How the calculator works
First, the calculator estimates how much energy your body uses before pregnancy. It uses the Mifflin–St Jeor equation — a widely used formula based on age, height and weight — and multiplies the result by your activity level. Then it adds the extra energy recommended by the Institute of Medicine for each stage of pregnancy and for breastfeeding.
| Stage | Extra calories a day (one baby) |
|---|---|
| First trimester | About 0 |
| Second trimester | About 340 |
| Third trimester | About 450 |
| Exclusive breastfeeding (0–6 months) | About 330 (on top of pre-pregnancy needs) |
Example: a 30-year-old who is 165 cm tall, weighed 60 kg before pregnancy and is lightly active needs about 1,815 calories a day before pregnancy — and about 2,160 a day in the second trimester.
What 340 extra calories looks like
The extra energy is smaller than many people expect. Around 340 calories is roughly a slice of wholegrain toast with peanut butter and a glass of milk, or a yogurt with fruit and a handful of nuts. “Eating for two” in quantity is not needed — but eating well for two is.
Nutrients matter more than calories
- Folic acid before and in early pregnancy, as recommended by health services.
- Iron from foods such as lean meat, beans and fortified cereals.
- Calcium and vitamin D for bones — dairy or fortified alternatives.
- Protein from a range of sources, and plenty of vegetables, fruit and whole grains.
- Follow local food-safety advice, such as avoiding certain fish, unpasteurised products and undercooked meat.
A guide, not a target
Calorie needs differ with metabolism, activity, nausea, twins and health conditions such as diabetes. Many people in the first trimester struggle simply to eat, and that is normal. Rather than counting calories, most guidance focuses on regular, balanced meals and healthy weight gain — see the Pregnancy Weight Gain Calculator. Talk to your midwife, doctor or a registered dietitian for advice that fits you.