Pregnancy BMI Calculator
Calculate pre-pregnancy BMI and get the IOM-recommended gestational weight gain range for underweight, normal, overweight, and obese starting BMI categories.
Gaining the right amount of weight during pregnancy is important for both mother and baby. Too little weight gain is linked to low birth weight and preterm birth. Too much weight gain increases the risk of gestational diabetes, cesarean delivery, and long-term weight retention.
How BMI Determines Weight Gain Guidelines
The Institute of Medicine (IOM) and most global health authorities use pre-pregnancy BMI to set personalized weight gain targets. BMI is calculated as:
BMI = weight (kg) ÷ height (m)²
Or in imperial: BMI = (weight in lbs × 703) ÷ height in inches²
IOM Recommended Gestational Weight Gain (Singleton Pregnancy)
| Pre-Pregnancy BMI | Category | Recommended Total Gain |
|---|---|---|
| Below 18.5 | Underweight | 12.5–18 kg (28–40 lbs) |
| 18.5–24.9 | Normal weight | 11.5–16 kg (25–35 lbs) |
| 25.0–29.9 | Overweight | 7–11.5 kg (15–25 lbs) |
| 30.0 and above | Obese | 5–9 kg (11–20 lbs) |
For twin pregnancies
| Pre-Pregnancy BMI | Recommended Total Gain |
|---|---|
| Underweight | No IOM guideline exists |
| Normal weight | 17–25 kg (37–55 lb) |
| Overweight | 14–23 kg (31–51 lb) |
| Obese | 11–19 kg (24–42 lb) |
The blank row is not an oversight. The IOM had too few twin pregnancies with an underweight starting BMI to derive a range, and said so rather than guessing. If that is your situation, the singleton figure is definitely too low and the number has to come from your provider.
Pound figures here are converted from the kilogram ranges, which is why they differ by a pound from some charts that round the other way.
Rate of gain
Gain is not meant to be linear. Most women gain only 0.5–2 kg (1–4 lb) across the entire first trimester, and almost all of it comes in the second and third. For a normal-weight woman the recommended rate after week 13 works out at roughly 0.4–0.5 kg (about 1 lb) per week; for overweight and obese categories it is meaningfully less. The calculator derives that weekly figure from your own total rather than quoting a fixed one, so the two numbers agree with each other.
Use pre-pregnancy weight
This matters enough to repeat. The IOM categories are defined on pre-pregnancy body composition, so entering your current weight partway through a pregnancy pushes you into a higher BMI band and produces a lower target than you actually need. If you do not know your pre-pregnancy weight, your first prenatal appointment record usually has it.
Individual health factors modify these guidelines, and your obstetrician or midwife may set something different for good reason.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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