BMI Calculator for Pregnancy
Calculate your BMI when pregnant using your pre-pregnancy weight (in kg, stones or pounds) and height — then see the IOM / NICE recommended total weight-gain range for a singleton or twin pregnancy. NHS-standard, free, UK-friendly.
Use your pre-pregnancy weight
BMI is calculated from your weight just before conception, not your current pregnant weight. The result gives you an IOM/NICE-recommended weight-gain range for the rest of the pregnancy.
Your weight in the days or weeks just before conception.
Expecting twins?
Different IOM weight-gain ranges apply for twin pregnancies.
IOM 2009 weight-gain ranges
The Institute of Medicine's 2009 ranges are the international standard adopted by NHS midwives and obstetricians.
| Pre-pregnancy BMI | Category | Singleton (kg) | Twins (kg) |
|---|---|---|---|
| Below 18.5 | Underweight | 12.5 – 18 | Discuss with consultant |
| 18.5 – 24.9 | Healthy weight | 11.5 – 16 | 17 – 25 |
| 25 – 29.9 | Overweight | 7 – 11.5 | 14 – 23 |
| 30 or above | Obese | 5 – 9 | 11 – 19 |
Source: Institute of Medicine, Weight Gain During Pregnancy (2009). NICE Public Health Guideline PH27.
How to calculate BMI when pregnant
The BMI formula doesn’t change in pregnancy — but the weight you plug into it does. Instead of your current pregnant weight, use your pre-pregnancyweight (the weight you were just before you conceived, or your booking weight at 8–10 weeks if you don’t remember). Then apply the standard formula:
Pregnancy BMI = pre-pregnancy weight (kg) ÷ height (m)²
Worked example. If you weighed 68 kg before pregnancy and you’re 1.65 m tall: 68 ÷ (1.65 × 1.65) = 68 ÷ 2.7225 = BMI 25.0. That puts you just into the overweight band, so your IOM-recommended total pregnancy gain is 7–11.5 kg.
In UK units: convert your stones-and-pounds weight (1 stone = 6.35 kg, 1 lb = 0.4536 kg) and feet-and-inches height (1 ft = 0.3048 m, 1 in = 0.0254 m). Or just enter them directly above — the calculator does the maths for you.
How the pregnancy BMI calculator works
Enter your pre-pregnancy weight
Use your weight just before conception, not your current pregnant weight. Kilograms or pounds — the calculator handles the conversion.
Enter your height
Type in cm or feet/inches. Height does not change in pregnancy, so use your normal measurement.
See your IOM / NICE weight-gain range
You'll see the recommended total gain, weekly gain in the 2nd/3rd trimester, and a separate range for twin pregnancies if you toggle the switch.
When to speak to your midwife
- •Booking BMI of 30 or above — triggers an NHS consultant-led referral
- •Gaining markedly more or less than the IOM range for your category
- •Loss of weight in any trimester after the first
- •Rapid gain (more than 1 kg per week) — may indicate gestational diabetes or pre-eclampsia
- •Hyperemesis or severe morning sickness limiting food intake
This calculator is a guidance tool, not a clinical assessment. Your midwife or obstetrician should always have the final say.
Pregnancy BMI — FAQs
Quick answers to the most common questions
BMI in pregnancy is calculated from your weight just before conception, because pregnancy weight gain is healthy and expected. Using your current pregnant weight would falsely classify almost every woman as overweight or obese. Your pre-pregnancy BMI sets the target weight-gain range for the whole pregnancy.
The Institute of Medicine (IOM 2009) and NICE recommend total pregnancy weight gain based on pre-pregnancy BMI. Underweight (BMI <18.5): 12.5–18 kg. Healthy (18.5–24.9): 11.5–16 kg. Overweight (25–29.9): 7–11.5 kg. Obese (30+): 5–9 kg. Twins add roughly 5–10 kg to these ranges.
The NHS uses your booking BMI (calculated at your first midwife appointment, around 8–10 weeks) to plan your maternity care. A booking BMI of 30 or above triggers a referral to a consultant-led team. The NHS broadly follows the same IOM weight-gain ranges, though it focuses more on monitoring rather than strict weekly targets.
A pre-pregnancy BMI below 18.5 carries a slightly increased risk of low birth weight and preterm birth. The IOM recommends a higher total gain of 12.5–18 kg to compensate. Your midwife may suggest extra calorie-dense, nutrient-rich foods and check for any underlying nutritional issues.
A starting BMI of 30 or above is common (around 1 in 5 UK pregnancies) and the IOM recommends a smaller total gain of 5–9 kg. Your NHS care team will monitor blood pressure and glucose more often, offer a glucose tolerance test, and may discuss vitamin D and folic acid supplementation. Do not actively diet during pregnancy — instead, aim for a small daily surplus through healthy meals.
Most weight gain happens after the first trimester. Per the IOM, healthy weekly gain in the second and third trimesters is about 0.35–0.5 kg if you started at a healthy BMI; 0.44–0.58 kg if underweight; 0.23–0.33 kg if overweight; and 0.17–0.27 kg if obese. The first trimester usually adds only 0.5–2 kg total.
Yes — toggle the “Expecting twins?” switch and the calculator will return IOM twin-pregnancy ranges. For a healthy starting BMI, twins typically require 17–25 kg total gain; for overweight, 14–23 kg; for obese, 11–19 kg. The IOM does not publish a twin range for underweight starting BMI — discuss this with your obstetrician.
Active weight loss is not recommended during pregnancy, even for women with a high starting BMI, because the fetus needs a steady supply of nutrients. The aim is to gain less, not lose. Focus on balanced meals, daily activity (walking, swimming, prenatal yoga), and avoiding empty-calorie snacks. Your midwife may refer you to a dietitian for a personalised plan.
Have another question? Browse our full article library or try a free calculator.
Related tools and reading
Standard BMI Calculator
For non-pregnancy use — adults aged 20 and over.
OpenCalorie Calculator
Find your daily calorie needs (an extra 200–300 kcal/day is typical in 2nd/3rd trimester).
OpenNHS BMI Calculator
Standard NHS-style BMI check for non-pregnancy use.
OpenPregnancy & weight articles
Browse evidence-based articles on pregnancy weight, nutrition, and post-natal recovery.
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