Pregnancy Weight Gain Calculator
See the weight gain the IOM 2009 guidelines suggest for your pre-pregnancy BMI, in total and week by week, and how your gain so far compares, for one baby or twins.
How to Use
- Choose One baby or Twins.
- Enter your height (cm, m, inches, or feet and inches such as 5'4") and your weight before pregnancy in kg, lb or stone (9 st 10 works).
- Enter how many weeks and days pregnant you are, and your current weight if you want to compare.
- Read the total gain the IOM 2009 guidelines suggest for your BMI, the range for this week and your gain so far. Switch kg or lb for the results.
- On the chart the green band is the suggested pace, the orange bar the total range at 40 weeks, and the dot is you.
Worked Example
One baby, 20 weeks. Height 165 cm, 60 kg before pregnancy: BMI = 60 ÷ 1.65² = 22.04, the IOM’s normal-weight group, so the total range is 11.5–16 kg (25–35 lb). The first trimester allows 0.5–2 kg; after 14 weeks the weekly range is 0.35–0.50 kg, so at 20 weeks 0 days the range is 0.5 + 0.35 × 6 = 2.6 kg to 2 + 0.50 × 6 = 5.0 kg. At 64.5 kg now, the gain of 4.5 kg is within it.
The common mistake: using today’s weight for the BMI. The range depends on weight before pregnancy. At 20 weeks this person weighs 64.5 kg, which gives a BMI of 23.69: still normal weight here, but for someone starting at 24.5 the same mistake would cross 25 and wrongly move them to the overweight range of 7–11.5 kg.
Twins. At 165 cm and 62 kg (BMI 22.77, normal weight) carrying twins, the IOM’s provisional total is 17–25 kg (37–54 lb) at term. There is no weekly band for twins, so the chart shows only that total at 37–42 weeks.
Show Work
Formulas and IOM 2009 Ranges
Where the Ranges Come From
The ranges come from Weight Gain During Pregnancy: Reexamining the Guidelines, published in 2009 by the Institute of Medicine and the National Research Council of the US National Academies, which replaced the IOM’s 1990 guidance. The committee adopted the World Health Organization BMI categories and set each range by weighing outcomes for mother and baby such as caesarean birth, weight kept on after the birth, preterm birth, babies small or large for their age, and obesity in childhood.
The report looked at special groups too. It found too little evidence to keep separate advice for women of short stature or for adolescents, and offered provisional ranges for twins based on the middle half of gains among women whose twins were born at 37–42 weeks averaging at least 2,500 g. In 1990 twin advice had been a single 16–20.5 kg for everyone. ACOG Committee Opinion 548 (reaffirmed 2026) describes the IOM guidelines as a basis for practice.
About This Calculator
This calculator works out your BMI before pregnancy from your height and weight in metric, US or stone units, picks the IOM 2009 range for one baby or twins, and shows the gain suggested by the week you are in, with your own gain on a chart. It shows the IOM’s figures exactly as published, in kilograms or pounds.
Your measurements stay in your browser; nothing is sent anywhere.
For general information only — not medical advice. Talk to a doctor or other qualified professional about your own health.
Related tools: Ovulation Calculator, hCG Doubling Time Calculator, and Corrected Age Calculator.
Frequently Asked Questions
How much weight should I gain in pregnancy?
The Institute of Medicine (IOM) 2009 guidelines base it on your BMI before pregnancy: 12.5–18 kg (28–40 lb) if underweight (BMI under 18.5), 11.5–16 kg (25–35 lb) at a normal weight (18.5–24.9), 7–11.5 kg (15–25 lb) if overweight (25.0–29.9) and 5–9 kg (11–20 lb) if obese (30.0 or more). At 165 cm and 60 kg the BMI is 22.0, so the range is 11.5–16 kg.
How much should I gain each week?
The IOM assumes 0.5–2 kg in the first trimester, then a weekly rate for the second and third: 0.44–0.58 kg if underweight, 0.35–0.50 kg at a normal weight, 0.23–0.33 kg if overweight and 0.17–0.27 kg if obese. At a normal weight that puts the range at 20 weeks at 0.5 + 0.35 × 6 = 2.6 kg to 2 + 0.50 × 6 = 5.0 kg.
What about twins?
The IOM gives provisional totals at term: 17–25 kg (37–54 lb) at a normal weight, 14–23 kg (31–50 lb) if overweight and 11–19 kg (25–42 lb) if obese. They come from the middle half of gains among women whose twins were born at 37–42 weeks. There was not enough data for an underweight range or for weekly rates.
Why does the weekly band not reach the total range at 40 weeks?
Because the IOM table gives the totals and the weekly rates separately, and their ends do not line up exactly. At a normal weight the weekly pace adds up to 9.6–15.0 kg by 40 weeks, while the total range is 11.5–16 kg. The chart shows both: the band for pace and the bar for the target.
Is the BMI category a diagnosis?
No. The IOM uses the World Health Organization BMI categories only to choose a weight-gain range; it says nothing about your health on its own. Weight gain also varies from week to week, so a single reading above or below the band is something to talk through with your midwife or doctor rather than a cause for alarm.
How do I use the Pregnancy Weight Gain Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Checking progress
At 165 cm and 60 kg, a gain of 4.5 kg at 20 weeks sits inside the 2.6–5.0 kg range for that week.
Pounds and feet
At 5′5″ and 165 lb (BMI 27.5, overweight), the total range is 15–25 lb and the range at 28 weeks is 8.2–14.6 lb.
Stone
At 5′4″ and 9 st 10 lb (BMI 23.3), 16 weeks pregnant, the range so far is 1.2–3.0 kg.
Twins
At 165 cm and 62 kg (BMI 22.8) carrying twins, the provisional total is 17–25 kg.
Higher BMI
At 170 cm and 95 kg (BMI 32.9), the total range is 5–9 kg and 3.2–6.3 kg by 30 weeks.
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