The formula
How to calculate pregnancy weight gain
Recommended weight gain in pregnancy depends on pre-pregnancy BMI: less if you started heavier, more if you started lighter. The Institute of Medicine ranges are the ones used in most guidance.
Gain is not linear. The first trimester typically adds only 1–2 kg; the rest accumulates steadily through the second and third trimesters at roughly 0.4–0.5 kg a week for someone in the normal BMI band.
The inputs, one by one:
- Pre-pregnancy weight (kg)
- Current weight (kg)
- Weeks pregnant (weeks)
- Target total gain (kg) — 12.5–18 kg if underweight, 11.5–16 normal, 7–11.5 overweight, 5–9 obese
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
Consistency in how the underlying measurements are taken matters more than precision to the last decimal — measuring at the same time of day, in the same state of rest or hydration, makes readings taken weeks apart genuinely comparable in a way that a single very precise reading on its own is not.
Why pregnancy weight gain matters
This kind of calculation gets used both out of general interest and ahead of a specific conversation — with a trainer, a midwife or a GP — where having an actual number ready, rather than a description, makes that conversation more concrete and specific.
It is most useful tracked over time rather than read once — a single reading tells you where you are today, while the same calculation repeated weeks or months apart, using the same method each time, shows the direction things are actually moving in.
It is also worth being honest about what a formula like this is actually built from. Most of the equations behind calculators like this one were derived from population studies — averages taken across large groups of people — and applied here as a general-purpose estimate rather than a measurement tailored to any one individual. That is a reasonable trade-off for a quick, free estimate, and it is also exactly why a result that looks unusual is worth discussing with a professional rather than acted on alone.
In practice, this kind of figure is often calculated more than once with slightly different inputs to see how sensitive it is — trying a measurement taken this morning against one taken yesterday evening, for instance, to see how much normal daily variation actually moves the result. That habit is a good one: it builds a feel for how much a single reading can be trusted on its own before drawing any conclusion from it.
Worked example
A concrete run-through, using the values already in the fields:
- Pre-pregnancy weight: 65 kg
- Current weight: 72 kg
- Weeks pregnant: 28 weeks
- Target total gain: 13.5 kg
That gives:
- Gained so far: 7 kg
- Expected by now: 8.17 kg
- Difference from expected: -1.17 kg
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
Being a kilogram or two either side of the curve is unremarkable. A sudden jump — more than 1 kg in a week in the third trimester — is worth mentioning to your midwife, as it can indicate fluid retention.
Where this goes wrong. The UK does not routinely weigh at every appointment, precisely because focusing on the number can cause more anxiety than it prevents problems. Follow your midwife's guidance; this page is general information only.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
Institute of Medicine ranges for a single baby: 12.5–18 kg if underweight before pregnancy, 11.5–16 kg if normal weight, 7–11.5 kg if overweight and 5–9 kg if obese.
For a typical 12.5 kg gain: baby around 3.4 kg, placenta 0.7, amniotic fluid 0.9, larger uterus 1, breast tissue 0.5, extra blood 1.8, and roughly 3–4 kg of fluid and fat stores.
The answer it gives you is gained so far. With 65 kg pre-pregnancy weight, 72 kg current weight and 28 weeks weeks pregnant, that comes to 7 kg. Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.