Weight gain calculator
With a pre-pregnancy BMI of 22, the recommended gain by week 20 is 4.5 to 6 kg, on the way to 11.5 to 16 kg at term. Enter your BMI before pregnancy and the current week to see where your range sits today. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Bmi
- 22
- Week
- 20
- Bmi category
- normal kg
- Category label
- Normal weight
- Guideline
- IOM/NRC 2009
- Pregnancy type
- singleton
A pregnancy weight-gain calculator turns pre-pregnancy body mass index and the current gestational week into the recommended kilogram range clinicians and educators cite from the Institute of Medicine guidelines. Enter BMI in kilograms per square metre and a week of gestation from zero through forty-two; the tool classifies the BMI band as underweight, normal weight, overweight, or obese, returns the full-term total recommended gain for a singleton pregnancy, the second- and third-trimester weekly rate band, and a cumulative recommended range interpolated to the week you supply.
What IOM pregnancy weight-gain ranges mean
Recommended gestational weight gain is not a single target number; it is a band that depends on pre-pregnancy body mass index.
The Institute of Medicine and National Research Council 2009 report for singleton pregnancies assigns underweight women with BMI under eighteen point five a total gain of twelve point five to eighteen kilograms, normal-weight women with BMI from eighteen point five up to but not including twenty-five a total of eleven point five to sixteen kilograms, overweight women from twenty-five up to thirty a total of seven to eleven point five kilograms, and women with obesity at BMI thirty or higher a total of five to nine kilograms.
Those totals apply near term, roughly forty weeks. For the second and third trimesters the same report publishes approximate weekly rates: about zero point four four to zero point five eight kilograms per week when underweight, zero point three five to zero point five zero when normal, zero point two three to zero point three three when overweight, and zero point one seven to zero point two seven when obese.
Early pregnancy contributes a smaller first-trimester segment; this calculator derives that segment so that week-forty cumulative gain matches the published totals, then ramps linearly from zero at week zero through the first thirteen weeks and applies the weekly rate afterward. From week forty through forty-two the returned range is the published full-term total, because the IOM table is a term target rather than an unbounded weekly sum. The result is a cumulative min–max range at any week from zero through forty-two, plus the full-term totals and rates for context.
The tool does not diagnose, prescribe diet, or replace clinical judgment for twins, multiples, or medical conditions that change targets. Always feed true pre-pregnancy BMI, never a mid-pregnancy BMI computed from current weight, when selecting the guideline row.
Practical tips, limits, and related searches
Use true pre-pregnancy BMI, not a mid-pregnancy BMI, when selecting the band; mixing current weight into the BMI field will misclassify the guideline row. Prefer numbers rather than pre-formatted strings when you control the client; string numbers are accepted only when they match a strict finite numeric pattern. Prefer this capability when you already have pre-pregnancy BMI and gestational age and only need the published range; if your source is height and weight, compute BMI first.
Keep requests small, pin the rounded kilogram bounds in tests, and you get a reproducible prenatal range without standing up a clinical spreadsheet of your own.
What you can do with it
Prenatal education worksheets
Educators enter pre-pregnancy BMI and current gestational week from case vignettes and compare hand-written IOM ranges against a deterministic cumulative min–max before class discussion.
FAQ
What inputs does the weight gain calculator need?
Pre-pregnancy BMI (kg/m²) and the current gestational week (0–42). The tool classifies BMI and returns IOM 2009 singleton recommended ranges in kilograms.
What happens if the week is greater than 42?
Gestation above 42 weeks is outside the supported schedule for this calculator.
Which guideline table is used?
Institute of Medicine / National Research Council 2009 singleton pregnancy total and second/third-trimester weekly weight-gain ranges by pre-pregnancy BMI category.
Does this cover twin or multiple pregnancies?
Twin and higher-order schedules use different published ranges and are out of scope.
Is the weight gain calculator free?
The browser tool is free and runs locally.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/health/weight-gain \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"bmi":22,"week":20}'const res = await fetch("https://api.kit.forhosting.com/health/weight-gain", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"bmi": 22,
"week": 20
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/health/weight-gain",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"bmi": 22,
"week": 20
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/weight-gain", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"bmi":22,"week":20}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"bmi":22,"week":20}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/weight-gain", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"bmi": 22,
"week": 20
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "health.weight_gain",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
min_bmi | 10 |
max_bmi | 60 |
max_week | 42 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Weeks above forty-two are rejected with invalid_input so post-term outliers never invent a schedule.
How to call the API and what comes back
Pre-pregnancy BMI must lie between ten and sixty kilograms per square metre. Fractional weeks are accepted for mid-week checks. Send a JSON body with two required finite numbers: bmi (aliases pre_pregnancy_bmi, prepregnancy_bmi, bmi_pre, pre_bmi) and week (aliases weeks, gestational_week, gestational_weeks, gestation_weeks, ga_weeks). Gestational week must be at least zero and at most forty-two; any value strictly greater than forty-two returns invalid_input. On success the response echoes rounded bmi and week, sets bmi_category to underweight, normal, overweight, or obese, category_label to a human English band name, guideline to IOM/NRC 2009, pregnancy_type to singleton, recommended_min_kg and recommended_max_kg to the cumulative recommended range at that week, total_min_kg and total_max_kg for full-term totals, weekly_rate_min_kg and weekly_rate_max_kg for the second- and third-trimester rates, first_trimester_min_kg and first_trimester_max_kg for the derived week-thirteen endpoints, unit as kg, message as a short English summary, and steps as an ordered array of classification and interpolation lines. Failed validation—missing fields, non-finite numbers, BMI outside limits, negative week, or week above forty-two—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item. Pair the numeric range with your own charting code if you need a trajectory plot; this endpoint returns the published kilogram band only and does not emit diet plans, calorie targets, or ultrasound estimates.
Practical tips, limits, and related searches
When teaching, pair examples: BMI twenty-two at week twenty (normal band, mid-pregnancy cumulative range), BMI seventeen at week forty (underweight full-term totals near twelve point five to eighteen kilograms), BMI twenty-eight at week twelve (overweight first-trimester ramp), BMI thirty-two at week forty (obese full-term totals of five to nine kilograms), and deliberate week equals forty-three to demonstrate the hard invalid_input path for gestation beyond forty-two. This landing targets people searching for a weight gain calculator, IOM gestational weight gain by BMI, or recommended pregnancy weight gain by week online; it is not a BMI calculator from height and weight, not a twin-pregnancy schedule, not a calorie planner, and not medical advice. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately set week above forty-two. Round-trip checks are easy: confirm the BMI category matches WHO cut-points, confirm full-term totals match the IOM table for that band, and at week forty confirm recommended_min_kg and recommended_max_kg equal the published totals within the same twelve-decimal rounding.
Integration use cases
Unit tests for maternity apps
Generate known-good underweight, normal, overweight, and obese fixtures plus deliberate week-over-42 rejections for coding kata expected values.
Pre-flight before charting weight trajectory
Pipelines fetch the recommended band first, then plot measured gain against recommended_min_kg and recommended_max_kg only when BMI and week pass validation.