How it's calculated
The starting point is the basal metabolic rate, the energy you use at complete rest. The most widely used estimation formula comes from Mifflin and St Jeor in 1990. It was developed from 498 people, 247 of them women, whose resting energy expenditure was measured in the lab. For women it reads: 10 × weight in kg + 6.25 × height in cm − 5 × age in years − 161.
An activity factor is applied to the basal rate, giving your daily need. Rhythmus takes it from the training days and intensity in your profile: 1.2 without training, 1.44 with three moderate training days a week, 1.75 with seven, and at most 1.9 for very intense training. We set these steps ourselves; they cover the usual range of such factors.
A worked example
A woman, 30 years old, 168 cm, 65 kg, trains moderately three times a week. Basal rate: 650 + 1,050 − 150 − 161 = 1,389 kcal. With the factor 1.44, that gives a daily need of around 2,000 kcal.
How accurate is the formula?
For women of normal weight, it's one of the most accurate. A 2019 study compared 45 estimation formulas with the measured resting rate in 132 young adults. In women of normal weight, the Mifflin formula was one of the two most accurate; in women with overweight or obesity, other formulas fitted better. So which formula works best depends on the body.
Even the best formula misses. In a 2021 study of 31 female students, it underestimated the resting rate on average, but stayed within the ±10 percent that counts as clinically acceptable. Even that acceptable deviation is a good 140 kcal of basal rate in the example above, and around 200 kcal of daily need, so anywhere between 1,800 and 2,200 kcal. For individual women it can be more.
So this approach makes sense: take the number as a starting point, eat roughly to it for two to three weeks and watch what happens. Your observation beats the formula, just as in training.
What the cycle has to do with it
In the second half of the cycle, the resting rate rises slightly. A 2026 review found seven studies from 2020 to 2025. Five saw higher expenditure in the luteal phase, estimated at 30 to 120 kcal a day, roughly 3 to 5 percent. The authors note themselves that this may be lost in normal day-to-day variation and measurement error. It's smaller than the formula's uncertainty, so we don't turn it into a separate number.
Appetite is more noticeable. A meta-analysis found an energy intake on average 168 kcal per day higher in the second half of the cycle than in the first. If you're hungrier in the luteal phase, you don't have a discipline problem. There's more in Cravings across the cycle.
More important than the calorie number
- Eat enough at all. Too little energy over a long time has consequences far beyond weight. There's more in When your period stops.
- Enough protein. For people who strength train, recommendations are well above what general reference values for adults state.
- Consistency. A roughly suitable intake you can keep up beats an exactly calculated one you give up after two weeks.