Weight Converter

Weight & Mass Conversions

Common Conversion Factors

Weight vs Mass

Technically, kilograms measure mass while pounds can be a unit of force. In everyday use, we treat them interchangeably for weight measurements.

Common Reference Points

Exact Conversion Factors

Since the 1959 International Yard and Pound Agreement, the pound is legally defined in metric terms: 1 lb = 0.45359237 kg exactly. Every cross-system conversion derives from exact definitions like this, not from rounded approximations.

Troy vs Avoirdupois: Two Ounce Systems

There are two different ounces, and mixing them up is expensive. The avoirdupois ounce (28.3495 g) is the everyday ounce for groceries and body weight. The troy ounce (31.1035 g) is used for gold, silver, and other precious metals — about 10% heavier. When you see gold priced at "$X per ounce", it is always troy ounces. Note that carat (200 mg, gemstone weight) is different from karat, which measures gold purity.

Weight vs Mass

Technically, kilograms measure mass while pounds can be a unit of force. In everyday use, we treat them interchangeably for weight measurements. In physics, mass (kg) is the amount of matter and stays constant everywhere, while weight (newtons, or pound-force) is the gravitational pull on that mass and varies by location — a 70 kg person has the same mass on the Moon but weighs one-sixth as much. Bathroom and kitchen scales display mass, corrected for local gravity.

Worked Examples

Example 1: Body Weight

75 kg in pounds: 75 × 2.20462 = 165.35 lb. In stones (UK format): 165.35 ÷ 14 = 11.81 stone — written as 11 st 11.4 lb.

Example 2: Airline Luggage

A 50 lb baggage limit in kilograms: 50 × 0.45359237 = 22.68 kg. Airlines often set roughly the same limit in different units — 50 lb in one place, 23 kg in another.

Example 3: Gold Pricing

Gold at $1,950 per troy ounce. One troy ounce is 31.1035 g, so the per-gram price is 1,950 / 31.1035 = $62.69/g. A 100 g bar costs 100 × 62.69 = $6,269, and 1 kg of gold is worth about $62,694.

Example 4: Body Mass Index

BMI divides body weight in kilograms by height in meters squared: BMI = 70 kg / (1.75 m)² = 70 / 3.0625 ≈ 22.9 — in the healthy range of 18.5–24.9. The formula only works in these units; with pounds and inches, multiply by 703 first: BMI = 703 × lb / in².

Practical Tips

• Quick mental math: multiply kg by 2.2 for pounds (accurate within 0.2%); divide pounds by 2.2 — or multiply by 0.45 — for kilograms.
• In the kitchen, weigh in grams: a cup of flour can weigh anywhere from 120 g to 165 g depending on how it was scooped. Recipes that specify 250 g of flour work for everyone; recipes that specify 2 cups do not.
• Shipping: carriers charge the greater of actual weight and dimensional weight — (L × W × H in cm) / 5,000. A light but bulky package is billed on volume, not mass.
• Confirm the ton: metric ton (1,000 kg), US short ton (907 kg), and UK long ton (1,016 kg) differ by up to 10% — check which one your contract uses.

Common Mistakes

• Confusing pounds and ounces: 1 lb = 16 oz — do not divide by 10 or by any other "round" number.
• Using fluid ounces for weight: fluid ounces (fl oz) measure volume, not weight. Converting cups to grams requires the ingredient's density.
• Mixing up the tons: a metric tonne is not a US ton. The difference is 10% — enough to break a shipping budget or a recipe at industrial scale.
• Treating carats as ordinary weight: jewelry carats follow their own conventions (200 mg exactly), and troy ounces differ from avoirdupois ounces.
• Forgetting gravity in "weight vs mass": for engineering and aerospace work, pound-mass and pound-force are different quantities.