Wondering if you’re gaining weight at a healthy pace this trimester? This pregnancy weight gain calculator uses your pre-pregnancy height and weight to work out your BMI, then shows you the recommended minimum and maximum weight for whatever week of pregnancy you’re in, based on the Institute of Medicine’s 2009 guidelines. It also has a twins option, so you’re not stuck comparing your numbers to a singleton chart that doesn’t apply to you.

How to use this pregnancy weight gain calculator

1. Enter your pre-pregnancy height and pick the unit you want to use: centimeters, meters, inches, feet, feet and inches, or meters and centimeters.
2. Enter your pre-pregnancy weight and choose a unit: kilograms, grams, pounds, ounces, or stones.
3. Check the BMI field. This fills in automatically once height and weight are entered, so you don’t need to calculate it separately.
4. Set the week of pregnancy you’re currently at, using the increment and decrement buttons (0 to 40 weeks).
5. Tick the Twins checkbox if you’re carrying twins, otherwise leave it unchecked for a singleton pregnancy.
6. Read the results: your pre-pregnancy BMI category, the minimum weight and maximum weight expected at that week, and the corresponding minimum weight gain and maximum weight gain in total.
7. Use Save as Image or Download PDF if you want to keep a record, or check the Saved Calculations section to review past entries (with a Clear All button if you want to start fresh).

What this pregnancy weight gain calculator does

The calculator takes your pre-pregnancy height and weight, converts them into a BMI figure, and sorts you into one of the four standard BMI categories: underweight, normal weight, overweight, or obese. Each category has its own recommended range of total weight gain by the end of pregnancy, and a rough pace at which that weight should be gained week by week. Rather than making you look up a chart and do the math yourself, the calculator plots where you should be at your current week and gives you a minimum and maximum weight, plus how much weight that represents gained since the start of pregnancy.

This is a reference tool, not a diagnostic one. It’s built to give you a general sense of whether your weight gain is tracking within a normal range, using the same population-level guidelines your OB or midwife is likely working from.

The formula behind the numbers

BMI is calculated the standard way, using metric units internally regardless of what you enter:

Once your BMI category is known, the calculator applies the total pregnancy weight gain range for that category from the IOM guidelines, then interpolates a week-by-week range between conception and week 40 to estimate where you should fall at your selected week. The minimum and maximum weight shown are your pre-pregnancy weight plus that estimated gain range, and the minimum and maximum weight gain fields are simply the difference from your starting weight.

IOM weight gain guidelines by pre-pregnancy BMI

That gap in the underweight/twins row isn’t a mistake. The IOM never published a twin pregnancy weight gain range for the underweight category because there wasn’t enough data to establish one when the guidelines were written. If you check the twins box while your pre-pregnancy BMI is underweight, the calculator will note this and fall back to the singleton figures as a rough reference only, but this is not a substitute for guidance from your doctor.

Why the calculator asks for pre-pregnancy weight, not current weight

All of the weight gain ranges in the IOM guidelines are anchored to your weight and BMI before you became pregnant, not your weight today. That’s why the height and weight fields at the top of this pregnancy weight gain calculator are labeled as pre-pregnancy figures. If you enter your current weight there by mistake, your BMI category and every result that follows will be off, since the whole calculation is built around your starting point.

Twin pregnancies

Ticking the Twins checkbox switches the underlying gain range from the singleton figures to the twin-specific figures where they exist. Twin pregnancies carry meaningfully higher recommended totals across every BMI category except underweight, reflecting the extra tissue, fluid, and fetal growth involved in carrying two babies.

This pregnancy weight gain calculator gives you a quick, BMI-based reference point, but every pregnancy is different, and factors like multiples, pre-existing conditions, or how a pregnancy is progressing can all shift what’s healthy for you individually. Treat the numbers here as a starting point for a conversation, not a verdict, and talk to your OB/GYN or midwife about what pace of weight gain makes sense for your specific situation.

Frequently asked questions