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Pregnancy Weight Gain Calculator

See the 2009 IOM recommended weight gain range for your pre-pregnancy BMI and how your gain so far compares at your current week.

Level: Grades 6–8

  • Common Core standard 6.EE.A.2.cEvaluate expressions at specific values of their variables. Include expressions that arise from formulas used in real-world problems. Perform arithmetic operations, including those involving whole-number exponents, in the conventional order when there are no parentheses to specify a particular order (Order of Operations).
Printable worksheet

Pregnancy Weight Gain Calculator

Inputs
Units

Completed weeks since the first day of your last period, as your provider counts them.

Result

Recommended total gain (singleton)

25–35 lb

For a pre-pregnancy BMI of 23.3, the 2009 IOM guidelines suggest a total gain of 25–35 lb for a single baby. These are general guidelines; your healthcare provider can advise on what is right for you. Twin pregnancies use different ranges, which this calculator does not model.

Pre-pregnancy BMI
23.3 kg/m²
BMI category used
Normal weight (BMI 18.5–24.9)
Approximate range at week 20
7.3–12.3 lb
Gain so far
12 lb
Compared with this week's range
Within the approximate range for this week

See the step-by-step solution ↓

Step-by-step solution

  1. Change height to meters and your weight before pregnancy to kilograms, since BMI uses metric units (1 in = 2.54 cm, 1 lb = 0.45359237 kg)

    (5×12+5)×0.0254=1.651 m; 140 lb×0.45359237=63.503 kg

  2. Find your BMI before pregnancy: weight in kilograms divided by height in meters, squared

    63.5031.6512=23.3

  3. Look up your BMI group. The 2009 IOM guidelines give a total gain range for each one

    Normal weight (BMI 18.5–24.9) → =25–35 lb

  4. After week 13, the range climbs at a steady pace from the first-trimester gain to the full range at week 40

    low: 1.1+(25−1.1)×(20−13)27; high: 4.4+(35−4.4)×(20−13)27=7.3–12.3 lb

  5. Take your weight before pregnancy away from your current weight to get your gain so far. Then compare it with this week's range

    152 lb−140 lb=12 lb

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

Please note: This calculator gives a general screening estimate, not a diagnosis. Talk to a qualified healthcare professional about your individual health.

What this calculator does

See the recommended weight gain range for your pregnancy, and how your gain so far compares. How much to gain depends mostly on where you started. In 2009 the US Institute of Medicine (now the National Academy of Medicine) published guidelines on this. They set a recommended total gain for a single-baby pregnancy based on your BMI before pregnancy. BMI (body mass index) is a measure of weight for your height. People who start out underweight get a larger range, and people who start with a higher BMI get a smaller one. These ranges are still widely used in prenatal care.

The calculator works out your BMI from your height and weight before pregnancy. It shows the matching total range. Then it estimates a range for your current week, so you can see roughly where your gain so far sits. It gives general guidelines only. Your midwife, doctor or other healthcare provider knows your situation and may recommend something different. Their advice comes first. Twin and other multiple pregnancies have different ranges, which aren't modeled here.

Where it's used

Why this math matters outside the classroom:

  • Pre-pregnancy BMI. Starting at 64 kg and 1.65 m gives a BMI of 64 ÷ 1.65², about 23.5, which picks the range the calculator shows.
  • Kilograms and pounds. Converting a 25 to 35 lb range to about 11.3 to 15.9 kg lets families compare it with a metric scale.
  • Straight-line estimates. Students see linear interpolation at work, since the weekly range rises in a straight line after the first trimester.
  • Prenatal visits. Expecting parents bring a log of weekly weights to prenatal visits to review the trend with their doctor.
  • Weekly check-ins. Some people set a weekly weigh-in reminder and note each result next to the range for that week.

Grade level and standards: Grades 6–8; Common Core State Standards for Mathematics 6.EE.A.2.c. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose US or metric units and enter your height.
  2. Enter your weight before pregnancy and your current weight. If you don't know your weight before pregnancy, use the one from your first prenatal visit.
  3. Enter the current week of pregnancy. It's counted the usual way, from the first day of your last period.
  4. Read the recommended total gain, the approximate range for this week, and how your gain so far compares.

