Let's cut through the noise: A calorie formula is a math equation that estimates your basal metabolic rate (BMR) — the calories you burn at complete rest. The two big ones are Harris-Benedict (1919) and Mifflin-St Jeor (1990). Mifflin-St Jeor is newer and generally more accurate for most adults, while Harris-Benedict still holds up in specific groups like older adults and those actively losing weight. Neither is perfect — they're estimates, not measurements. This guide breaks down which one fits your situation and how to use them without overthinking the math.
Developed by: BMI Calculator Web Editorial Team — content aligned with NIH dietary reference intake methodologies and Academy of Nutrition and Dietetics standards.
What Is a Calorie Formula?
It's a predictive equation for your BMR: the calories your body spends on breathing, heartbeat, and cell repair. You can't change it, but you can estimate it.
Multiply BMR by an activity factor, and you get your total daily energy expenditure (TDEE) — your maintenance calories. Eat that, stay the same. Eat more, gain. Eat less, lose.
Skip the math entirely with our Adult BMI Calculator to get a baseline for your body size.
The Two Formulas That Actually Matter
Harris-Benedict Equation (1919)
The original. Built from just 239 adults and revised in 1984.
Men: BMR = 66.47 + (13.75 × kg) + (5.003 × cm) − (6.755 × age)
Women: BMR = 655.1 + (9.563 × kg) + (1.85 × cm) − (4.676 × age)
Honestly? It's a century old. Still useful for older adults and weight-loss research, but take it as a rough guide.
Mifflin-St Jeor Equation (1990)
The modern standard, published in the American Journal of Clinical Nutrition. Cleaner math:
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Here's what nobody tells you: these formulas were built on averages. And averages don't know you.
Which One Is More Accurate?
Mifflin-St Jeor wins for most healthy adults. A 2018 study found it predicted resting metabolic rate within 10% of measured values in 71% of participants. Harris-Benedict lagged behind in that group.
But a 2023 review showed Harris-Benedict had the smallest bias during the first three months of active weight loss (about 103 kcal/day difference). So context matters.
Both slightly overestimate metabolism in adults over 65. They're maps, not GPS.
How to Use Them in Real Life
Step 1: Start with Mifflin-St Jeor.
Step 2: Multiply by your activity factor: 1.2 (sedentary), 1.375 (light), 1.55 (moderate), 1.725 (active), 1.9 (very active). These are standard multipliers in nutrition science.
Step 3: Use our calorie calculator to skip the mental math.
Step 4: Track your weight for 2–3 weeks. Adjust by 100–200 calories if needed.
Across thousands of U.S. users, the average gap between formula output and real-world maintenance is ~150 calories — right in line with published research.
When to Ignore the Math
Athletes/muscular: Formulas underestimate your burn. Muscle needs more fuel.
Over 65: Both overestimate. Subtract 100 calories.
Post-weight loss: Metabolism adapts. Let your scale decide, not the equation.
Pair with a body fat calculator if you carry more muscle than average.
Frequently Asked Questions
Can I use a calorie formula to build muscle?
Yes. Calculate maintenance, then add 250–500 calories daily. Pair that with progressive resistance training and enough protein — roughly 1.6–2.2 g per kg of body weight. The formula gives you the floor; your training builds the ceiling.
My Fitbit or Apple Watch says 2200 calories, but the formula says 1800. Which is right?
Both are estimates. Wearables use motion sensors; formulas use body size. The truth is in between. Track your weight for two weeks to find your real number.
Why do different calculators give different results?
They use different formulas. Harris-Benedict, Mifflin-St Jeor, and WHO equations were built from different populations. A 150-calorie gap between tools is normal.
How accurate are these compared to lab tests?
Lab tests measure actual oxygen use. Formulas only predict. Expect to be off by 10–15% for many people.
Do these work for older adults?
They overestimate. If you're over 65, shave 100 calories off the calculator result and fine-tune from there.
What's the difference between BMR and TDEE?
BMR is your resting burn. TDEE adds movement, digestion, and daily activity. TDEE is the number you actually eat to.
Sources
This article is for informational purposes only. It does not replace professional medical or nutrition advice. Always consult a qualified provider. The BMI Calculator Blog Team maintains editorial independence.