BMR Calculator: Step-by-Step Formula With Worked Examples

Updated 2026-07-12 · 10 min read

Your basal metabolic rate (BMR) is the number of calories your body burns at complete rest to sustain vital functions like breathing, circulation, and cell production. To calculate BMR manually, use the Mifflin-St Jeor equation: for men, BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) + 5; for women, BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) − 161. This formula gives your daily resting calorie expenditure, which you can then adjust with an activity factor to find your total daily energy expenditure (TDEE).

The Mifflin-St Jeor Formula Explained Term by Term

The Mifflin-St Jeor equation is the most accurate BMR formula for the general population, according to the Academy of Nutrition and Dietetics. Let's break down every component.

For men:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5

For women:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

What each variable means

• Weight (kg): Your body mass in kilograms. To convert from pounds, divide by 2.205.
• Height (cm): Your height in centimeters. To convert from inches, multiply by 2.54. From feet, multiply total inches by 2.54.
• Age (years): Your current age in whole years.
• Constant term: The +5 for men and −161 for women account for average differences in muscle mass and body composition between sexes.

The BMR calculator using the Mifflin-St Jeor method outputs calories per day. Note that these are estimates and individuals should consult a healthcare professional for personalized dietary advice.

How to Calculate BMR Manually: Step-by-Step Example

Let's walk through a realistic worked example so you can see every arithmetic step.

Notice how we handled the arithmetic: multiply first, then add and subtract left to right. The BMR calculator for men follows this exact sequence — no rounding until the final result.

A Second Worked Example: BMR Calculator for Women Over 40

Many women over 40 ask about how to calculate BMR manually because metabolism can shift with age. Here's a second example with different numbers.

A BMR calculator for women over 40 would produce the same number as this manual method, but without the risk of arithmetic errors. The age term (−5 × age) naturally reduces the BMR each year, matching the typical metabolic slowdown.

Common Mistakes People Make When Calculating BMR by Hand

Even with a straightforward formula, errors creep in. Here are the most frequent pitfalls.

• Forgetting unit conversions: The most common error. You must use kilograms and centimeters, not pounds and inches. Mixing units throws the result off by hundreds of calories.
• Using the wrong constant: Adding 5 when you need −161, or vice versa. This shifts the result by 166 calories — a meaningful difference for anyone using a BMR calculator to lose weight safely.
• Rounding too early: If you round 83.9 kg to 84 kg and 177.8 cm to 178 cm, the final BMR changes by roughly 10–15 calories. Small errors compound.
• Confusing BMR with TDEE: BMR is resting only. Total daily energy expenditure (TDEE) includes activity. A BMR calculator vs TDEE difference explained: BMR × activity factor = TDEE. Many people mistakenly diet at BMR levels, which is too low.
• Forgetting the order of operations: Multiply before adding and subtracting. The formula is linear, but calculating 10 × weight first keeps things clean.

A free calculator eliminates all of these issues. But knowing the manual method helps you understand what the number actually means.

When Manual Calculation Is Fine vs. When You Should Use a Calculator

Manual calculation is fine when you're learning the formula or don't have internet access. It builds understanding. However, for daily use, clinical settings, or weight management plans, a BMR calculator is far more practical.

Consider this: a single arithmetic error when converting height or weight can shift your BMR by 50–100 calories. Over a week, that's a 350–700 calorie miscalculation — enough to derail fat loss or gain goals. The free calculator on this page computes the identical formula with perfect precision every time.

Use manual calculation if you're learning or verifying. Use a calculator when you need speed, accuracy, and the ability to quickly test different scenarios (what if you lose 10 pounds? what if you gain muscle?).

Manual vs. Calculator: Comparison Table

The BMR calculator using the Harris-Benedict vs Mifflin-St Jeor method is also available on many tools, but the free calculator here exclusively uses Mifflin-St Jeor because it is the current clinical standard.

BMR vs. TDEE: What's the Difference and Why It Matters

A common point of confusion is the BMR vs TDEE difference explained. BMR is the calories you burn at complete rest — imagine lying still all day. TDEE (Total Daily Energy Expenditure) adds all movement: walking, exercise, digestion, fidgeting, even standing.

