Weight Converter — mass & weight unit conversion

A reliable weight converter saves you from doing awkward mental math whenever a recipe, shipping label, gym plate, or product spec uses a unit you don't think in every day. This tool converts between ten common mass and weight units — milligrams, grams, kilograms, tonnes, ounces, pounds, stone, US tons, imperial tons, and carats — instantly, with exact conversion factors instead of rounded shortcuts.

Arb Digital built this weight converter as one of our free everyday utilities, alongside our length, temperature, and volume converters. It's designed to be fast, accurate, and genuinely useful whether you're cooking, shipping a package internationally, tracking body weight, or pricing out gold and gemstones by the carat.

What This Weight Converter Does

Enter a value, pick your starting unit and your target unit, and the tool immediately recalculates the result. Behind the scenes, every unit is defined relative to the kilogram, the SI base unit of mass, so the tool converts your input into kilograms first and then out to your target unit. That two-step routing keeps every conversion mathematically consistent, whether you're going from milligrams to carats or from US tons to metric tonnes.

Below the calculator you'll find a quick-reference table of the most frequently searched weight conversions, so you can scan for a common answer — like how many pounds are in a kilogram — without touching a single input field.

How to Use the Weight Converter

1. Enter your value. Type the number you want to convert into the "Value to convert" field.
2. Choose your starting unit. Select the unit your value is currently measured in from the "From unit" dropdown.
3. Choose your target unit. Select the unit you want to convert into from the "To unit" dropdown.
4. Read your result instantly. The converted value appears immediately, with quick equivalents in kilograms, pounds, grams, and ounces shown below it.
5. Swap when needed. Click "Swap Units" to flip the conversion direction without retyping your number.

The Formula Behind Every Weight Conversion

Every unit in this converter is anchored to the kilogram, the internationally recognized SI base unit of mass as defined by the National Institute of Standards and Technology (NIST). Each supported unit carries a fixed "kilograms per unit" factor: one pound equals exactly 0.45359237 kilograms, one ounce equals exactly 0.028349523125 kilograms, one stone equals exactly 6.35029318 kilograms (14 pounds), one US short ton equals 907.18474 kilograms, one imperial long ton equals 1,016.0469088 kilograms, and one carat — used for gemstones and precious metals — equals exactly 0.0002 kilograms (200 milligrams). To convert, the tool multiplies your entered value by the "from" unit's factor to get kilograms, then divides that kilogram figure by the "to" unit's factor. Because every unit routes through the same fixed anchor, the tool never needs a separate formula for each possible pair of units, which is what keeps the math both simple to verify and free of the small inconsistencies that creep into converters built from dozens of direct pairwise ratios.

Mass vs. Weight: A Distinction Worth Knowing

In everyday language, "mass" and "weight" are used interchangeably, and for practical purposes on Earth, that's fine — this is why a kitchen scale reading in kilograms and a bathroom scale reading in pounds both feel like they're measuring "how heavy" something is. But physically, the two are different quantities. Mass is the amount of matter in an object and never changes regardless of location; weight is the force gravity exerts on that mass, and it does change depending on where you are. An astronaut's mass stays constant on the Moon, but their weight there is only about one-sixth of what it is on Earth, because lunar gravity is weaker. All of the units in this converter — kilograms, pounds, ounces, stone, tons, and carats — are, strictly speaking, units of mass, even though several of them (pounds and ounces in particular) are colloquially called "weight" units in everyday US and UK speech. For any conversion happening on Earth's surface, this distinction doesn't change the numbers you need, but it's worth knowing if you ever see the terms used precisely in a physics or engineering context.

Common Weight Conversion Reference Table

Why the US Ton and Imperial Ton Aren't the Same

One of the most common weight-conversion mistakes involves "tons," because the word means different things in different countries. A US short ton (used throughout the United States) equals 2,000 pounds, or 907.18474 kilograms. A UK imperial long ton equals 2,240 pounds, or 1,016.0469088 kilograms — about 12% heavier than the US ton. And a metric tonne, used almost everywhere outside the US, equals exactly 1,000 kilograms, which sits between the two. This tool separates all three explicitly, labeled "tonne (metric ton)," "US ton (short ton)," and "imperial ton (long ton)," so you never have to guess which "ton" a shipping quote, cargo manifest, or industrial spec sheet is referring to. Getting this wrong on a freight invoice or a bulk-material order can mean a meaningful cost difference, so it's worth double-checking which ton definition applies before you commit to a number.

Why Carats Matter for Gemstones and Precious Metals

The carat is a small but precisely defined unit equal to exactly 200 milligrams (0.2 grams), standardized internationally for weighing diamonds, gemstones, and pearls. It's unrelated to "karat" (spelled with a k), which measures gold purity rather than weight. Jewelers, appraisers, and gemological labs rely on the carat because gemstone prices scale dramatically with weight — a single extra carat can multiply a diamond's price many times over — so precision to the hundredth of a carat genuinely matters in that industry. This converter lets you move a carat weight straight into grams, milligrams, or ounces if you need to cross-reference it against a different measurement system, such as when converting an appraisal document into a shipping insurance declaration that requires grams.

Weight Conversion in Shipping, Fitness, and the Kitchen

Weight conversion shows up constantly outside a physics classroom. International shipping is one of the biggest examples: a courier quote from a US carrier may bill by the pound, while the manufacturer's invoice from an overseas supplier lists the item in kilograms, and getting the two mixed up can throw off a landed-cost calculation or trigger an unexpected customs fee. Gym-goers who train with plates marked in kilograms but track their lifts in pounds (or the reverse) need quick, accurate conversions to log progress correctly, especially since a 20-kilogram barbell is noticeably different from a 20-pound one. Home cooks following a recipe from a source that measures ingredients by weight rather than volume — common in European and professional baking — often need to convert grams to ounces or vice versa, particularly for ingredients like butter, flour, or chocolate where precision affects the outcome. Medical and veterinary dosing sometimes requires converting a patient's or animal's weight between kilograms and pounds to match a dosage chart printed in the other unit, which makes accuracy especially important in that context.

Postal and freight services also frequently apply different weight brackets depending on which unit system a shipment originates from, so travelers packing for an international flight often convert a baggage limit from kilograms to pounds to check whether a suitcase is compliant before reaching the airport counter. In every one of these situations, the core requirement is the same: an exact, trustworthy conversion factor rather than a rough mental estimate, since even a small error can compound into a real financial or practical consequence.

Common Mistakes to Avoid

• Mixing up US and imperial tons. A US ton (2,000 lb) and an imperial/long ton (2,240 lb) differ by over 100 kilograms — always confirm which is being used before ordering bulk freight or materials.
• Confusing carat with karat. Carat measures gemstone weight; karat measures gold purity. They are not interchangeable despite the similar spelling.
• Rounding too early in multi-step conversions. If you're converting through several units, keep full decimal precision until the final step.
• Treating "weight" and "mass" as always identical off-Earth. On Earth's surface the distinction rarely affects everyday conversions, but it matters in physics, aerospace, and any calculation involving different gravitational fields.
• Forgetting stone is a UK-specific unit. Stone (14 lb) is common for body weight in the UK and Ireland but rarely used elsewhere — confirm the context before assuming it applies.

Related Free Tools From Arb Digital

Pair this weight converter with our Length Converter for distance units, our Temperature Converter for Celsius, Fahrenheit, and Kelvin, and our Volume Converter for liquid measurements including the US-vs-imperial gallon difference. See the full lineup on our free online tools hub.

Frequently Asked Questions

This tool runs entirely in your browser — no data is sent to any server, and all calculations happen instantly on your device.