General information, not medical advice. Check with a doctor or registered dietitian before a big diet change, especially if you have a medical condition, are pregnant, or have a history of disordered eating.
The Harris-Benedict equation is the original scientific formula for estimating basal metabolic rate (BMR) — the calories your body burns at rest — from your age, sex, height, and weight. Published in 1919 and given a coefficient overhaul in 1984, it predates the Mifflin-St Jeor formula this site uses by default, yet it reads almost identically: multiply your weight, height, and age by fixed constants, add them up, and you get a daily calorie floor. This page lays out both versions — the original 1919 constants and the revised 1984 Roza-Shizgal constants, for men and women — then walks a real person through the arithmetic all the way to a full TDEE number. It also explains, honestly, why it now runs a little high for modern bodies.
TDEE Calculator
Total Daily Energy Expenditure — your real maintenance calories.
Biological sex
BMR formulas use biological sex, not gender identity.
Leave blank if unsure — we'll fall back to Mifflin-St Jeor (the standard).
Most people overestimate this. When in doubt, drop one tier.
In 1919, James Arthur Harris and Francis Gano Benedict published a biometric study of basal metabolism at the Carnegie Institution of Washington. They measured resting energy expenditure in 239 subjects — 136 men and 103 women, studied between 1909 and 1917 — and fit a linear equation to the data. That equation became the first widely-used way to predict how many calories a person burns at rest without putting them in a metabolic chamber, and for the better part of a century it was the standard taught in dietetics programs and printed in physiology textbooks.
Structurally it's simple. You start with a constant, add a coefficient times your weight in kilograms, add another coefficient times your height in centimeters, and subtract a third coefficient times your age in years. Men and women get different constants and coefficients because average body composition differs between the sexes. The output is BMR in kcal/day.
For decades this was the number hospitals used to plan feeding for patients, the number dietitians started from, and the number that quietly seeded a generation of early online calorie calculators. Its longevity is exactly why it still shows up everywhere despite being over a century old.
The version most online tools use today isn't actually the 1919 original. In 1984, Roza and Shizgal reanalyzed the Harris-Benedict data set against body cell mass and republished refined coefficients — the numbers you'll see labeled "revised Harris-Benedict." Both live side by side in the wild, which is one reason two calculators can hand you slightly different BMRs from the same inputs. I compare the whole equation head-to-head with the modern standard on the Mifflin-St Jeor vs Harris-Benedict page; here the goal is just to show you the formula and make the math concrete.
The Harris-Benedict equation constants
Every version of the Harris-Benedict equation is the same shape: constant + (weight coefficient × kg) + (height coefficient × cm) − (age coefficient × years). Only the four numbers change. Here they are for both the original 1919 study and the revised 1984 Roza-Shizgal reanalysis, split by sex.
Term
Men — original (1919)
Women — original (1919)
Men — revised (1984)
Women — revised (1984)
Constant (kcal)
66.473
655.096
88.362
447.593
Weight coefficient (per kg)
+13.752
+9.563
+13.397
+9.247
Height coefficient (per cm)
+5.003
+1.850
+4.799
+3.098
Age coefficient (per year)
−6.755
−4.676
−5.677
−4.330
Read a column top to bottom to build one formula. Men, revised (1984): BMR = 88.362 + 13.397·W + 4.799·H − 5.677·A. Women, revised (1984): BMR = 447.593 + 9.247·W + 3.098·H − 4.330·A. W = weight in kg, H = height in cm, A = age in years.
Working a real example: from BMR to TDEE
Let's take a concrete person: a 30-year-old man, 5'10" (178 cm), 175 lb (79.4 kg). I'll run him through both versions so you can see how close they land.
So the two Harris-Benedict versions agree within about 10 kcal here — the original edges slightly higher. That's your BMR, the calories to keep the lights on at complete rest. It is not your eating target. To get TDEE (total daily energy expenditure) you multiply BMR by an activity factor: 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active, 1.725 for very active, and 1.9 for extremely active. If this man trains a few times a week — moderately active — you'd use 1.55:
TDEE = 1,836 × 1.55 ≈ 2,846 kcal/day
That multiply-by-activity step is the same one described in detail on the how to calculate TDEE page, and it's exactly what the calculator at the top of this page does automatically. Here's the catch worth noticing: run the same man through Mifflin-St Jeor and BMR comes out around 1,762 kcal, for a TDEE near 2,730 — roughly 115 kcal/day lower. That gap is the whole reason people moved on from Harris-Benedict, and it's the theme of the next two sections.
Field note from Sukie
The winter I checked the equation against my own logbook
In February 2024 I got curious about how far off Harris-Benedict actually was for me, because I'd been reading conflicting things online. I'm 38, 5'6" (168 cm), and at the time I was 143 lb (65 kg). So I ran the revised 1984 women's formula by hand: 447.593 + (9.247 × 65) + (3.098 × 168) − (4.330 × 38) = about 1,405 kcal BMR. Mifflin-St Jeor gave me 1,349 — 56 kcal lower.
