From checking your body weight at the doctor's office to calculating shipping costs for an online business, weight conversions are a daily necessity. The challenge? The United States uses pounds and ounces while most of the world uses kilograms and grams — and the relationship between these systems isn't always intuitive.
This comprehensive guide covers every weight conversion you need, complete with exact formulas, worked examples, practical tips, and a quick-reference table you can bookmark for instant access.
Before diving into conversions, it's worth clarifying a distinction that often causes confusion. In everyday language, "weight" and "mass" are used interchangeably. In physics, they're different: mass is the amount of matter in an object, while weight is the force of gravity acting on that mass.
For practical conversion purposes, this distinction rarely matters. Whether you're weighing groceries or shipping a package, the conversions in this guide will give you the right numbers. But if you're doing precision scientific work, remember that the metric system technically measures mass (grams, kilograms) while the imperial system measures force (pounds-force, ounce-force).
The US customary system uses three primary units for weight: ounces, pounds, and tons. The relationships are straightforward within the system.
| Unit | Symbol | Equivalent |
|---|---|---|
| Ounce | oz | 1/16 pound |
| Pound | lb | 16 ounces |
| Short Ton | T | 2,000 pounds |
One important note: the US uses the "short ton" (2,000 pounds), while the UK uses the "long ton" (2,240 pounds). When you see "ton" in an American context, it's the short ton. The metric tonne (1,000 kg) is yet another value, approximately 2,204.62 pounds.
The metric system is beautifully consistent. All units are based on the gram, scaled by powers of 10.
| Unit | Symbol | Equivalent |
|---|---|---|
| Milligram | mg | 0.001 grams |
| Gram | g | 1 gram (base unit) |
| Kilogram | kg | 1,000 grams |
| Metric Tonne | t | 1,000 kilograms |
These are the exact conversion factors you need. The relationship between pounds and kilograms is defined internationally and is exact.
The first two formulas are the most important. Once you know how pounds relate to kilograms, you can derive any other imperial-metric conversion by working through the chain.
Imperial → Imperial: There are 16 ounces in a pound and 2,000 pounds in a (short) ton. Multiply or divide accordingly.
Metric → Metric: Move the decimal point. Kilograms to grams? Multiply by 1,000. Grams to milligrams? Multiply by 1,000 again.
| From | To | Multiply By |
|---|---|---|
| Pounds | Kilograms | 0.4536 |
| Kilograms | Pounds | 2.2046 |
| Ounces | Grams | 28.3495 |
| Grams | Ounces | 0.0353 |
| Pounds | Ounces | 16 |
| Ounces | Pounds | 0.0625 |
| Kilograms | Grams | 1,000 |
| Grams | Kilograms | 0.001 |
| Stones | Kilograms | 6.3503 |
| Kilograms | Stones | 0.1575 |
Body weight is one of the most common reasons people convert between pounds and kilograms. Gym equipment, BMI calculators, and medical records may use different systems. If you're 180 pounds and your BMI calculator expects kilograms, you'll need to know you're 81.6 kg. Most digital scales let you toggle between units, but understanding the conversion helps you interpret data from any source.
American recipes use cups and ounces, while European recipes use grams. This is especially important in baking, where precision matters. A recipe calling for 200 grams of flour is not the same as 200 milliliters — but many people confuse weight and volume. When converting cooking measurements, always use weight (grams) rather than volume for dry ingredients.
Shipping costs are calculated by weight, and international shipments require conversions. USPS, FedEx, and UPS all use pounds and ounces domestically but may quote international rates in kilograms. If you're shipping a 5 lb package internationally, you'll need to declare it as 2.27 kg on customs forms.
Drug dosages are calculated per kilogram of body weight. If a medication requires 5 mg per kg and the patient weighs 150 pounds, you must convert: 150 lb = 68.04 kg, so the dose is 340.2 mg. Errors in this conversion can be medically dangerous — always double-check.
In the UK and Ireland, personal body weight is commonly expressed in "stones and pounds." One stone equals 14 pounds (approximately 6.35 kg). So a person who weighs "11 stone 4" weighs 11 × 14 + 4 = 158 pounds, or about 71.7 kg. If you encounter this unit in British media, fitness apps, or health discussions, now you know how to convert it.
Not exactly. One kilogram equals approximately 2.20462 pounds. Using 2.2 as a shortcut is fine for rough estimates, but it introduces an error of about 0.2%. For precise work, use the full factor.
"Lb" comes from the Latin word "libra," which was a unit of weight in ancient Rome. The pound symbol (£) also derives from libra, which is why the British pound sterling uses that symbol.
A regular (avoirdupois) ounce measures weight — 28.35 grams. A fluid ounce measures volume — about 29.57 milliliters. They're different measurements with the same name. When a recipe calls for "2 oz of water," it usually means fluid ounces (volume), but "2 oz of flour" almost always means weight.
Divide pounds by 2,204.62 (the number of pounds in a metric tonne). Alternatively, convert to kilograms first (multiply by 0.4536), then divide by 1,000. So 5,000 lb ÷ 2,204.62 = 2.268 metric tonnes.
Weight conversions don't need to be a source of confusion. With the formulas and tables in this guide, you can convert between pounds, kilograms, ounces, grams, and other units with confidence. Bookmark this page for quick reference, and use Risetop's free weight converter tool when you need instant, precise results.
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