Pre-Pregnancy BMI Category Calculator
Pre-pregnancy body mass index (BMI) is the starting point for individualised pregnancy weight-gain guidance. This calculator computes your BMI from height and weight, assigns the World Health Organization weight category, and returns the Institute of Medicine recommended total weight-gain range for a singleton pregnancy in that category. Enter US units (pounds and inches) or metric (kilograms and centimetres). The results are general references; your prenatal care provider should tailor weight-gain goals to your full health history.
BMI formula and categories
Metric: BMI = weight(kg) / height(m)^2
US: BMI = 703 * weight(lb) / height(in)^2
Underweight: BMI < 18.5
Normal: 18.5 to 24.9
Overweight: 25.0 to 29.9
Obesity: BMI >= 30.0
The recommended singleton total gain ranges are 28 to 40 lb (underweight), 25 to 35 lb (normal), 15 to 25 lb (overweight), and 11 to 20 lb (obesity).
Pregnancy weight context
- BMI category is set from pre-pregnancy weight, not current weight.
- The IOM gain ranges apply to singleton pregnancies; twin ranges are higher.
- Gaining within the recommended range supports healthier outcomes.
- Weight gain pattern by trimester also matters, not just the total.
- Individual targets should be set with a prenatal care provider.
Pre-pregnancy BMI: frequently asked questions
Why does pre-pregnancy BMI matter?
Pre-pregnancy body mass index (BMI) is used to set a recommended total weight-gain range during pregnancy. The Institute of Medicine (now the National Academy of Medicine) publishes gain ranges by BMI category to support healthy outcomes for the pregnant person and baby.
How is BMI calculated?
BMI equals weight in kilograms divided by height in metres squared. In US units, BMI equals weight in pounds times 703, divided by height in inches squared. This calculator accepts metric or US units and uses the standard WHO category cut-offs.
What are the WHO BMI categories?
The World Health Organization defines underweight as a BMI below 18.5, normal weight as 18.5 to 24.9, overweight as 25.0 to 29.9, and obesity as 30.0 or above. These are the cut-offs used to set pregnancy weight-gain guidance.
What are the IOM total weight-gain ranges?
For a singleton pregnancy, the Institute of Medicine recommends 28 to 40 lb for underweight, 25 to 35 lb for normal weight, 15 to 25 lb for overweight, and 11 to 20 lb for people with obesity. Ranges differ for twin pregnancies.
Does this replace medical advice?
No. This calculator is for general information. Recommended weight gain should be individualised with your prenatal care provider based on your full health history.
Official sources
- U.S. National Heart, Lung, and Blood Institute: Body mass index.
- U.S. Centers for Disease Control and Prevention: Reproductive health.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.