" and pronounced the same way, refers to two other, non-metric units that are each a different size again. The US short ton equals 2,000 pounds, which works out to about 907.18 kilograms — noticeably lighter than the metric tonne. The UK long ton, used historically in British shipping and industry, equals 2,240 pounds, or about 1,016.05 kilograms — heavier than the metric tonne. None of the three is interchangeable with either of the others, and this converter's tonne unit is specifically the 1,000-kilogram metric version; a US shipping manifest quoting "tons" is very likely using the 907.18-kilogram short ton instead, and applying this converter's factor to that figure would introduce a real, non-trivial error.

Why People Convert Weight and Mass

Mass conversion shows up across a wide range of everyday and professional situations:

• Cooking and baking. A recipe written in grams needs converting for a kitchen scale calibrated in ounces, or vice versa, and precision matters more in baking than in most other cooking, where ratios of flour to liquid can make or break a result.
• Tracking body weight across regions. A US doctor's chart in pounds, a UK reading in stone, and a European scale in kilograms all describe the same physical quantity — converting between them is routine for anyone splitting time between countries or reading international health guidance.
• Shipping and postage. Courier services often price by weight bracket, and international shipments frequently require the sender to state a package's mass in both kilograms and pounds depending on origin and destination.
• Scientific and laboratory work. Milligram-to-gram precision matters in pharmacology, chemistry, and nutrition science, where a decimal point in the wrong place changes a dose or formulation entirely.
• Industrial and commodity quantities. Tonnes appear in shipping manifests, agricultural yields, and construction material orders, often needing conversion to or from pounds for cross-border paperwork.

The Seven Units, Side by Side

Unit	Equivalent in Kilograms	Typical Use
Milligram (mg)	0.000001 kg	Pharmaceutical doses, lab chemistry
Gram (g)	0.001 kg	Cooking, small parcels, jewelry (non-precious)
Kilogram (kg)	1 kg (base unit)	Body weight, groceries, most of the world's everyday use
Tonne	1,000 kg	Shipping manifests, bulk materials, vehicle ratings
Ounce (oz, avoirdupois)	≈0.0283495 kg	Food packaging, US recipes, postage
Pound (lb)	0.45359237 kg	Body weight in the US, produce, general commerce
Stone	6.35029318 kg	Body weight in the UK and Ireland

An honest limitation worth stating plainly: this tool converts mass, not force, and it has no concept of gravity — a value entered as "1 kilogram" converts the same way whether you're describing an object on Earth, on the Moon, or in orbit, because mass itself doesn't change with location even though weight, in the strict physics sense, does.

A Weight Converter vs. Other Ways to Get There

A few alternatives exist for the same task, each with its own tradeoffs:

• A kitchen or postal scale with a unit-switch button. Handy when you're weighing something physically in front of you, but useless when you already have a number from a recipe or shipping label and just need the converted figure.
• Rough mental approximations. "A kilogram is about 2.2 pounds" is close enough for a quick gut check, but the error compounds on larger numbers, and it doesn't help at all with less familiar units like stone or troy weights.
• A spreadsheet formula. Reliable if you're already converting a whole column of values as part of a larger data set, but overkill for a single quick conversion.
• This converter. All seven units in one place, updating as you type, with no setup required for a one-off conversion.

How Your Data Is Handled

The number and units you select are processed entirely inside your browser — nothing you type is sent to a server to compute the result, and there's no account, login, or upload step involved anywhere in the process. The same math runs whether you're online or offline once the page has loaded, since the conversion logic is plain JavaScript executing on your own device. For developers who need to run the identical mass conversion from external code — say, normalizing a batch of product weights pulled from different supplier feeds — the same logic is exposed as a documented API endpoint at /docs.

Weight and Mass Conversion Questions

Is a "weight converter" really converting mass instead of weight?

Yes, in the strict physics sense — every unit here (kilogram, pound, ounce, stone, and the rest) measures mass, which stays constant regardless of gravity, rather than weight, which is a force that changes with location. The everyday, commercial use of "weight" to mean mass is so universal that this tool follows that common convention rather than the technical distinction.

What's the real difference between avoirdupois and troy ounces?

The avoirdupois ounce (used in this converter) weighs about 28.35 grams and covers everyday goods, food, and body weight. The troy ounce, used specifically for precious metals like gold and silver, weighs about 31.10 grams — roughly 10% heavier. Quoting a metal's price "per ounce" almost always means the troy ounce, so confirm which system applies before converting bullion weights by hand.

How many kilograms is a stone, exactly?

Exactly 6.35029318 kilograms, since one stone equals 14 avoirdupois pounds and the pound itself has an exact, internationally fixed value. The stone is used almost exclusively in the UK and Ireland for reporting body weight.

Why do American and European recipes list ingredients differently?

American recipes traditionally lean on volume measures (cups, tablespoons) alongside ounces, while European and many international recipes specify mass directly in grams, which tends to be more precise since it doesn't depend on how tightly an ingredient like flour is packed into a cup. Converting between the two systems for baking specifically benefits from checking density-based conversions rather than a pure mass converter, because equal volumes of different ingredients land at noticeably different masses on a kitchen scale.

Does this tool handle troy ounces for gold and silver?

No — the ounce unit here is strictly the avoirdupois ounce used for everyday goods. Converting bullion or jewelry weights requires a troy-specific conversion (1 troy ounce ≈ 31.10 grams), which is a different unit from the one this tool provides, by design, to avoid the two being confused.

Related Tools

Mass is only one axis of the conversions people commonly need alongside it. If your numbers also involve distance rather than mass, the Length Converter applies the same instant-conversion approach to meters, feet, and miles. Property or land measurements pair naturally with the Area Converter for square meters, acres, and hectares. Anyone tracking body composition alongside body weight might also find the BMI Calculator useful, since it takes a weight and height together to compute a single health metric. Weather or lab readings that accompany a mass figure convert cleanly with the Temperature Converter, and digital file sizes — a different kind of "how much" question entirely — are handled by the Data Storage Converter.