At light activity (×1.375) that's a Harris-Benedict TDEE of ~1,932 versus a Mifflin TDEE of ~1,855. The useful part is that I already had the answer sitting in a spreadsheet: six weeks of weighed food and a daily morning weigh-in from that winter, held dead flat within about a pound. My real maintenance that logged out to roughly 1,875 kcal/day — under the Harris-Benedict estimate by ~57 kcal, and within spitting distance of the Mifflin number.
One person is not a study, and 57 kcal is genuinely small. But it matched what the comparison research says at scale: for a modern, lightly-built adult woman, Harris-Benedict tends to sit a notch high. I've used Mifflin as my anchor ever since and just let two weeks of scale data correct the rest.
Where the Harris-Benedict equation drifts today
The equation isn't broken — it usually lands within about 14% of a measured value. But it was built on early-20th-century bodies, and that shows up in predictable places:
It was fit to 1919 subjects who were, on average, leaner than today's population, so it tends to overestimate BMR by roughly 5–15% for many modern adults — the drift is largest at higher body weights.
The revised 1984 Roza-Shizgal coefficients narrowed the gap but didn't fully close it; comparison studies still found it running high versus indirect calorimetry.
It uses total body weight, so it can't tell muscle from fat. Very muscular people are under-served — for them, Katch-McArdle (which works from lean body mass) is a better fit.
It isn't well-validated at the extremes: very high body weights and adults over about 80 both stretch its reliability.
It was never validated for pregnancy or breastfeeding, where energy needs rise substantially and no resting-metabolism formula should be trusted as a target.
Thyroid dysfunction, long aggressive diet history, and other metabolic-adaptation situations move real BMR away from what any prediction equation returns.
None of this makes the number useless — it makes it a starting estimate to verify against two to three weeks of real weight-trend data, never a figure to obey.
Should you still use the Harris-Benedict equation?
For setting your own diet in 2026, I'd reach for Mifflin-St Jeor first — the accuracy edge is small but it consistently points the right way, and there's no cost to using the better default. That said, the Harris-Benedict equation is far from worthless. If you're following an older program, a textbook, or a study that was built around it, computing it for reference is completely reasonable, and its output is close enough that no one eating to it will be wildly misled.
Where Harris-Benedict genuinely falls short is muscular bodies, because it only sees total weight. If you know your body fat percentage reliably, Katch-McArdle will describe you better — that's the fork covered on the Katch-McArdle vs Mifflin-St Jeor page. And if the whole BMR-versus-TDEE distinction still feels fuzzy, the TDEE vs BMR explainer untangles which number is your floor and which is your target.
There's also a middle path I like: compute both Harris-Benedict and Mifflin-St Jeor, take the two TDEE figures, and start at the lower of the pair. Because Harris-Benedict skews high, the lower number is statistically likelier to be closer to your truth, and it's easier to add calories back after a stalled loss than to claw a surplus back down. That single habit removes most of the anxiety about which formula is "right."
Whatever equation you land on, treat it the way I treat mine: as an educated first guess. Plug your numbers into the calculator above or the standalone BMR calculator, pick an honest activity factor, then eat at that TDEE for two to three weeks while tracking your weight trend. If the scale holds steady, the formula was right for you. If it drifts, adjust by 100–150 kcal and retest. Real data beats any equation — Harris-Benedict, Mifflin, or otherwise — because it's measuring you instead of predicting a population average from a century ago.
What's the difference between the original and revised Harris-Benedict equation?+
Same structure, different numbers. The 1919 original was fit by Harris and Benedict to their study subjects. In 1984, Roza and Shizgal reanalyzed that data against body cell mass and published refined coefficients — the 'revised' version most modern calculators use. For an average person the two land within about 10–20 kcal of each other.
Is the Harris-Benedict equation accurate?+
Reasonably — it typically falls within about 14% of a measured resting metabolic rate. But it was built on leaner 1919 bodies and tends to run 5–15% high for modern adults, especially at higher body weights. That's why Mifflin-St Jeor has largely replaced it as the default.
Does the Harris-Benedict equation give BMR or TDEE?+
BMR — your resting calorie burn. To reach TDEE (what you actually burn in a day), multiply that BMR by an activity factor between 1.2 (sedentary) and 1.9 (extremely active). The calculator on this page does both steps for you.
Which activity multipliers go with the Harris-Benedict equation?+
The standard set: 1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active, 1.9 extremely active. These are the same multipliers used with Mifflin-St Jeor — the activity factor is independent of which BMR formula you chose.
Should I use Harris-Benedict or Mifflin-St Jeor?+
For a modern adult, Mifflin-St Jeor. It's the equation recommended by the Academy of Nutrition and Dietetics and it sits closer to measured values. Use Harris-Benedict for reference or if a program you're following specifies it, and use Katch-McArdle instead if you're muscular and know your body fat.
Why does my Harris-Benedict result differ from an online calculator?+
Usually because the calculator uses a different version (original 1919 vs revised 1984), rounds the constants differently, or converts your pounds and inches to kilograms and centimeters with slightly different rounding. Small input differences compound into a 10–30 kcal gap.