# BMI Calculator — Pre-Pregnancy BMI & Healthy Range _Find your BMI and healthy weight range — the number that sets your pregnancy weight-gain targets._ - Source: https://proba.baby/en/tools/bmi-calculator - Language: English - Last verified: v1.0.4 · 7 Sep 2026 - Last reviewed: 2026-07-15 ## How it 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 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. ## FAQ **What is a normal BMI?** 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. **How is BMI calculated?** 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. **Why does pre-pregnancy BMI matter during pregnancy?** 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. **What is a healthy weight for my height?** 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. **What are the limitations of BMI?** 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. ## Sources - WHO — Body Mass Index Classifications — World Health Organization: https://www.who.int/data/gho/data/themes/topics/topic-details/GHO/body-mass-index - IOM 2009 — Weight Gain During Pregnancy: Reexamining the Guidelines — Institute of Medicine, 2009: https://www.ncbi.nlm.nih.gov/books/NBK32813/ --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.