Pregnancy Weight Gain Calculator
See the weight gain recommended for your pregnancy from your height and pre-pregnancy weight: the total, the range for this week and a week-by-week table, in pounds and kilograms, for one baby or twins.
Why pregnancy weight gain has a recommended range
Gaining weight is a normal, healthy part of pregnancy, and how much is right depends mostly on your weight beforehand. The Institute of Medicine (IOM, now the National Academy of Medicine) set a range for each pre-pregnancy BMI category in its 2009 report Weight Gain During Pregnancy: Reexamining the Guidelines. The ranges rest on observational data showing better outcomes for women who gained within earlier guidelines, and aim to balance risks such as a baby growing too large or too small, preterm birth and weight kept on after the birth.
ACOG’s Committee Opinion 548 uses these guidelines as the basis for practice, advising providers to check your BMI at the first prenatal visit and discuss weight gain, diet and exercise through pregnancy. The CDC notes that the first trimester needs no extra calories, then about 340 extra a day in the second trimester and 450 in the third.
How to use this calculator
Enter your height, your weight before pregnancy (your best estimate is fine), how many completed weeks pregnant you are (1–42; enter 20 for 20 weeks 3 days) and whether you’re expecting one baby or twins. The units switch converts what you’ve typed between metric and imperial.
How it’s calculated
BMI. Weight in kilograms divided by height in metres squared (703 × pounds ÷ inches²), rounded to one decimal. The category comes from that rounded figure, using the bands in our BMI calculator, which the IOM also uses: underweight below 18.5, healthy (normal) weight 18.5–24.9, overweight 25.0–29.9 and obesity 30.0 or more.
Total and weekly rate. Straight from the IOM table below, in the report’s own pounds and kilograms, which aren’t exact conversions (25–35 lb is listed as 11.5–16 kg).
Gain so far. The IOM gives totals and weekly rates, not a range for every week. Its rates assume 1.1–4.4 lb (0.5–2 kg) over the first trimester, and its own chart rises gently to week 13, then climbs in a straight line to the total at 40 weeks. The calculator does the same, and shows the full total from 40 weeks. Because the published totals and rates don’t line up exactly, the line runs slightly off the printed rate — about 0.89–1.13 lb a week for a healthy BMI, against 0.8–1 lb — but always ends on the IOM total.
Twins. The IOM’s twin ranges are provisional totals, based on twins born at 37–42 weeks, with no weekly rates. It found insufficient information to set a range when pre-pregnancy BMI is underweight.
Worked example
Maria is 5 ft 5 in, weighed 140 lb before pregnancy and is 20 weeks pregnant with one baby. Her BMI is 703 × 140 ÷ 65² = 23.3 (healthy weight), so her recommended total is 25–35 lb (11.5–16 kg), gained at about 1 lb a week (range 0.8–1 lb), or 0.42 kg (0.35–0.50 kg), in the second and third trimesters. At 20 weeks the calculator shows 7.3–12.3 lb (3.4–5.6 kg) so far, and at 28 weeks 14.4–21.4 lb (6.6–9.8 kg). With twins, her provisional total would be 37–54 lb (17–25 kg).
IOM weight gain guidelines by pre-pregnancy BMI
The figures the calculator uses. Weekly rates show the average, with the range in brackets.
| Pre-pregnancy BMI | Category | Total gain, one baby | Weekly rate, 2nd & 3rd trimesters | Total gain, twins |
|---|---|---|---|---|
| Below 18.5 | Underweight | 28–40 lb (12.5–18 kg) | 1 lb (1–1.3) · 0.51 kg (0.44–0.58) | Insufficient information |
| 18.5–24.9 | Healthy weight | 25–35 lb (11.5–16 kg) | 1 lb (0.8–1) · 0.42 kg (0.35–0.50) | 37–54 lb (17–25 kg) |
| 25.0–29.9 | Overweight | 15–25 lb (7–11.5 kg) | 0.6 lb (0.5–0.7) · 0.28 kg (0.23–0.33) | 31–50 lb (14–23 kg) |
| 30.0 or more | Obesity | 11–20 lb (5–9 kg) | 0.5 lb (0.4–0.6) · 0.22 kg (0.17–0.27) | 25–42 lb (11–19 kg) |
Limitations & when to talk to your provider
These are population guidelines, not a personal target, and the calculator can’t see how your pregnancy is going. BMI doesn’t distinguish muscle from fat, and the IOM found too little evidence to adjust the ranges for adolescents or women of short stature. ACOG notes that the guidelines don’t differentiate between classes of obesity, and that if someone who is overweight or obese gains less than recommended while the baby grows well, there’s no evidence that encouraging more gain helps. There isn’t enough data for triplets or more.
A single weigh-in above or below the line isn’t a cause for worry. If you’re consistently well outside the range, gain weight suddenly or can’t keep food down, contact your midwife, obstetrician or doctor, and check with them before starting any weight-loss diet. This is general information, not medical advice or a diagnosis.
Frequently asked questions
How much weight should I gain during pregnancy?
For one baby, the Institute of Medicine recommends 28–40 lb if your pre-pregnancy BMI was under 18.5, 25–35 lb for 18.5–24.9, 15–25 lb for 25.0–29.9 and 11–20 lb for 30.0 or more. Your prenatal care provider may tailor this to you.
How much weight should I gain in the first trimester?
Not much. The IOM’s weekly rates assume about 1.1–4.4 lb (0.5–2 kg) over the whole first trimester, whatever your BMI, and the CDC notes that no extra calories are needed then. Most of the gain comes later.
How much weight should I gain each week?
In the second and third trimesters the IOM’s average is about 1 lb a week if you started underweight (range 1–1.3 lb) or at a healthy weight (range 0.8–1 lb), 0.6 lb (range 0.5–0.7 lb) if overweight and 0.5 lb (range 0.4–0.6 lb) with obesity.
How much weight should I gain with twins?
The IOM’s provisional twin ranges are 37–54 lb (17–25 kg) for a healthy pre-pregnancy BMI, 31–50 lb (14–23 kg) if overweight and 25–42 lb (11–19 kg) with obesity. It found insufficient information to set a range for underweight, so ask your provider.
Why does my weight before pregnancy matter?
The amount of gain linked with the best outcomes for parent and baby differs between BMI groups, so people who start with a lower BMI are advised to gain more and those with a higher BMI less.
What if I’m gaining more or less than the recommended range?
Weight often rises in fits and starts, so try not to worry about a single weigh-in. If you’re consistently above or below the range, talk to your prenatal care provider, who can look at your health, eating, activity and how your baby is growing.
Sources & references
- Institute of Medicine — Weight Gain During Pregnancy: Reexamining the Guidelines (2009) — Table S-1 ranges, weekly rates and first-trimester assumption; twin guidance.
- Institute of Medicine — Report brief (May 2009) — the summary table and Figure 1, the week-by-week chart whose shape the calculator follows.
- ACOG — Committee Opinion No. 548, Weight Gain During Pregnancy — adopts the IOM guidelines; individualized care.
- CDC — Weight Gain During Pregnancy — recommended ranges for single and twin pregnancies, and extra calories by trimester.
Last updated: September 2026 ·How we calculate
BriskToolbox provides estimates for general information only and is not medical advice. Talk to a healthcare professional about your individual health.