Weekly Pregnancy Weight Gain Calculator

Track Recommended Weight Gain by Pregnancy Week

Weekly Pregnancy Weight Gain Formula

Formula

\[\mathrm{BMI}=\frac{W_0}{(H/100)^2}\] \[G_{\min}(w)=G_{\min}(40)\frac{w}{40},\qquad G_{\max}(w)=G_{\max}(40)\frac{w}{40}\]

Where:

\(W_0\) — weight before pregnancy (kg).
\(H\) — height (cm).
\(w\) — current week of pregnancy.
\(G_{\min}(40)\) and \(G_{\max}(40)\) — the lower and upper weight-gain values assigned to the pre-pregnancy BMI category (kg).
\(G_{\min}(w)\) and \(G_{\max}(w)\) — the calculated gain range at week \(w\) (kg).

The calculator selects a total gain range from pre-pregnancy BMI, then scales both ends of that range in direct proportion to the pregnancy week. It converts the results to pounds using 1 kg = 2.20462 lb.

Weekly Pregnancy Weight Gain – Calculation Example

Example: A person is 160 cm tall, weighed 55 kg before pregnancy, weighs 57 kg now, and is at week 12.

First, calculate pre-pregnancy BMI and the change in weight:

\[\mathrm{BMI}=\frac{55}{(160/100)^2}\approx21.5,\qquad 57-55=2\ \mathrm{kg}\]

A BMI of 21.5 is in the normal-weight category, so the calculator selects a week-40 gain range of 11.5–16 kg.

\[G_{\min}(12)=11.5\times\frac{12}{40}=3.45\ \mathrm{kg},\qquad G_{\max}(12)=16\times\frac{12}{40}=4.8\ \mathrm{kg}\]

Result: The calculator reports a gain of 2 kg so far, a week-40 range of 11.5–16 kg (25.4–35.3 lb), and a week-12 range of 3.4–4.8 kg (7.5–10.6 lb). The 3.4 kg display reflects the calculator’s floating-point rounding of the 3.45 kg midpoint.

How to use the calculator

Enter your Height in centimeters.
Enter your Weight before pregnancy in kilograms. Use the best available pre-pregnancy measurement rather than your current weight.
Enter your Current weight in kilograms, ideally measured under conditions similar to earlier weigh-ins.
Enter the Current week of pregnancy, from 1 to 40. If you are unsure of the week, use the How Many Weeks Pregnant Are You? calculator.
Read the pre-pregnancy BMI category, your change in weight so far, and the estimated gain ranges for week 40 and your selected week. Both ranges appear in kilograms and pounds.

Understanding pregnancy weight gain

Weight gained during pregnancy includes the baby, placenta, amniotic fluid, increased blood volume, and changes in maternal tissues. An appropriate total gain depends partly on BMI before pregnancy. This calculator uses four BMI categories: underweight (below 18.5), normal weight (18.5 to below 25), overweight (25 to below 30), and obese (30 or above).

The corresponding week-40 gain ranges are 12.5–18 kg, 11.5–16 kg, 7–11.5 kg, and 5–9 kg, respectively. These are commonly used total pregnancy weight-gain ranges for a singleton pregnancy. The calculator’s “Comparing to current weight you were” result is simply current weight minus pre-pregnancy weight: it says whether you gained weight, lost weight, or had no change. It does not assess whether that change is medically appropriate.

The week-specific result needs more care. It is calculated by multiplying the full-term range by your week number divided by 40. It is therefore a straight-line estimate of cumulative gain, not a clinical recommendation to gain the same amount every week. Weight gain is often modest in the first trimester and generally faster later. Being outside the displayed range at one weigh-in does not, by itself, establish a problem.

Assumptions and practical considerations

Singleton ranges: These total-gain ranges should not be applied to twins or higher-order pregnancies. Individual advice may also differ because of medical history or pregnancy complications.
Week-40 scaling: The calculator assumes gain progresses evenly from week 0 to week 40. Actual pregnancy weight gain usually does not follow a straight line, so treat the weekly figure as a calculation rather than a target to pursue.
Rounding: Week-specific kilogram values are rounded to one decimal place before conversion to pounds. At an exact rounding midpoint, the calculator’s floating-point arithmetic can produce a result that differs by 0.1 kg from ordinary decimal rounding, as in the example above.
Tracking: Look at the pattern across several measurements rather than a single day. Clothing, time of day, meals, and fluid retention can affect the number on the scale.
Personal guidance: Ask your prenatal care professional about an appropriate gain range for your circumstances, particularly if your weight changes suddenly or you have concerns about eating, swelling, or fetal growth.

If you need to check your pregnancy timing, the Gestational Age Calculator and Pregnancy Due Date Calculator can help you identify the week to enter.

Frequently Asked Questions

How do I calculate my average weekly pregnancy weight gain by hand?

Subtract your pre-pregnancy weight from your current weight, then divide by the number of weeks pregnant. For example, a gain of 4 kg by week 20 averages 0.2 kg per week. This is an average since the start of pregnancy, not a recommended weekly target; gain often changes between trimesters.

How much weight gain is typical in the first trimester?

Many people gain about 0.5–2 kg (1–4.5 lb) during the first trimester, though nausea and appetite changes can affect this. The calculator’s week-specific range may look higher because it spreads the full-term range evenly across 40 weeks. Ask your prenatal care professional about persistent weight loss or difficulty eating and drinking.

Should I recalculate my BMI using my current pregnancy weight?

No. Use your pre-pregnancy weight to determine the BMI category for pregnancy weight-gain guidance. Your current weight includes pregnancy-related changes, so recalculating BMI with it could move you into a different category without changing the guidance intended for your starting weight. If your pre-pregnancy weight is uncertain, discuss the best estimate with your prenatal care professional.

Does weight lost early in pregnancy count when I calculate total gain?

Yes. Total gain is measured relative to your pre-pregnancy weight, so early weight loss remains part of the overall change. For example, if you lose 2 kg and later regain 3 kg, your net gain is 1 kg. Tell your prenatal care professional if vomiting, poor intake, or ongoing weight loss is a concern.

How do I enter measurements taken in pounds, feet, and inches?

Convert pounds to kilograms by dividing by 2.20462. Convert your height to total inches, then multiply by 2.54 to get centimeters. For example, 5 ft 4 in is 64 in, or 162.56 cm. Convert both your pre-pregnancy and current weights before entering them; mixing pounds and kilograms will give an incorrect result.

When should I seek advice about a sudden weight change during pregnancy?

Contact your prenatal care team if your weight rises or falls suddenly, especially if the change persists or you cannot keep food or fluids down. A rapid increase accompanied by swelling of the face or hands, a severe headache, or vision changes needs prompt medical assessment. A calculator cannot determine the cause of a weight change.