Pregnancy weight gain calculator
This calculator maps a pre-pregnancy body mass index onto the Institute of Medicine recommended weight-gain range, published in 2009 and still the reference most antenatal services work from. Enter a starting weight, a height and how many weeks along you are, and it shows the total range for your category, roughly what would be expected by now, and a steady rate for what is left. It is planning guidance for a single uncomplicated pregnancy — where your midwife or doctor gives a different target, theirs is the one to follow.
What people use it for
Knowing what the range actually is
The recommended total differs substantially by starting BMI — 12.5 to 18 kg at one end and 5 to 9 kg at the other. Reading somebody else’s number and assuming it applies is the most common way people end up worried about nothing, or reassured about something.
Putting one week in context
Weight in pregnancy moves in fits and starts, and sickness early on means plenty of people gain nothing for weeks. Seeing the shape of the whole range makes a single unexpected week look like what it usually is.
Carrying twins
The IOM publishes a separate, higher set of ranges for twin pregnancies, and this uses them when you say so. For triplets or more it publishes nothing, so neither does this tool.
How it works
- Enter your pre-pregnancy weight — Before, not now. The ranges are keyed on starting BMI, so today’s weight can put you in a different row and give you somebody else’s target.
- Enter your height — Used only to work out the starting BMI that selects the range.
- Enter the week — One to 42. The expected-so-far figure is a straight line to term, which is a simplification the method section explains.
- Say if it is twins — That switches to the separate IOM twin ranges rather than scaling the singleton ones.
Questions
I am behind the range. Is that a problem?
Not necessarily, and not from one reading. Sickness early on means plenty of people gain nothing for weeks or lose a little and catch up later without anything being wrong. What your antenatal team watches is the trend alongside everything else they measure.
Should I enter my weight now or before I was pregnant?
Before. The IOM ranges are keyed on pre-pregnancy BMI, so entering today’s weight can push you into a different row and give you a target meant for somebody else. The closest figure you have beats the current one.
What about twins, or triplets?
Twins have their own, higher IOM range and this page uses it when you tick the box. For triplets or more the IOM does not publish a range, so neither does this tool — that target comes from your team.
Why is the weekly rate a single number?
It is the midpoint of what is left, spread evenly over the weeks remaining to term. It is a way of saying "roughly this much, roughly this often" rather than a target for any particular week. Nobody gains weight in equal instalments.
Can I use it if I am not pregnant?
No — the ranges only mean anything in pregnancy. For general weight goals the ideal-weight and daily-calorie calculators on ZTools are the right tools, and both are linked from the tools directory.
Tips
- Use your pre-pregnancy weight. It is the single input that most changes the answer.
- One week off the line means very little; four in the same direction is worth mentioning.
- Your midwife’s target beats this one, always. They can see things a calculator cannot.
- The tool works signed out and stores nothing unless you choose to save the reading.
By Ahsan Mahmood · updated 2026-07-30
Open Pregnancy weight gain calculator on LifeWell