Mifflin-St Jeor BMR Calculator Formula vs Harris-Benedict

Use the Mifflin-St Jeor equation if you want a BMR calculator formula. In my three test cases it ran 5% to 12% below Harris-Benedict, the older equation it’s usually compared with. A systematic review found it within 10% of measured resting energy for more adults than any other common equation. Its own authors found Harris-Benedict overestimated measured resting energy by 5%.

What is the Mifflin-St Jeor equation?

Mifflin-St Jeor is a formula that estimates resting energy expenditure from weight, height, age and sex. Mifflin and colleagues published it in 1990. They measured resting energy by indirect calorimetry in 498 healthy adults, 247 women and 251 men aged 19 to 78. Then they fitted the equation to those measurements. The two simplified versions are:

Sex	Mifflin-St Jeor equation (kcal/day)
Men	10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
Women	10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161

Each kilogram adds 10 kcal, each centimeter adds 6.25 kcal, and each year of age takes off 5 kcal. Sex alone moves the answer by 166 kcal, the gap between +5 and −161.

The same paper reports an R² of 0.71, so weight, height, age and sex explain most of the variation between people, though not all of it, and that leftover is why I check any result against my own scale.

What is the Harris-Benedict equation?

Harris-Benedict is the older formula. James Arthur Harris and Francis Benedict published it in 1918. A table in a 2023 paper in Metabolites describes its source as 239 white, normal-weight volunteers aged 16 to 63. In 1984, Roza and Shizgal refitted it with 98 more subjects of a wider age range, for 337 people in all. They reported a precision of 14% in normally nourished adults.

The 2023 paper still calls these among the most frequently used equations in daily practice, and the coefficients below come from its table.

Version	Men (kcal/day)	Women (kcal/day)
Original (1918)	66.47 + 13.75 × kg + 5.003 × cm − 6.755 × age	655.1 + 9.563 × kg + 1.850 × cm − 4.676 × age
Revised (1984)	88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age	447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age

These coefficients look nothing like Mifflin-St Jeor’s, and every term differs. Compare them on real people instead.

Mifflin-St Jeor vs Harris-Benedict: how far apart are the answers?

About 5% in two of my three cases and 12% in the third. I put three adults through all three formulas in a spreadsheet, using the coefficients above. Their details are in the first column.

Person	Mifflin-St Jeor	Harris-Benedict original	Harris-Benedict revised	Original vs Mifflin-St Jeor
Woman, 35, 165 cm, 70 kg	1,395 kcal	1,466 kcal	1,455 kcal	+71 kcal (+5%)
Man, 40, 178 cm, 85 kg	1,768 kcal	1,856 kcal	1,854 kcal	+88 kcal (+5%)
Woman, 60, 155 cm, 55 kg	1,058 kcal	1,187 kcal	1,177 kcal	+129 kcal (+12%)

Harris-Benedict came out higher every time. For the first two people the gap is the same 5% that Mifflin’s team found when it compared Harris-Benedict with measured values. The 60-year-old woman, the smallest and oldest of the three, saw 12%. Three cases make no rule, but I wouldn’t assume 5% if you are small or older.

That gap, 70 to 130 kcal, is modest. NASEM’s whole-day equations are typically off by 246 to 339 kcal for an adult. That is a different measurement, but it shows where the larger uncertainty sits. Even the most accurate TDEE calculator carries it.

How accurate is the Mifflin-St Jeor equation?

Better than the other three in that review. Not exact, though. The 2005 review compared the four equations in common clinical use: Harris-Benedict, Mifflin-St Jeor, Owen and the WHO/FAO/UNU equation. Mifflin-St Jeor won, with the narrowest error range. The reviewers still warned of “noteworthy errors” for single people. They also found older adults and US ethnic minorities underrepresented in the development and validation of the equations.

The formula knows nothing about body composition. In Mifflin’s data, fat-free mass was a better single predictor of resting energy (R² 0.64) than weight alone (R² 0.56). Two people with the same height, weight, age and sex get the same answer, even if one carries far more muscle. If you know your body fat, a TDEE calculator with body fat can use it.

These equations were built on healthy adults. If you’re pregnant, breastfeeding or managing a medical condition, a doctor or midwife sets your energy needs, not a formula.

Which Mifflin-St Jeor BMR calculator formula works with pounds, inches and kJ?

The equation takes kilograms and centimeters, so convert before you plug in anything else. Multiply pounds by 4.536 and inches by 15.875. That’s the same formula with the factors folded in: 4.536 × pounds + 15.875 × inches − 5 × age, then +5 or −161. The 35-year-old woman at 154.3 lb and 65 in comes out at 1,396 kcal. One kcal off, which is rounding. For kilojoules, multiply by 4.184, which turns her 1,395 kcal into 5,837 kJ.

Your result is a resting number, not a daily one, and BMR vs TDEE explains the gap. To turn it into calories to eat, you need an activity level, and that’s where I’d skip the multiplier. Typical daily totals are in how many calories per day adults need. NASEM fitted its adult TDEE equations straight to doubly labeled water data from 5,456 adults, so they give a daily total without a BMR step. For the same 35-year-old woman they return 2,103 to 2,642 kcal across four activity levels. Multiplying her Mifflin-St Jeor result by the 1.4 to 2.05 NASEM uses in its examples gives 1,953 to 2,860 kcal. That range is wider and cruder.

For a daily figure rather than a resting one, put your details into the calorie calculator, which applies the NASEM 2023 equations. Its result stays above the 1,200 and 1,500 calorie floors.

I use Mifflin-St Jeor when I want a resting number. For a daily one, NASEM. Either way, I check the result against two or three weeks of my own weigh-ins before I trust it. If the two disagree, I go with the scale.