BMI Calculator — Pre-Pregnancy BMI & Healthy Range
Find your BMI and healthy weight range — the number that sets your pregnancy weight-gain targets.
The tool
Want this in your pocket? Get the iOS app:
How this works
Enter your weight and height in whichever units you prefer — kilograms or pounds, centimeters or feet and inches. The calculator converts everything with exact factors (1 lb = 0.45359237 kg, 1 in = 2.54 cm) and updates your BMI instantly as you move the sliders.
Your result comes with the WHO category — underweight (below 18.5), normal (18.5–24.9), overweight (25–29.9), or obese (30 and above) — plus the healthy weight range for your exact height, shown in whichever weight unit you're using, and practical recommendations for your category.
If you're planning a pregnancy or already pregnant, this number matters more than usual: the Institute of Medicine's 2009 guidelines use your pre-pregnancy BMI category to set your recommended weight-gain range for the whole pregnancy. Once you know your BMI, our Pregnancy Weight Gain Calculator shows the band that applies to you.
Keep in mind that BMI is a screening tool, not a diagnosis. It estimates body fat from height and weight alone and doesn't account for muscle mass, age, or gender — so treat it as a starting point for a conversation with your healthcare provider.
The math behind it
The formula is the standard one used by the World Health Organization: BMI = weight (kg) ÷ height (m)². For example, at 65 kg and 165 cm: 65 ÷ 1.65² = 65 ÷ 2.7225 = 23.9.
If you enter pounds or feet and inches, we convert with exact factors before computing: kilograms = pounds × 0.45359237, and centimeters = (feet × 12 + inches) × 2.54. For showing kilogram results in pounds we use 1 kg = 2.20462 lbs.
Category thresholds follow the WHO classification: below 18.5 is underweight, 18.5 to below 25 is normal, 25 to below 30 is overweight, and 30 or above is obese.
The healthy weight range simply inverts the formula: minimum = 18.5 × height² and maximum = 24.9 × height², with height in meters. At 165 cm that's 18.5 × 2.7225 = 50.4 kg to 24.9 × 2.7225 = 67.8 kg.
Nothing is rounded until display — the BMI you see is the raw result shown to one decimal place, exactly as computed in the ProbaBaby iOS app this tool is ported from.
In the ProbaBaby app
iPhone · iPad · Apple WatchFrom one BMI number to a weight trend your doctor can read
Your pre-pregnancy BMI sets your weight-gain range. The app keeps going from there: it tracks your weight against that range all the way through.
The app also
- A Detailed mode adds your waist-to-height ratio and a body-fat estimate to the BMI.
- Weight tracking in the pregnancy module, charted against the IOM 2009 range for your pre-pregnancy BMI, with twin ranges too.
- Your weight trend is on the one-page Doctor Visit Summary PDF you take to appointments.
- Iron, caffeine and water calculators beside it, among 32 free tools in 7 categories.
Free to download and use: pregnancy, baby and mom tracking, 32 tools, PDF reports, widgets, Apple Watch and family sharing cost nothing. Only AI baby predictions use credits.
On the App Store
Sheet 1 of 1
Sample Doctor Visit Summary for a twin pregnancy at week 32: delivery timeline adjusted for twins, weight, kicks, contractions, symptoms, screenings and your questions on one sheet. Fictional patient.
Frequently asked questions
A BMI between 18.5 and 24.9 is classified as normal by the WHO. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.
BMI equals your weight in kilograms divided by your height in meters squared. If you enter pounds or feet and inches, the calculator converts them first using exact factors: 1 lb = 0.45359237 kg and 1 in = 2.54 cm.
The Institute of Medicine's 2009 guidelines set recommended pregnancy weight-gain ranges based on your pre-pregnancy BMI category. Knowing your BMI is step one; our Pregnancy Weight Gain Calculator then shows the range that applies to you.
The healthy range runs from 18.5 × height² to 24.9 × height², with height in meters. For 165 cm that works out to about 50.4–67.8 kg. The calculator shows the range for your exact height, in kg or lbs.
BMI is a screening tool that uses height and weight to estimate body fat. It doesn't directly measure body fat or account for muscle mass, age, or gender, so a muscular person can score as overweight. Discuss your result with a healthcare provider.