Pregnancy Weight Gain Calculator

What it is

This calculator turns your pre-pregnancy height and weight into a BMI category and shows the weight gain recommended by the 2009 Institute of Medicine (IOM) report, both for the whole pregnancy and as a reference range for the week you are in. Add your current weight and it tells you whether the gain so far is below, within or above that range. It handles one baby or twins, and for one baby it draws the week-by-week range as a chart with a table underneath. The boxes open with an example (height 160 cm, pre-pregnancy weight 55 kg, week 28, current weight 62.5 kg) so you see a result straight away. Everything is calculated in your browser and nothing you type is sent anywhere. It is a rough reference, not a diagnosis or treatment.

How to use

1. Choose one baby or twins, then enter your height and your weight before pregnancy.
2. Enter the current week as a whole number from 0 to 42.
3. Optionally enter your current weight to compare the gain so far with the range.
4. Read the total recommended gain, the range for your week, the chart or table and the math lines.
5. Follow the target your own obstetric care team gives you.

How it works

• BMI is pre-pregnancy weight in kg divided by height in meters squared, rounded to one decimal. WHO categories apply: below 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, 30 or more obese. These are not the Korean cut-offs of 23 and 25.
• Totals for one baby are 12.5 to 18 kg (underweight), 11.5 to 16 kg (normal), 7 to 11.5 kg (overweight) and 5 to 9 kg (obese). Weekly gain in the second and third trimesters averages 0.51, 0.42, 0.28 and 0.22 kg, with ranges of 0.44 to 0.58, 0.35 to 0.50, 0.23 to 0.33 and 0.17 to 0.27 kg (1 (1 to 1.3), 1 (0.8 to 1), 0.6 (0.5 to 0.7) and 0.5 (0.4 to 0.6) lb in the table).
• Twin totals are 17 to 25 kg (normal), 14 to 23 kg (overweight) and 11 to 19 kg (obese). Underweight is not given in the source, and there are no weekly rates for twins.
• The range by week uses the IOM assumption of 0.5 to 2 kg gained in the first trimester, shown as is through week 13. From week 14 on, low = 0.5 + low rate × (week − 13) and high = 2 + high rate × (week − 13). These are derived values.
• At week 40 the low end of this curve sits slightly below the total range (9.95 to 15.5 kg against 11.5 to 16 kg for normal weight). The high end is capped at the top of the total range (obese: 9 kg instead of the computed 9.29 kg). The total is the guideline, so from week 37 the current gain is also compared with the total range on its own row.
• The gain so far is current weight minus pre-pregnancy weight, compared at one decimal. Limits: height 120 to 220 cm, weight 30 to 250 kg, week 0 to 42.

Examples

Height 160 cm, pre-pregnancy weight 55 kg (BMI 21.5, normal), one baby at week 28.

Item	Math	Result
Recommended total	normal category	11.5 to 16 kg
Week 28 low	0.5 + 0.35 × (28 − 13)	5.75 kg
Week 28 high	2 + 0.50 × (28 − 13)	9.5 kg
Current weight 62.5 kg	62.5 − 55	7.5 kg, within range

Switch to twins and the same BMI shows only the 17 to 25 kg total.

Source and good to know

Based on the Institute of Medicine 2009 report Weight Gain During Pregnancy: Reexamining the Guidelines, summary Table S-1, looked up 2026-10-04. The right gain depends on your starting weight, health and how the pregnancy is going, and triplets or more are not covered. This is a rough reference and not a diagnosis or treatment, so talk to your care team and get checked if your weight rises or falls suddenly.