To get TDEE from BMR, multiply by an activity factor:

• Sedentary (little or no exercise): BMR × 1.2
• Lightly active (1–3 days/week): BMR × 1.375
• Moderately active (3–5 days/week): BMR × 1.55
• Very active (6–7 days/week): BMR × 1.725
• Extra active (physical job + training): BMR × 1.9

Using James from earlier (BMR 1,780): if he exercises 4 days per week (moderate activity), his TDEE is 1,780 × 1.55 = 2,759 calories per day. That is the number to use for weight maintenance, not the BMR itself.

Why Is My BMR So Low? Common Explanations

If you've used a BMR calculator and the number seems low, several factors may be at play. Age is the largest — BMR drops naturally as you age due to muscle loss, with an average decline of about 2–3% per decade after age 20. Body composition also plays a role: muscle tissue burns roughly 6 calories per pound per day at rest, while fat burns only 2 calories. Lower muscle mass equals a lower BMR.

Other factors include genetics, hormonal changes (especially thyroid function), and caloric restriction itself. When you severely restrict calories, your body adapts by lowering BMR to conserve energy — one reason why aggressive dieting often backfires. If you're wondering "why is my BMR so low explained," it's typically a combination of age, muscle mass, and dietary history. A free calculator can help you track changes over time as these variables shift.

BMR Calculator With Body Fat Percentage: Does It Improve Accuracy?

Some BMR calculators incorporate body fat percentage using the Katch-McArdle formula: BMR = 370 + (21.6 × lean body mass in kg). This can be more accurate for athletes or people with very high or low body fat, because it directly accounts for metabolically active muscle tissue.

However, body fat percentage is itself an estimate (calipers, bioelectrical impedance, DEXA scans all vary in accuracy). For most people, the Mifflin-St Jeor formula used in this guide is sufficiently accurate — within about 10–15% of measured BMR in laboratory conditions. If your body fat is known precisely, the Katch-McArdle BMR calculator with body fat percentage is worth considering, but for general use, Mifflin-St Jeor remains the gold standard.

Harris-Benedict vs. Mifflin-St Jeor: Which BMR Calculator Should You Use?

The original Harris-Benedict equation (developed in 1919 and revised in 1984) was the standard for decades. However, the Mifflin-St Jeor formula, developed in 1990, is now preferred because it was validated on a more diverse and modern population. Studies show Mifflin-St Jeor is accurate within 10% of measured BMR in about 70–80% of people, while Harris-Benedict tends to overestimate BMR by 5–15%, especially in overweight individuals.

For the Harris-Benedict vs Mifflin-St Jeor calculator decision: use Mifflin-St Jeor unless you are working with a professional who specifically requests the older equation. The free calculator on this page uses Mifflin-St Jeor exclusively. Both formulas are estimates and should not replace professional medical advice.

How Many Calories Do I Burn at Rest? Setting Realistic Expectations

The question "how many calories do I burn at rest" is another way of asking for BMR. For a typical adult, the range is approximately 1,200–1,800 calories per day for women and 1,500–2,200 for men, depending on age, size, and body composition.

A 30-year-old woman who is 5'4" and 140 lb has a BMR around 1,360 calories. A 40-year-old man who is 5'10" and 180 lb has a BMR around 1,750 calories. These are estimates — actual resting metabolism can vary by up to 200–300 calories due to genetic differences, even between people of identical size.

If you're using a BMR calculator to lose weight safely, the goal is to eat between your BMR and your TDEE, creating a moderate 300–500 calorie deficit. Dropping below BMR for extended periods can trigger metabolic adaptation and muscle loss. The free calculator shows both BMR and estimated TDEE ranges to help you set appropriate targets.

What About BMR for Athletes, Pregnant Women, or People Over 65?

The Mifflin-St Jeor formula is still a good starting point for most people, but certain populations may need adjustments. Athletes with very high muscle mass may find the Katch-McArdle formula (which uses body fat percentage) more accurate. Pregnant women have elevated BMR, especially in the second and third trimesters (roughly 200–300 additional calories per day). People over 65 may have slightly lower BMR than the formula predicts, though the age variable already accounts for some of this decline.

In all cases, these are estimates. A healthcare provider or registered dietitian can offer personalized guidance. The free calculator is a starting point, not a diagnosis.