Pregnancy Weight Gain Calculator

This calculator estimates pregnancy weight gain with IOM/NASEM ranges used in U.S. prenatal counseling and consistent with ACOG guidance. Enter pre-pregnancy weight and height (or BMI), the week, and — if you want — today’s weight to compare with the band. At BMI 22 in week 20 the midpoint is about 4.3 kg (3.4–5.6). At BMI 32 in week 30 about 4.8 kg. This is an orientation guide, not a prescription from your clinician.

Your numbers

Start with pre-pregnancy weight and height, then the week of pregnancy. Add today’s weight only if you want to compare your gain with the band — you will also need pre-pregnancy weight. You can enter BMI directly, or leave it blank and calculate it from weight and height.

Quick examples

BMI 22 · week 20 BMI 32 · week 30 137 lb → 146.6 lb · week 20 Twins · BMI 19
1. Before pregnancyPre-pregnancy weight and height, or BMI alone
2. NowGestational week. Current weight — only if you want to compare with the IOM band
3. Options (twins, activity)Click to expand — twins and activity (optional)

Abbreviations

  • BMI (body mass index) — weight (kg) ÷ height² (m).
  • IOM/NASEM — Institute of Medicine / National Academies, source of pregnancy weight-gain ranges.
  • ACOG — American College of Obstetricians and Gynecologists (U.S. twin guidance).

An orientation guide — it does not replace talking with your obstetrician or midwife.

Results

Enter pre-pregnancy weight and height (or BMI) and the week of pregnancy from 1 to 42. The result will appear here. Current weight is optional and only works together with pre-pregnancy weight.

IOM/NASEM ranges depend on pre-pregnancy BMI. The result is not a grade. You can check adult mass category on the BMI calculator. Ovulation is covered on the ovulation calculator. Due date: due date calculator.

IOM gain corridor for a normal pre-pregnancy BMI Chart from week 0 to 40: IOM band, midpoint line, and a marker at week 20 near 4.3 kg. ≈ 4.3 kg · week 20 0 week 20 40 kg
For a normal BMI (18.5–24.9) the IOM total is 11.5–16 kg. At mid-pregnancy the corridor midpoint is about 4.3 kg — a population guide, not a clinic weigh-in target.

How the results are calculated

BMI 22 · week 20 · IOM band → ≈ 4.3 kg (midpoint)
BMI22
Categorynormal
Week20
Gain≈ 4.3 kg

This calculator estimates cumulative pregnancy weight gain using IOM/NASEM ranges. The category comes from pre-pregnancy BMI: underweight below 18.5; normal 18.5–24.9; overweight 25–29.9; obesity from 30. For a normal BMI the usual total by delivery is 11.5–16 kg; for obesity it is 5–9 kg. For twins the calculator uses separate IOM/ACOG totals (for a normal BMI usually 16.8–24.5 kg).

Through week 13 typical gain is small, about 0.5–2 kg. From week 14 the rest of the band is spread linearly toward week 40. At BMI 22 in week 20 the midpoint is about 4.3 kg (range 3.4–5.6). At BMI 32 in week 30 it is about 4.8 kg (3.3–6.4).

The easiest path is pre-pregnancy weight and height — then BMI is calculated for you. Gestational week is 1–42. If you want to compare your gain so far with the usual band for that week, enter today’s weight together with pre-pregnancy weight. BMI alone, without pre-pregnancy weight, is not enough for that comparison. Activity slightly adjusts the pace in the second and third trimester; you can leave it blank.

The result is an IOM-style range, not a grade of “too much / too little” and not a diet plan. Week-to-week swings are normal. A large gap belongs in a conversation with your prenatal clinician.

Formula

Category = f(BMIpre) with IOM cut-offs (18.5 / 25 / 30).

Through week 13: gain ≈ (0.5…2 kg) × (week / 13).

From week 14: gain ≈ startT1 + (week − 13) × (IOM target − startT1) / 27 × kactivity.

Model: IOM/NASEM singleton ranges and IOM/ACOG twin totals. This is not an individualized nutrition plan.

How to use

  1. Enter pre-pregnancy weight and height, for example 62 kg and 165 cm, or BMI, for example 22.
  2. Enter the week, for example 20. Optionally add today’s weight together with pre-pregnancy weight to compare with the band.
  3. For twins, open Options and check twin pregnancy. Activity can stay blank.
  4. Click Calculate. At BMI 22 in week 20 the midpoint should be about 4.3 kg (3.4–5.6).
  5. Compare with BMI 32 in week 30 (about 4.8 kg) — a lower IOM total under obesity.

