The metric (SI) mass system
The metric system uses the kilogram as the base unit of mass and powers-of-ten prefixes for the rest: 1 t = 1,000 kg, 1 kg = 1,000 g, 1 g = 1,000 mg. The kilogram was originally defined in 1799 as the mass of one liter of water at 4°C. In 2019, BIPM redefined it in terms of the Planck constant - a fundamental physical constant - making it independent of any physical artifact for the first time. Saudi Arabia, like most of the world, uses metric for everyday weights.
The imperial / US-customary mass system
Imperial mass units (ounce, pound) derive from medieval English commerce. The avoirdupois pound used today is defined since 1959 as exactly 0.45359237 kg, and the ounce as exactly 1/16 pound. The system is still used in the US for everyday weights (groceries, body weight), and in Saudi Arabia primarily for imported products labeled in pounds and ounces (US foods, fitness equipment, baby items).
Why 1 pound is exactly 0.45359237 kg
Until 1959, the British pound, US pound, and Canadian pound were each defined slightly differently - close but not identical. The International Yard and Pound Agreement of 1959 unified them as exactly 0.45359237 kg. Every English-speaking country signed on. This is why every metric ↔ imperial weight factor in this calculator is exact and not an approximation. The figure has nine significant digits and is the legal definition used by NIST, BIPM, and every metrology authority worldwide.
Common Arabic terms for mass units
Arabic speakers in Saudi Arabia use a mix of metric and imperial terms. كجم (kg), جم (g), ملغ (mg), and طن (t) are universal in groceries, pharmacies, and shipping. For imperial: «رطل» (raṭl) and «باوند» (band) both mean pound - رطل is the Classical Arabic word, باوند is the anglicized form used informally and often on imported labels. «أونصة» (oz) is standard for ounce. In the gym, body weight is almost always given in kilograms; supplement labels often use grams and milligrams.
Common use cases
Grocery shopping: imported US products often label pack sizes in ounces and pounds (peanut butter, cereal, meat). Gym / fitness: dumbbells and plates labeled in kg locally but in lb on US-imported equipment. Medication: doses in milligrams (mg) - converting between mg, g, and lb is critical for parents administering child medication. Shipping: international parcels charge by kg or lb depending on the carrier. Cooking: many international recipes call for ounces and pounds where Saudi cooks usually have grams. This converter handles every common case.
Mass vs weight - a quick clarification
Strictly speaking, 'weight' (the force gravity exerts on an object) and 'mass' (the amount of matter in an object) are different physical quantities. Weight changes between Earth, the Moon, and space - mass does not. In everyday language, including in this calculator, 'weight' usually means mass. The kg, g, lb, and oz on grocery scales and bathroom scales are all units of mass, even though we call the result a 'weight'.
Frequently asked questions
Since the 1959 International Yard and Pound Agreement, the avoirdupois pound is defined by international treaty as exactly 0.45359237 kilograms. It's the legal definition used by NIST, BIPM, and every metrology authority worldwide - not an approximation.
Multiply kilograms by 2.2 (exact factor is 2.20462). So 70 kg ≈ 154 lb, 100 kg ≈ 220 lb. For the reverse, multiply pounds by 0.45. This calculator uses the exact factor; the head-math shortcut is about 0.2% off.
Yes, both refer to the avoirdupois pound (0.45359237 kg). 'رطل' is the Classical Arabic word; 'باوند' is the anglicized form used informally and often on imported product labels in Saudi Arabia. They are interchangeable in everyday speech.
The metric tonne (t) - used here and in most of the world - is exactly 1,000 kg. The US short ton is 2,000 lb (907.185 kg). The UK long ton is 2,240 lb (1,016.047 kg). In Saudi Arabia, 'طن' (ton) almost always means the metric tonne.
Not strictly. Weight is the force of gravity on an object; mass is the amount of matter. Your mass stays the same on the Moon, but your weight there is about 1/6 of what it is on Earth. In everyday language, including in this calculator, 'weight' usually means mass - which is what kg, g, lb, and oz all measure.
Sources
- The International System of Units (SI) - 9th edition— Bureau International des Poids et Mesures (BIPM)
- NIST Special Publication 811 - Guide for the Use of SI— National Institute of Standards and Technology (NIST)
- Avoirdupois system - pound and ounce definitions— Reference summary
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