Weight Calculators

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In everyday usage, 'weight' refers to how heavy an object is — typically measured in kilograms (kg), pounds (lb), or grams (g). In physics, weight is a force (W = mg, where m is mass and g is gravitational acceleration ≈ 9.8 m/s²), measured in Newtons. For most scientific and medical purposes, mass (kg) is the more precise term. Body weight is a central clinical and physiological measurement used to calculate drug doses, BMI, caloric needs, fluid requirements, and ideal body weight. Unit conversions between kg, lb, g, and oz are among the most common calculations in everyday life.

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Weight vs. Mass

Mass (kg): the amount of matter in an object — constant regardless of location. Weight (N): the gravitational force on an object — W = m × g. On Earth's surface, g ≈ 9.81 m/s². A 70 kg person weighs 70 × 9.81 = 686.7 N on Earth. On the Moon (g = 1.62 m/s²), the same person weighs 70 × 1.62 = 113 N — but mass is still 70 kg.

Common Unit Conversions

  • 1 kg = 2.2046 lb
  • 1 lb = 0.4536 kg
  • 1 kg = 1000 g
  • 1 oz = 28.35 g
  • 1 lb = 16 oz = 453.6 g
  • 1 stone (UK) = 14 lb = 6.35 kg

Ideal Body Weight Formulas

Devine formula: Men: IBW = 50 + 2.3 × (inches over 5 ft). Women: IBW = 45.5 + 2.3 × (inches over 5 ft). Units: kg. Used for drug dosing, ventilator settings, and nutritional assessment.

BMI and Weight

BMI = weight (kg) / height² (m). Normal: 18.5–24.9; overweight: 25–29.9; obese: ≥ 30. BMI uses mass, not weight (Newtons). On food labels, 'calories per serving' depend on serving mass (grams).

Glossary

Mass vs. Weight
Mass (kg) = amount of matter, constant everywhere; weight (N) = gravitational force = m × g; everyday scales report mass; in science, use mass; in medicine, weight means mass in kg.
Ideal Body Weight (IBW)
Estimated optimal weight for height: men = 50 + 2.3×(inches above 5 ft); women = 45.5 + 2.3×(inches above 5 ft), in kg; used for drug dosing adjustments in pharmacology.
Adjusted Body Weight (ABW)
ABW = IBW + 0.4×(actual−IBW); used for weight-based dosing of drugs in obese patients where actual weight overestimates distribution volume; intermediate between IBW and actual weight.

Frequently Asked Questions

Mass (measured in kg) is the amount of matter — constant everywhere. Weight (measured in Newtons or pounds-force) is the gravitational force on an object: W = mg. On Earth (g = 9.81 m/s²), a 70 kg person 'weighs' 686 N. In everyday life, scales report mass in kg or lb, not true weight in Newtons. On the Moon, a person's mass is unchanged (70 kg) but their weight is only ~113 N (1/6 of Earth). In science: use mass (kg); in medicine and everyday language, 'weight' means mass as measured on a scale.

1 kg = 2.2046 lb; 1 lb = 0.4536 kg. To convert kg to lb: multiply by 2.2046. Example: 80 kg × 2.2046 = 176.4 lb. To convert lb to kg: divide by 2.2046 (or multiply by 0.4536). Example: 150 lb × 0.4536 = 68.0 kg. Quick approximation: kg × 2.2 ≈ lb; lb / 2.2 ≈ kg. This is important in medicine where drug doses in mg/kg require the weight in kg — always convert lb to kg before dose calculation.

Ideal body weight estimates optimal weight for a given height. Devine formula: men = 50 + 2.3 × (inches above 5 feet); women = 45.5 + 2.3 × (inches above 5 feet), in kg. Example: 5'10" (10 inches above 5 ft) male: IBW = 50 + 2.3×10 = 73 kg. IBW is used in clinical pharmacology for dosing drugs whose distribution correlates better with lean body mass than actual body weight (aminoglycosides, certain chemotherapy agents). Adjusted body weight (ABW = IBW + 0.4×(actual − IBW)) is used when actual weight significantly exceeds IBW.

Most drug doses are calculated per kg of body weight: total dose = weight (kg) × dose rate (mg/kg). Always use the patient's actual weight unless dosing guidelines specify IBW or ABW. Convert lb to kg: divide by 2.2. Example: prescribe amoxicillin 25 mg/kg to a 44 lb child: weight = 44/2.2 = 20 kg; dose = 25 × 20 = 500 mg. For obese patients, using actual weight for certain drugs (aminoglycosides, vancomycin, some chemotherapy) can cause overdosing and toxicity — always check dosing guidelines for weight adjustment recommendations.