Pre-pregnancy BMI and IOM ranges

Cumulative gain depends on pre-pregnancy BMI and gestational week. BMI 22 at week 20 is about 4.3 kg; BMI 32 at week 30 about 4.8 kg.

BMIpre
Body-mass index before pregnancy, not from today’s weight. Enter it directly or compute it from mass and height (e.g. 62 kg and 165 cm ≈ 22.8). The separate BMI calculator computes the index alone.
IOM
Institute of Medicine — source of the delivery totals. Normal BMI usually 11.5–16 kg total; obesity 5–9 kg.
NASEM
National Academies — the same guideline line as IOM. The underweight / normal / overweight / obesity categories come from those tables.
week
Gestational week from 1 to 42. Through week 13 gain is small; afterward the pace follows the category band.

Usage examples

Example 1

  • BMI: 22
  • Week: 20
  • Singleton, moderate activity

~4.3 kg (3.4 kg - 5.6 kg)

A typical course for a normal BMI at mid-pregnancy — total recommended by delivery is 11.5 kg - 16 kg.

Example 2

  • BMI: 32
  • Week: 30
  • Singleton, moderate activity

~4.8 kg (3.3 kg - 6.4 kg)

At a higher BMI, guidelines recommend a smaller total gain (5 kg–9 kg by delivery). That range is normal under the guidelines.

Example 3

  • BMI: 19 (normal weight)
  • Week: 24
  • Twin pregnancy (IOM twin band)

~9.0 kg (7.1 kg – 11.2 kg)

Twins use a higher IOM total (at a normal BMI usually 16.8–24.5 kg by delivery).

Example 4

  • Pre-pregnancy weight: 62 kg
  • Height: 165 cm
  • Current weight: 66.5 kg
  • Week: 20

Gain: +4.5 kg · band ≈ 3.4 kg–5.6 kg

After comparing with today’s weight, the gain sits inside the usual IOM band for this week.

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FAQ

How much weight should I gain during pregnancy per IOM?

The total range depends on pre-pregnancy BMI. For a normal BMI the usual total is about 11.5–16 kg by delivery. At BMI 22 in week 20 the midpoint is about 4.3 kg within 3.4–5.6 kg. An estimate, not a prescription.

What is the pregnancy weight gain calculator on this page?

It shows an approximate weight-gain range from IOM/NASEM tables using pre-pregnancy BMI and gestational week. At BMI 22 in week 20 the midpoint is about 4.3 kg within 3.4–5.6 kg. An educational estimate, not a clinical prescription.

Where do the gain ranges come from?

From IOM/NASEM guidelines based on pre-pregnancy BMI. Underweight, normal, overweight, and obesity have different totals by delivery. A normal BMI usually aims at 11.5–16 kg total; obesity at 5–9 kg.

Why pre-pregnancy BMI, not today’s weight?

The guidelines start from mass before pregnancy. That number sets the category and band, no matter how much you have already gained.

How does the calculator spread gain across weeks?

Through week 13 typical gain is small, about 0.5–2 kg. From week 14 the rest of the IOM band is spread linearly. At BMI 22 in week 20 the midpoint is about 4.3 kg (range 3.4–5.6).

Do I need to enter BMI if I have mass and height?

No. You can leave BMI blank and enter pre-pregnancy mass and height — the calculator computes BMI. 62 kg and 165 cm give about 22.8. Week alone, without one of those paths, is not enough.

How do twins change the result?

The calculator uses separate IOM/ACOG twin totals, not a simplified ×1.6 factor. At a normal BMI the usual total is 16.8–24.5 kg by delivery. Underweight twin data are limited in IOM — then we show the normal-weight twin band as orientation only.

Why enter current weight?

To compare your gain so far with the usual IOM band for this week. You also need pre-pregnancy weight — BMI alone is not enough for the comparison. Without current weight, you still get the recommended corridor. The comparison is not a grade or a diet plan.

Does activity change the number?

It slightly adjusts the pace in the second and third trimester. That is an orientation, not a training plan. Activity in pregnancy belongs in a conversation with your clinician.

Does this mean I am gaining too much or too little?

No. We show a typical IOM-style range, not a grade. Week-to-week swings are normal. A large gap belongs with your prenatal clinician.

How is this different from regular BMI and ovulation?

Adult BMI categories do not assess pregnancy weight gain. Ovulation is estimated from the first day of the last menstrual period (LMP) on a separate page. Here the inputs are the pre-pregnancy BMI category and the gestational week.

· Hubert Ligocki

At BMI 22 in week 20 the midpoint ≈ 4.3 kg (3.4–5.6). Total by delivery for a normal BMI: 11.5–16 kg.

Knowledge sources

The gain ranges come from IOM/NASEM guidelines. They do not replace a clinic chart.

Page updated in 2026.