What is BMR?
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest, just to keep you alive. Breathing, pumping blood, holding body temperature, replacing cells, running the brain — all of that costs energy, and BMR is the bill. For most adults, BMR accounts for 60–75% of daily calorie burn. The rest comes from movement, exercise, and digesting food.
BMR matters because it's the floor of any honest weight-management math. Eat below it for too long and your body interprets the deficit as a famine signal — metabolism slows, hunger climbs, and the diet stops working. Eat at or slightly above it, add activity on top, and you have a sustainable plan. The BMR Calculator gives you that floor in about three seconds, in metric or imperial, with no signup.
One example. A 35-year-old man, 80 kg, 180 cm, comes out to about 1,800 calories per day at rest. That's what his body needs to do nothing. The day he walks 8,000 steps, hits the gym, and digests three meals, he burns closer to 2,800. The difference between those two numbers is where every "how much should I eat?" question actually lives.
How to use the BMR Calculator
The calculator runs the Mifflin-St Jeor equation, which is the current ACSM and Academy of Nutrition and Dietetics standard, and shows the older Harris-Benedict number alongside it for reference. Nothing leaves your browser. Switch between metric (kg, cm) and imperial (lb, ft, in) with the toggle.
- Pick your biological sex. The formula constants differ between male and female because of average body-composition differences — more muscle mass burns more calories at rest, and the equations bake that average in.
- Enter age, weight, and height. BMR drops about 1–2% per decade after 30, mostly from muscle loss. Staying lean and active slows that curve.
- Read the Mifflin-St Jeor number — that's your BMR in calories per day.
- Pick an activity level from the dropdown. The TDEE shown below it is your maintenance calorie target.
- To lose weight: eat 300–500 below TDEE. To gain: eat 300–500 above. Don't drop below BMR for extended stretches.
The result updates as you change the inputs. No Calculate button.
The Mifflin-St Jeor formula
Two formulas show up in almost every BMR conversation online. Here they both are, in plain form.
Mifflin-St Jeor (1990, current standard):
Men: BMR = 10·kg + 6.25·cm − 5·age + 5
Women: BMR = 10·kg + 6.25·cm − 5·age − 161
Harris-Benedict (revised 1984):
Men: BMR = 88.362 + 13.397·kg + 4.799·cm − 5.677·age
Women: BMR = 447.593 + 9.247·kg + 3.098·cm − 4.330·age
Mifflin-St Jeor is roughly 5% more accurate for modern populations. Harris-Benedict came from a 1919 sample that doesn't look much like today's average body composition; the 1984 revision helped but didn't fix the underlying problem. The reason the older formula still shows up everywhere is inertia — most online calculators were written years ago and never updated. The BMR Calculator displays both numbers so you can see exactly where the difference comes from.
Worked example. The 35-year-old man, 80 kg, 180 cm, computed both ways:
- Mifflin-St Jeor: 10·80 + 6.25·180 − 5·35 + 5 = 800 + 1125 − 175 + 5 = 1,755 calories/day
- Harris-Benedict: 88.362 + 13.397·80 + 4.799·180 − 5.677·35 = 88.362 + 1071.76 + 863.82 − 198.7 = 1,825 calories/day
About a 70-calorie gap — that's the 4% spread between the two formulas in this case. Either one rounds to "about 1,800 calories per day at rest." For most practical purposes, treat the number as ±100.
BMR by age and sex
BMR scales with body size and drops with age. The table below shows Mifflin-St Jeor estimates for average-height adults at three weight points, by decade. It's a sanity-check, not a target — use the calculator with your real numbers for anything that matters.
