How Many Calories Should a Woman Eat? The Equation

A maintenance figure is only useful with the equation that produced it and the error attached to that equation. The Mifflin-St Jeor equation, published in 1990, is the one most clinical dietetics uses, and it comes in two forms. For women: 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161. For men: the same terms with +5 instead of −161. The 166 kcal difference between those constants is not a statement that women and men of identical height, weight and age differ by 166 kcal. It is a fitted adjustment for the fact that, at the same height and weight, the two groups differ on average in lean mass, and lean mass is what drives resting expenditure.

Run the same body through both and the gap is visible. A 40-year-old at 165 cm and 70 kg:

Women's constant:
  10 × 70      = 700
  6.25 × 165   = 1031.25
  5 × 40       = 200
  700 + 1031.25 − 200 − 161 = 1370 kcal/day

Men's constant:
  700 + 1031.25 − 200 + 5   = 1536 kcal/day

Difference: 166 kcal/day

That is resting metabolic rate, not maintenance. Multiply by an activity factor to reach total daily expenditure. Sedentary is roughly 1.2, lightly active 1.375, moderately active 1.55, very active 1.725. At 1.375, the women’s-constant figure is 1,884 kcal and the men’s-constant figure is 2,112 kcal. The 166 kcal gap survives the multiplication unchanged.

The band around the figure

The Mifflin-St Jeor equation predicts resting metabolic rate with a standard error of roughly 10% in the populations it was fitted to. That is not a rounding error; it is the spread of individual measurements around the line. A predicted 1,370 kcal carries a band of about 1,233 to 1,507. The 166 kcal sex-constant difference sits inside that band. For any one person, the equation cannot tell you which constant is correct, because it does not know your lean mass. It knows only that the two groups differ on average.

This is why a woman and a man of the same height, weight and age can have the same measured resting expenditure, and why a lean woman can outspend a heavier man. The constant is a population adjustment, not a personal one.

What the figure depends on

Worked maintenance, with the band

Take the same 40-year-old woman, 165 cm, 70 kg, lightly active. The arithmetic above gives a resting figure of 1,370 kcal. Multiply by 1.375:

Resting (Mifflin-St Jeor, women's constant): 1370 kcal
Activity factor (lightly active):            × 1.375
Maintenance estimate:                        1884 kcal

Standard error band (±10%):
  1884 × 0.90 = 1696 kcal
  1884 × 1.10 = 2072 kcal

So: 1,700–2,100 kcal/day, midpoint 1,884

A moderately active version (1.55) gives 2,124 kcal, band 1,912–2,336. The midpoint moves with the activity factor, which is the least precise input in the whole calculation. Self-reported activity levels are notoriously unreliable; the difference between “lightly active” and “moderately active” is 250 kcal in this example, larger than the sex-constant difference.

Why the two constants exist

The +5 and −161 are not physiological constants. They are regression intercepts fitted to a dataset in which women and men differed in body composition at the same height and weight. Lean tissue is metabolically active; adipose tissue is far less so. At matched height and weight, women carry more fat mass and less lean mass on average, so their resting expenditure is lower on average. The equation encodes that average difference in a single number.

The consequence is that the equation is wrong in a predictable direction for individuals who do not match the average. A lean, muscular woman will have a higher resting expenditure than the equation predicts with the women’s constant. A man with high fat mass and low lean mass will have a lower one than the men’s constant predicts. The equation cannot see either of them.

What people get wrong

The common mistake is to treat the sex constant as a biological fact about the reader. It is a fitted average, and the 10% standard error around the prediction is wide enough to swallow it. A woman who runs the men’s constant and gets a higher number has not discovered a hidden truth; she has moved from one point estimate to another, both inside the same band.

The second mistake is to treat the maintenance figure as fixed. Maintenance falls as body mass falls, because moving a smaller body costs less and because resting expenditure scales with lean mass. A 10 kg loss lowers maintenance by roughly 100–200 kcal/day at the same activity level, depending on what the lost mass was. The equation must be re-run at the new weight; extrapolating a straight line from the starting figure overestimates maintenance at the end of a diet.

The third mistake is to confuse the 7,700 kcal per kilogram figure with body weight. That number describes the energy in adipose tissue, not in the whole body. Early weight loss includes water and glycogen, which carry far less energy per kilogram, so the scale moves faster than the arithmetic predicts. Later, maintenance has fallen, so the same intake produces a smaller deficit. The figure is wrong in both directions, for two different reasons.

Where the figure stops being useful

A predicted maintenance of 1,884 kcal with a band of 1,700–2,100 is a planning figure, not a measurement. It is accurate enough to set a starting intake and adjust from observed weight change over two to three weeks. It is not accurate enough to justify an intake below 1,200 kcal for women or 1,500 kcal for men without clinical supervision, because the band around the prediction is wider than the margin those floors protect.

This page is not medical advice. Anyone with a history of disordered eating, or who finds that tracking intake produces distress rather than information, should work with a clinician rather than an equation. In the United States, the National Eating Disorders Association helpline is available; in the UK, Beat. The equation is a tool for arithmetic, not a substitute for care.