Pregnancy Weight Gain Calculator
The Institute of Medicine (now the NAM) splits total recommended gain by pre-pregnancy BMI into four bands: under 18.5 gains 12.5–18 kg, 18.5–24.9 gains 11.5–16, 25–29.9 gains 7–11.5, and 30 and above gains 5–9. The first trimester accounts for only 0.5–2 kg, with the remainder spread evenly from week 13 to week 40, so a BMI of 22 at week 24 gives a window of 4.98–7.70 kg, or 6.3 kg at the midpoint. This is a range and not a single figure, and the judgement of a midwife or doctor who can see the scans and the rest of the picture takes precedence over the table.
Your numbers
Recommended gain by now
6.3kg
Lower bound now
4.98kg
Upper bound now
7.7kg
Full-term minimum
11.5kg
Full-term maximum
16kg
What it means
A pre-pregnancy BMI of 22 sits in the 18.5–24.9 band: 11.5–16 kg by full term, and 0.41–0.52 kg a week after week 13.
Formula
Lower bound now = 0.5 + (Full-term minimum − 0.5) × (Weeks pregnant − 13) ÷ 27
A single weigh-in settles nothing. Swelling and morning sickness both fall outside what this table was built on, and twin pregnancies use a different band entirely — 16.8–24.5 kg for a BMI of 18.5–24.9.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Pre-pregnancy BMIBMI from your weight before pregnancy — it selects which gain range applies. | 22 | 0 ~ 60 |
| Weeks pregnant (wk)Weeks counted from the first day of your last period; term is 40 weeks. | 24 | 0 ~ 42 |
Step by step
Quick reference table
Results when only Pre-pregnancy BMI changes and everything else stays put.
| Pre-pregnancy BMI | Recommended gain by now (kg) | Lower bound now (kg) | Upper bound now (kg) |
|---|---|---|---|
| 11 | 7 | 5.39 | 8.52 |
| 16.5 | 7 | 5.39 | 8.52 |
| 22 | 6.3 | 4.98 | 7.7 |
| 33 | 3.6 | 2.33 | 4.85 |
| 44 | 3.6 | 2.33 | 4.85 |
What each result means
| Result | At default values |
|---|---|
| Recommended gain by now (kg)The midpoint of the gain expected to have accumulated by this week. | 6.3 |
| Lower bound now (kg)The lower end of this week’s range; below it is not automatically a problem. | 4.98 |
| Upper bound now (kg)The upper end of the range for this week. | 7.7 |
| Full-term minimum (kg)The recommended minimum total gain by full term. | 11.5 |
| Full-term maximum (kg)The recommended maximum total gain by full term. | 16 |
Common mistakes
A single weigh-in settles nothing. Swelling and morning sickness both fall outside what this table was built on, and twin pregnancies use a different band entirely — 16.8–24.5 kg for a BMI of 18.5–24.9.
Glossary
- Pre-pregnancy BMI
- BMI from your weight before pregnancy — it selects which gain range applies.
- Weeks pregnant
- Weeks counted from the first day of your last period; term is 40 weeks.
- Recommended gain by now
- The midpoint of the gain expected to have accumulated by this week.
- Lower bound now
- The lower end of this week’s range; below it is not automatically a problem.
- Upper bound now
- The upper end of the range for this week.
- Full-term minimum
- The recommended minimum total gain by full term.
- Full-term maximum
- The recommended maximum total gain by full term.
Frequently asked questions
Q. How is Pregnancy Weight Gain Calculator calculated?
Lower bound now = 0.5 + (Full-term minimum − 0.5) × (Weeks pregnant − 13) ÷ 27 — The Institute of Medicine (now the NAM) splits total recommended gain by pre-pregnancy BMI into four bands: under 18.5 gains 12.5–18 kg, 18.5–24.9 gains 11.5–16, 25–29.9 gains 7–11.5, and 30 and above gains 5–9. The first trimester accounts for only 0.5–2 kg, with the remainder spread evenly from week 13 to week 40, so a BMI of 22 at week 24 gives a window of 4.98–7.70 kg, or 6.3 kg at the midpoint. This is a range and not a single figure, and the judgement of a midwife or doctor who can see the scans and the rest of the picture takes precedence over the table.
Q. Can you walk through an example?
With Pre-pregnancy BMI 22, Weeks pregnant 24wk, the answer is Recommended gain by now 6.3kg.
Q. What do I need to enter?
Enter Pre-pregnancy BMI, Weeks pregnant. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Pre-pregnancy BMI moves the answer to Pre-pregnancy BMI 11 → Recommended gain by now (kg) 7 and Pre-pregnancy BMI 44 → Recommended gain by now (kg) 3.6. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
A single weigh-in settles nothing. Swelling and morning sickness both fall outside what this table was built on, and twin pregnancies use a different band entirely — 16.8–24.5 kg for a BMI of 18.5–24.9.
Related calculators
These calculators help you understand health figures; they do not replace diagnosis or prescription. See a clinician if you have symptoms.