| Age | Sex | Height | Weight | BMR (cal/day) |
|---|---|---|---|---|
| 25 | Male | 180 cm (5'11") | 75 kg (165 lb) | 1,755 |
| 25 | Female | 165 cm (5'5") | 60 kg (132 lb) | 1,371 |
| 35 | Male | 180 cm | 80 kg (176 lb) | 1,755 |
| 35 | Female | 165 cm | 65 kg (143 lb) | 1,371 |
| 45 | Male | 180 cm | 85 kg (187 lb) | 1,755 |
| 45 | Female | 165 cm | 70 kg (154 lb) | 1,371 |
| 55 | Male | 178 cm | 85 kg | 1,683 |
| 55 | Female | 163 cm | 70 kg | 1,308 |
| 65 | Male | 176 cm | 82 kg | 1,608 |
| 65 | Female | 161 cm | 68 kg | 1,233 |
The pattern is clean. Within a single decade, weight changes the number more than anything else — every extra kilogram adds 10 calories to BMR. Across decades, the age penalty is small per year but compounds: roughly 50 calories per decade in the formula, and probably a bit more in practice because most people quietly lose muscle as they age.
From BMR to TDEE — the activity multiplier
BMR is what you'd burn lying in bed all day. Total Daily Energy Expenditure (TDEE) is what you actually burn living a normal life. The two are related by a single multiplier:
TDEE = BMR × activity factor
The five standard activity factors are below. Pick honestly — most people underestimate sedentary and overestimate active, which is why "I'm eating my maintenance and gaining weight" is such a common refrain.
| Level | Multiplier | What it actually means |
|---|---|---|
| Sedentary | ×1.2 | Desk job, no exercise, less than 5,000 steps/day. Most office workers who don't go to the gym. |
| Lightly active | ×1.375 | Desk job + light exercise 1–3 days a week, or a moderately active job (teacher, retail). |
| Moderately active | ×1.55 | Exercise 3–5 days a week at meaningful intensity. The realistic average for committed gym-goers. |
| Very active | ×1.725 | Hard exercise 6–7 days a week, or a physical job (construction, nursing, warehouse). |
| Extra active | ×1.9 | Physical job AND daily training. Endurance athletes, military, manual labor + gym. |
For the 35-year-old man with a 1,755 BMR:
- Sedentary: 1,755 × 1.2 = 2,106 cal/day
- Moderately active: 1,755 × 1.55 = 2,720 cal/day
- Very active: 1,755 × 1.725 = 3,027 cal/day
That's a 900-calorie spread depending on activity level alone — roughly two extra meals. If you're picking the wrong bucket, you'll be off by enough that the scale won't move the way you expect. When in doubt, choose one level below what feels right and adjust based on actual weight changes over 4–6 weeks. The scale is more honest than your estimate.
How to use BMR for weight goals
Once you have BMR and TDEE, the math is short.
- Maintain — eat at TDEE.
- Lose weight — eat 300–500 below TDEE. That's a 0.5–1 lb/week loss. Sustainable, doesn't crash metabolism, doesn't burn through muscle.
- Gain weight — eat 300–500 above TDEE. Pair with strength training so the gain is muscle rather than fat.
Bigger deficits work short-term and rebound. Your body adapts by lowering BMR (metabolic adaptation) and ramping hunger hormones. The 1,200-calorie diet is famous for a reason: it sits below BMR for most women, which is why it produces fast initial loss followed by reliable regain. The slower path is the more reliable one.
Recompute after 8–12 weeks. As you lose weight, your TDEE drops — the same calorie intake creates a smaller deficit, and progress stalls. Update the number, adjust intake, keep going.
If you're stuck on a plateau and the scale won't budge: first, check that you've actually been eating what you think. Most plateau cases come down to portion drift, not metabolic mystery. A weekend of honest food logging usually finds the missing 300 calories. After that, recompute TDEE at your new weight, and consider dropping the activity multiplier one level — most people overestimate it.
Where BMR estimates fall short
The formula is a population average. Your individual BMR can sit ±10% from the prediction depending on body composition, thyroid function, and genetics. Specific cases worth knowing about:
- Very muscular people — BMR is higher than the formula predicts because muscle burns ~6 calories/lb/day at rest while fat burns ~2. The Katch-McArdle formula uses lean body mass instead of total weight and is more accurate if you know your body fat percentage from a DEXA scan or accurate measurement.
