Free Tools
Pregnancy Weight Gain Calculator
See your IOM-recommended weight gain range by pre-pregnancy BMI, week by week — for singleton and twin pregnancies.
Want this in your pocket? Get the iOS app:
How this works
Set your pre-pregnancy weight (kg or lbs), your height, your current pregnancy week, and whether you're expecting one baby or twins. The calculator first works out your pre-pregnancy BMI — that single number decides which recommendation band applies to you.
From your BMI, the calculator looks up the Institute of Medicine (IOM) 2009 guideline range: a total recommended gain for the whole pregnancy and a weekly gain rate for the second and third trimesters. Toggle to twins and the same BMI thresholds map to the higher twin-pregnancy ranges instead.
Your current week turns those ranges into a concrete target: how many kilograms (or pounds) you'd typically have gained by now. Weight gain in the first trimester is minimal — the model assumes about 1.5 kg across the first 12 weeks — and the steady weekly gain only starts after week 12.
Everything updates live as you move the sliders, and your inputs are stored in the page URL so you can share or bookmark your result. All math runs in kilograms; pounds are converted exactly at the boundaries.
The math behind it
Step 1 is your pre-pregnancy BMI: BMI = weight (kg) ÷ height (m)². For example, 65 kg at 165 cm gives 65 ÷ 1.65² = 23.9. If you enter pounds, they're converted with the exact factor 1 lb = 0.45359237 kg before any math happens.
Step 2 looks up the IOM 2009 band. For a single baby: BMI under 18.5 → 12.5–18.0 kg total, 0.44–0.58 kg/week; BMI 18.5 to under 25 → 11.5–16.0 kg total, 0.35–0.50 kg/week; BMI 25 to under 30 → 7.0–11.5 kg total, 0.23–0.33 kg/week; BMI 30 and above → 5.0–9.0 kg total, 0.17–0.27 kg/week.
For twins, the same BMI thresholds map to higher ranges: under 18.5 → 22.7–28.1 kg total, 0.57–0.70 kg/week; 18.5 to under 25 → 16.8–24.5 kg total, 0.42–0.61 kg/week; 25 to under 30 → 14.1–22.7 kg total, 0.35–0.57 kg/week; 30 and above → 11.3–19.1 kg total, 0.28–0.48 kg/week. One honest caveat: the official IOM table gives no twin recommendation for the underweight class — the 22.7–28.1 kg range shown here is a provisional value carried over from the ProbaBaby app.
Step 3 is the current-week target. The model assumes 1.5 kg gained across the first trimester. Through week 12: target = 1.5 × (week ÷ 12), with a ±20% window (× 0.8 for the low end, × 1.2 for the high end). After week 12: target = 1.5 + (week − 12) × weekly rate, using the band's minimum and maximum rates for the two ends of the range.
The engine never rounds internally — only the displayed values are formatted. Results shown in pounds are converted from kilograms at 1 kg = 2.20462 lbs for display only.
Frequently asked questions
It depends on your pre-pregnancy BMI. The IOM 2009 guidelines recommend 12.5–18.0 kg total if your BMI is under 18.5, 11.5–16.0 kg for BMI 18.5 to under 25, 7.0–11.5 kg for BMI 25 to under 30, and 5.0–9.0 kg for BMI 30 and above. These are singleton ranges — twin pregnancies have higher targets.
Twin ranges are substantially higher: 16.8–24.5 kg total for a BMI of 18.5 to under 25, 14.1–22.7 kg for BMI 25 to under 30, and 11.3–19.1 kg for BMI 30 and above. The official IOM table gives no twin recommendation for the underweight class (BMI under 18.5); this calculator shows a provisional 22.7–28.1 kg range there.
Very little. First-trimester weight gain is typically minimal — around 1–2 kg. This calculator models it as roughly 1.5 kg across the first 12 weeks, with the steady weekly gain rate only starting after week 12.
The four standard bands from the IOM guidelines, based on your pre-pregnancy BMI: under 18.5 (underweight), 18.5 to under 25 (normal), 25 to under 30 (overweight), and 30 or above (obese). BMI is calculated as weight in kilograms divided by height in meters squared.
The total and weekly ranges come from the Institute of Medicine's 2009 report 'Weight Gain During Pregnancy: Reexamining the Guidelines', which ACOG endorses in Committee Opinion No. 548. They are population-level guidelines, not individual prescriptions — consult your healthcare provider for personalized guidance.
Yes — toggle the weight unit to lbs and both the input slider and all results switch to pounds. Under the hood all math runs in kilograms: pounds are converted in with the exact factor 1 lb = 0.45359237 kg, and results are converted out at 1 kg = 2.20462 lbs for display.