What the inputs mean

Weight before pregnancy
Your usual weight just before you became pregnant. The BMI category, and so the whole range, is based on this number.
Current week of pregnancy
Completed weeks of pregnancy as your provider counts them. The due date calculator can work this out from your last period.

Formula and method

Pre-pregnancy BMI = weight (kg) ÷ height (m)²
Total gain (IOM 2009, one baby): underweight 28–40 lb (12.5–18 kg); normal 25–35 lb (11.5–16 kg); overweight 15–25 lb (7–11.5 kg); obese 11–20 lb (5–9 kg)
Weeks 1–13: range = week ÷ 13 × first-trimester gain (1.1–4.4 lb or 0.5–2 kg)
Weeks 14–40: range = first-trimester gain + (total − first-trimester gain) × (week − 13) ÷ 27

BMI is weight in kilograms divided by height in meters, squared. It's rounded to one decimal before the category is chosen. The category uses the standard adult cutoffs of 18.5, 25 and 30. In metric mode, the calculator uses the kilogram figures from the IOM report instead of converting the pound figures. So the two systems differ slightly.

The week-by-week range is an estimate built by this calculator, not a table from the guidelines. The report assumes a small gain over the first trimester (the first 13 weeks) and a fairly steady gain after that. So the calculator draws a straight line from zero up to the first-trimester gain at week 13. Then it draws another on to the full range at week 40. After week 40, the full range is shown.

Worked example

Someone 5 ft 5 in tall weighed 140 lb before pregnancy and weighs 152 lb at week 20.

  1. Height 65 in = 1.651 m, and 140 lb = 63.503 kg. BMI = 63.503 ÷ 1.651² = 23.3, which is in the normal range.
  2. The IOM total gain for a normal pre-pregnancy BMI is 25–35 lb.
  3. Week 20 range: low 1.1 + (25 − 1.1) × 7 ÷ 27 = 7.3 lb; high 4.4 + (35 − 4.4) × 7 ÷ 27 = 12.3 lb.
  4. Gain so far: 152 − 140 = 12 lb.

Result: The recommended total gain is 25–35 lb. A 12 lb gain at week 20 is within the approximate range of 7.3–12.3 lb.

Understanding the result

Being a little above or below the approximate range in a given week is common. Weight shifts from week to week with fluid, nausea, appetite and when you weigh yourself. The overall pattern across the pregnancy matters more than any single reading.

If your gain looks well outside the range, or changes suddenly, mention it at your next appointment. That's better than changing your diet on your own. Report sudden swelling or fast weight gain to your provider promptly.

Weight gain is only one of the things your care team watches. It isn't a measure of the baby's growth or wellbeing.

Assumptions and limitations

  • A single-baby (singleton) pregnancy.
  • The weight before pregnancy you enter is accurate.
  • Gain is fairly steady after the first trimester, which is a simplification.
  • It doesn't give ranges for twins or other multiples. Those differ and should come from your provider.
  • The week-by-week range is an estimate, not part of the published guidelines.
  • BMI categories don't account for build, ethnicity or personal medical circumstances.
  • It isn't meant for people under 18 or for any medical decision.

Frequently asked questions

What if I'm expecting twins?

The IOM guidelines give separate, higher ranges for twin pregnancies, marked as provisional (not yet final). The right target depends on your situation. This calculator doesn't model them, so please ask your healthcare provider.

Why is the recommended gain lower if my BMI was higher?

The guidelines balance the needs of the pregnancy against the extra weight already there. The range is smaller for higher BMIs before pregnancy, but it's still a gain. The guidelines don't recommend losing weight during pregnancy.

I lost weight in the first trimester. Is the calculator still useful?

Yes. Early weight loss, often from nausea, is common. The calculator will show a gain below the range. Many people catch up later. Talk to your provider if you're worried or if nausea is severe.

Why don't the metric and US ranges match exactly?

The IOM report gives its own rounded kilogram figures, for example 11.5–16 kg next to 25–35 lb. The calculator uses those directly.

Is my information stored?

No. Everything is worked out in your browser, and this calculator doesn't create a shareable link with your details.

How this calculator is checked

Method: Institute of Medicine (2009), Weight Gain During Pregnancy: Reexamining the Guidelines — total gain ranges by pre-pregnancy BMI for singleton pregnancies and the assumed first-trimester gain of 0.5–2 kg; weekly interpolation is this calculator's linear approximation. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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