Average BMR by Age

There are two reasons a resting burn tends to shrink over the years, and only one of them is in the equation. The first is the age term itself: Mifflin-St Jeor simply subtracts five calories for each year you have lived, a flat, mechanical adjustment that builds in a slow downward slope regardless of anything else. That is pure arithmetic, not a sign of anything going wrong. The second reason lives outside the formula but matters just as much in real life — as people age they typically lose lean muscle, and because muscle is more metabolically active than fat, losing it lowers the resting burn beyond what the age term alone predicts. This is why the steady 50-calorie-per-decade step in the reference chart usually understates what happens to a real person: the chart holds weight and height fixed and only moves age, so it captures the first effect cleanly while leaving the muscle-loss effect out of the picture. Neither is a pathology; both are ordinary, and the muscle part is the one you can do something about.

The chart above is built from a single, fixed reference body — the same weight and height at every age — so that the only thing changing from one row to the next is the number of years. That is a deliberate simplification: it lets you see the shape of the age effect on its own, without it being tangled up with differences in size. Read down the columns and you will notice the men’s and women’s figures fall in identical 50-calorie steps each decade, separated by a constant gap that comes from the equation’s sex term. What the chart is not is a forecast of your personal BMR, because your weight and height are almost certainly different from the reference body’s. To see your own number rather than the average, enter your real stats in the calculator above — it runs exactly the same equation, just with your figures instead of the reference ones.

If the age term is fixed, the lever you can actually pull is lean mass. Muscle is the most metabolically demanding tissue you carry, so the more of it you keep, the higher your resting burn sits for your age — and the muscle loss that often comes with the years is exactly what makes real-world BMR fall faster than the flat age line suggests. The arithmetic on this page cannot show that, because it only moves the age input; to put a figure on the muscle side, the Lean Body Mass Calculator estimates how much of your weight is fat-free mass. And once you have your resting figure, the TDEE Calculator turns it into the daily total you actually burn.