Pregnancy Weight Gain Calculator

How This Calculator Works

How much weight is recommended to gain during pregnancy depends mainly on your pre-pregnancy body mass index (BMI) — a higher starting BMI generally comes with a lower recommended total gain, and vice versa. This calculator finds your pre-pregnancy BMI category, looks up its recommended total gain range from the Institute of Medicine’s published guidelines, and gives a rough “on track” estimate of how much you’d typically have gained by your current week.

This calculator is scoped to singleton pregnancies — the IOM publishes separate, higher recommended ranges for twins and other multiple pregnancies, which aren’t modeled here.

The Formula

Pre-pregnancy BMI uses the same formula as the standalone BMI Calculator:

$$\text{BMI} = \frac{\vA{\text{Weight (kg)}}}{\vB{\text{Height (m)}}^2}$$

That BMI value places you into one of four categories, each with its own IOM-published total gain range and typical 2nd/3rd-trimester weekly rate:

The “on track” estimate treats first-trimester gain as a simple straight-line ramp toward about 3 lbs by week 13 (a commonly-cited first-trimester figure, regardless of BMI category), then applies the category’s own typical weekly rate from week 13 onward — a reasonable approximation, not a precise week-by-week prediction.

Worked Example

For someone 165 cm (5’5”) tall, weighing 65 kg (about 143 lbs) before pregnancy, at week 20:

1. Pre-pregnancy BMI: $\vA{65} \div \vB{1.65}^2 = \vA{65} \div 2.7225 \approx \mathbf{23.88}$ — Normal weight.
2. Recommended total gain for Normal weight: 25–35 lbs.
3. On-track estimate at week 20: $3 + 1.0 \times (20 - 13) = \mathbf{10 \text{ lbs}}$.