Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest just to keep you alive — typically 60 to 70 percent of the calories you use in a day, before any movement, food, or exercise is added. To calculate BMR calories, researchers plug four simple inputs — sex, age, weight in kilograms, and height in centimeters — into a published prediction equation, and the result is an estimate of how many calories your body needs at rest. The two equations most often used are Mifflin-St Jeor (1990) and the revised Harris-Benedict (1984), and the BMR Calculator runs both at once so you can see your resting calorie number as a small range rather than a single point. Understanding the calorie figure — not just generating it — is what turns a number into a useful reference for daily energy planning.
What BMR Calories Actually Measure
The calorie figure your equation returns represents the energy your body uses to power non-stop, involuntary work: keeping your heart beating, your lungs exchanging air, your kidneys filtering blood, your brain running, and your core temperature stable. It also covers the constant background cost of cell repair and protein turnover. None of this involves walking, lifting, digesting a meal, or shivering in the cold — those are layered on top of BMR, not folded into it.
For most adults the resting calorie number is the single largest slice of the day's total energy budget. According to commonly cited ranges in metabolic literature, BMR typically accounts for 60 to 70 percent of total daily energy expenditure, with physical activity and the thermic effect of food making up the remainder. The exact share depends on how active you are: a desk worker will lean closer to the 70 percent end, while a manual laborer or daily runner will lean toward 60 percent because activity burns a bigger share.
Because the BMR calorie number is so dominant in the daily total, getting a reasonable estimate of it is the natural first step before anyone starts thinking about how much to eat, whether the goal is to lose, maintain, or gain weight. Everything else builds on top of it.
How to Calculate BMR Calories
You do not need to do the arithmetic by hand. The BMR Calculator runs both equations for you in your browser and shows the two calorie numbers side by side. Walk through these steps:
- Decide on metric units — kilograms for weight, centimeters for height, whole years for age. Convert pounds to kilograms (divide by 2.2046) and inches to centimeters (multiply by 2.54) if needed.
- Open the BMR Calculator.
- Select your sex and enter your age, weight in kilograms, and height in centimeters.
- Read your Basal Metabolic Rate from the Mifflin-St Jeor equation, which is shown as the primary calorie result.
- Compare it with the revised Harris-Benedict estimate to see your resting calorie range.
- Treat the smaller of the two as a conservative reference and the larger as an upper reference; the true resting figure for your body sits somewhere in that window.
Everything runs locally — nothing you type is uploaded or stored, which matters when the inputs include age and weight.
Mifflin-St Jeor vs. Revised Harris-Benedict
Both equations accept the same four inputs and return a calorie figure per day, but they were fit from different samples and use different coefficients. Here is how the two structures compare:
| Feature | Mifflin-St Jeor (1990) | Harris-Benedict, revised 1984 |
|---|---|---|
| Men formula | 10 × kg + 6.25 × cm − 5 × years + 5 | 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × years |
| Women formula | 10 × kg + 6.25 × cm − 5 × years − 161 | 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × years |
| Year published | 1990 | 1984 (Roza & Shizgal re-fit of the 1919 original) |
| Clinical guideline status | Preferred by most modern clinical and dietetic guidelines | Older form, retained for comparison |
| Typical output vs. other equation | Often slightly lower | Often slightly higher for the same adult |
The Mifflin-St Jeor equation is the one most current clinical and dietetic guidelines recommend for predicting resting energy, because it validated within about 10 percent of measured resting metabolic rate for a larger share of people than the older alternatives (see Wikipedia: Basal metabolic rate). Harris-Benedict, in its 1984 revised form (Roza & Shizgal), was re-fit to a larger and more representative sample than the 1919 original and remains widely cited (see Wikipedia: Harris–Benedict equation).
Reading the Calorie Number
The honest answer to "what is my BMR?" is a range, not a single figure. Individual metabolism is shaped by body composition, thyroid function, genetics, and adaptation to dieting — none of which appear in a height-and-weight formula. Two respected equations disagreeing by roughly 50 to 100 calories for the same adult is a useful visual cue that the calorie estimate is a population average, not a personal measurement.
To make this concrete, take a 30-year-old woman who weighs 65 kg and stands 165 cm tall. The Mifflin-St Jeor calorie estimate is:
BMR = 10 × 65 + 6.25 × 165 − 5 × 30 − 161 = 650 + 1031.25 − 150 − 161 = 1,370 calories per day
For the same person the revised Harris-Benedict equation returns a calorie number a little higher than the Mifflin-St Jeor figure. The gap is typical — it is the structural reason this calculator shows both side by side.
Once you have your resting calorie number, treat it as a starting reference, not a verdict. Watch your actual weight trend over two to three weeks at a stable calorie intake and adjust upward or downward from there. If you want a quick reference for where your weight sits relative to your height, the BMI Calculator gives you the WHO weight category in the same units. To turn the resting calorie figure into a daily target that includes movement and goals, the next step is a full TDEE calculation.
From BMR to Daily Calorie Targets
A BMR figure by itself is not a meal plan — it is the baseline before any activity is added. To get a daily calorie target that includes walking, training, work, and digestion, you multiply BMR by an activity factor and then add or subtract a goal-driven calorie adjustment. The BMR Calculator intentionally stops at the resting number; layering activity multipliers and goals onto it is the job of the Calorie Calculator, which uses the same Mifflin-St Jeor BMR and then walks you through activity level and weight goal to produce a per-day target. Keeping the two calculators separate lets you understand your true baseline first before any multiplier is applied.
Two reminders worth keeping in mind as you use either tool. First, every prediction equation assumes a healthy adult and does not adjust for pregnancy, athletic body composition, illness, or medications that change metabolic rate. Second, the calorie figures are population averages — the documented realistic error for any individual sits at roughly 10 percent or more. Use the number as a starting reference, not a prescription, and consult a qualified healthcare professional before making health, diet, or medical decisions.