Weight Converter

How to convert between weight units?

Weight conversion spans metric precision and imperial tradition. Here's how to get accurate results for any scenario:

• Enter your weight value in the input field. You can type whole numbers like 50, decimals like 72.5, or scientific notation like 1.5e3 for large values.
• Select your source unit from the first dropdown: metric options include milligram (mg), gram (g), kilogram (kg), and metric tonne (t); imperial options include ounce (oz), pound (lb), stone (st), and short ton.
• Choose your target unit from the second dropdown. The result appears instantly — for example, entering 1 kg shows 2.20462 lbs, 35.274 oz, and 0.15747 stones.
• Hit the Swap button to reverse the conversion direction without retyping. This is especially useful when comparing package weights across shipping carriers that use different systems.
• Copy the result using the copy icon next to the output. Results display up to 10 decimal places of precision, which you can round as needed.

Understanding the metric-imperial divide in weight measurement

The metric system defines weight through the kilogram, which was historically based on a physical platinum-iridium cylinder kept in France but redefined in 2019 using Planck's constant for permanent stability. One kilogram equals exactly 1000 grams, and one tonne equals exactly 1000 kilograms. The imperial system evolved from medieval trade practices: the pound originated from Roman libra (about 328.8 grams originally, standardized to 453.59237 grams in 1959). The ounce is 1/16th of a pound, and the stone (still used in the UK for body weight) equals 14 pounds or 6.35029 kg. These irregular ratios create real-world friction — a pharmaceutical company exporting medication must ensure dosage labels show both mg and grains (1 grain = 64.79891 mg exactly), while an American athlete competing internationally needs to understand that 165 lbs equals approximately 74.84 kg, not 75 kg.

Real-world scenarios where weight conversion accuracy matters

• International shipping: FedEx and UPS calculate rates by actual weight. A 2.5 kg package shipped from Germany to the US is billed at 5.51 lbs — understanding this prevents surprise surcharges at the weight bracket boundary.
• Nutrition tracking: Food databases often mix metric (grams per 100g) and imperial (ounces per serving). Converting 250g of chicken breast gives 8.82 oz, essential for macro counting apps.
• Precious metals trading: Gold is quoted in troy ounces (31.1035 g) rather than avoirdupois ounces (28.3495 g). A 10g gold bar weighs 0.3215 troy oz, not 0.3527 standard oz — a meaningful price difference.
• Medical dosing: Pediatric drug doses are calculated per kilogram of body weight. A child weighing 45 lbs needs their weight converted to 20.41 kg before applying mg/kg dosage guidelines.
• Aerospace engineering: Fuel load calculations require precise conversions. The Mars Climate Orbiter was lost in 1999 partly because one team used pound-force seconds while another used newton-seconds — a unit conversion failure costing $125 million.