Pregnancy Weight Gain Calculator
Compare your weight gain with the Institute of Medicine ranges for your pre-pregnancy BMI, week by week, for a singleton or twin pregnancy.
Recommended total range
The IOM ranges are the widely used US guideline and are what most maternity services in the US work from. Other countries use different guidance and your own midwife or obstetrician may set a different target — if so, switch to “Use my own range” and enter theirs.
Recommended gain by week 28
6.6 kg – 9.8 kg
Rate of gain after the first trimester
For your group the guideline works out at 0.4 kg – 0.5 kg per week from week 13 to week 40, on top of a first-trimester gain of 0.5 kg – 2.0 kg in total.
The weekly rate here is derived from the published total, so it lands exactly on the recommended total at week 40. The IOM’s own published mean rates (0.51, 0.42, 0.28 and 0.22 kg per week for the underweight, healthy, overweight and obese groups) are rounded and sit very close to these numbers.
Cumulative range week by week
| Week | Lowest of range | Highest of range |
|---|---|---|
| 4 | 0.2 kg | 0.6 kg |
| 6 | 0.2 kg | 0.9 kg |
| 8 | 0.3 kg | 1.2 kg |
| 10 | 0.4 kg | 1.5 kg |
| 12 | 0.5 kg | 1.8 kg |
| 14 | 0.9 kg | 2.5 kg |
| 16 | 1.7 kg | 3.6 kg |
| 18 | 2.5 kg | 4.6 kg |
| 20 | 3.4 kg | 5.6 kg |
| 22 | 4.2 kg | 6.7 kg |
| 24 | 5.0 kg | 7.7 kg |
| 26 | 5.8 kg | 8.7 kg |
| 28 | 6.6 kg | 9.8 kg |
| 30 | 7.4 kg | 10.8 kg |
| 32 | 8.2 kg | 11.9 kg |
| 34 | 9.1 kg | 12.9 kg |
| 36 | 9.9 kg | 13.9 kg |
| 38 | 10.7 kg | 15.0 kg |
| 40 | 11.5 kg | 16.0 kg |
Estimates only, and not medical advice. These are population guidelines for uncomplicated pregnancies; they are not adjusted for your individual health, and weight gain in pregnancy is normally uneven week to week rather than a smooth line. Sitting outside the range is common and is not automatically a problem. Your midwife or obstetrician knows your history and should be the one to advise you — bring any concern about weight, and any sudden change, to them. Everything here is calculated in your browser and nothing is uploaded.
What is the Pregnancy Weight Gain Calculator?
The Institute of Medicine recommends a total gain of 12.5–18 kg for an underweight pre-pregnancy BMI, 11.5–16 kg for healthy weight, 7–11.5 kg for overweight and 5–9 kg for obese, with 0.5–2 kg of that in the first trimester.
- Institute of Medicine 2009 ranges for all four pre-pregnancy BMI groups
- Singleton and twin pregnancies, with the provisional twin ranges
- Cumulative recommended range for every week, not just the term total
- Below / within / above status against your current weight
- Weekly rate of gain shown next to the IOM's published mean rates
- Fully editable range for anyone working to different national guidance
How to use the Pregnancy Weight Gain Calculator
- 1
Choose metric or imperial units and whether this is a singleton or twin pregnancy.
- 2
Enter your height and your pre-pregnancy weight so the calculator can work out your pre-pregnancy BMI.
- 3
Enter your current week of pregnancy, and your current weight if you want a comparison.
- 4
Read the recommended range for that week and whether your gain is below, within or above it.
- 5
Switch the range source to 'Use my own range' if your midwife or obstetrician has given you different numbers.
About the Pregnancy Weight Gain Calculator
The ByteTools Pregnancy Weight Gain Calculator plots the recommended cumulative gain against your current week of pregnancy. It starts from your pre-pregnancy BMI, picks the matching Institute of Medicine range, and shows the band you would be inside at each week — not just the total at the end.
Both singleton and twin pregnancies are covered, using the IOM's provisional twin ranges where they exist. The first-trimester assumption of 0.5–2 kg is applied before the weekly rate takes over, and the resulting rate is shown alongside the IOM's own published figures so you can see how close they are. If your midwife or obstetrician has given you a different target, switch the range source to custom and enter theirs.
This is a population guideline, not medical advice and not a judgement of your pregnancy. Everything is calculated in your browser; nothing you type is uploaded. Anything that worries you belongs with the clinician who knows your history.
Frequently asked questions
How much weight should I gain during pregnancy?
It depends on your pre-pregnancy BMI. The Institute of Medicine recommends 28–40 lb (12.5–18 kg) if you started underweight, 25–35 lb (11.5–16 kg) at a healthy weight, 15–25 lb (7–11.5 kg) if overweight and 11–20 lb (5–9 kg) if obese. Your own clinician may set a different target.
How much weight gain is normal in the first trimester?
Very little — the IOM assumes 1.1 to 4.4 lb (0.5 to 2 kg) across the whole first trimester for every BMI group. Many people gain nothing at all, and some lose a little through nausea, which is usually not a problem in itself.
How much should I gain per week in the second and third trimester?
Roughly 1 to 1.3 lb a week if you started underweight, 0.8 to 1 lb at a healthy weight, 0.5 to 0.7 lb if overweight and 0.4 to 0.6 lb if obese. Real weight gain is uneven week to week, so a single weigh-in outside the band is not a signal on its own.
What are the recommended ranges for twins?
The IOM's provisional twin ranges are 37–54 lb (16.8–24.5 kg) at a healthy pre-pregnancy BMI, 31–50 lb (14.1–22.7 kg) if overweight and 25–42 lb (11.3–19.1 kg) if obese. No range was issued for an underweight BMI because there was not enough evidence — in that case use the range your clinician gives you.
What if my weight gain is outside the recommended range?
It is common and it is not automatically a problem. These are population guidelines built for uncomplicated pregnancies and they do not know your health history, your nausea, your fluid retention or your baby's growth. Bring it up at your next appointment rather than trying to correct it yourself.
Do these ranges apply outside the United States?
Not necessarily. The IOM ranges are US guidance and are widely used there, but other countries issue different advice or none at all. That is exactly why the range in this calculator is editable — enter whatever your own maternity service recommends.
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