- Very lean or very heavy people — Mifflin-St Jeor was calibrated on average builds and gets less accurate at the extremes.
- Thyroid conditions — hypothyroidism can lower real BMR by 10–20% below the formula's prediction; hyperthyroidism raises it. If your weight isn't responding to a sensible plan, ask your doctor about a thyroid panel.
- After significant weight loss — research on participants from The Biggest Loser shows BMR can stay suppressed for years after major weight loss, well below what the formula predicts. The body remembers.
- Pregnancy and lactation — both raise calorie needs by hundreds per day; don't use the standard BMR formula as a target during either.
None of this makes the calculator useless. It just means the output is a starting point — a sane first guess to build a plan around, then refine using what actually happens on the scale over a few weeks.
Related calculations
BMR fits into a small family of body-composition tools that work well together:
- Calorie Calculator — same engine wrapped in a different question: "how many calories should I eat to reach a goal weight by a target date?"
- BMI Calculator — quick screening number based on height and weight alone. Useful as a sanity check; not a substitute for body-composition data.
- Weight Converter — kilograms to pounds, pounds to stones, and back, for when the labels on your scale and your nutrition app disagree.
Frequently asked questions
What's the difference between Mifflin-St Jeor and Harris-Benedict?
Both are equations derived from observed-and-measured calorie burn at rest. Harris-Benedict came from a 1919 study (revised in 1984), Mifflin-St Jeor from a 1990 study with more diverse subjects. Mifflin-St Jeor is generally about 5% more accurate for modern populations and is what the ACSM and the Academy of Nutrition and Dietetics recommend today. The calculator shows both because Harris-Benedict is still widely cited — if another site gave you a different number, that's probably why.
What's the difference between BMR and TDEE?
BMR is the calories your body burns at complete rest. TDEE is what you burn in a normal day, including walking, work, exercise, and digesting food. TDEE = BMR × activity factor. Activity factors range from 1.2 (sedentary) to 1.9 (extra active). For most people who exercise three to five days a week, 1.55 is the closest match.
Can I eat below my BMR?
Not for long. Below-BMR diets trigger metabolic adaptation: the body slows non-essential processes (immune function, hair growth, libido, hormones) to fit the lower intake. Short term it works. Long term you regain the weight when you stop, often above starting weight, because your maintenance calories have shifted downward. The 1,200-calorie-a-day diet sits below BMR for most women, which is why its long-term success rate is so poor.
Why does my BMR differ from another calculator?
Different formulas give different numbers — a 5–10% spread between Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle is normal. Katch-McArdle is more accurate if you know your body-fat percentage because it uses lean body mass. For most people without a DEXA scan, Mifflin-St Jeor is the best estimate. The only truly accurate BMR measurement is done with indirect calorimetry in a lab — accurate but rarely worth the cost outside of research.
Does muscle mass change my BMR?
Yes, but less than the popular advice suggests. A pound of muscle burns about 6 calories/day at rest; a pound of fat burns about 2. Adding 10 lb of lean muscle adds roughly 40 calories/day to BMR. Real, but not enormous. The bigger reason muscle helps weight management is that building it is metabolically expensive (you eat more during gain phases) and the act of training itself burns calories — both of those show up in the activity multiplier, not in BMR.
How often should I recompute?
Every 8–12 weeks if you're actively trying to change your weight. BMR drops as you lose weight (less tissue to keep alive) and rises as you gain. Using a stale TDEE is the most common reason a previously-working diet stops working: the deficit shrank without you noticing.
Is the activity multiplier accurate?
Roughly. Most people underestimate sedentary and overestimate active. If you sit at a desk 8 hours and don't exercise, you're sedentary (×1.2), not lightly active. If you train hard 3–5 times a week, moderately active (×1.55) is right. If you train daily and have a physical job, extra active (×1.9). When in doubt, pick one level below what you think — better to overestimate your deficit than underestimate. Real-world TDEE varies by ±10% within an activity bucket, so treat the number as a starting point and adjust based on actual scale changes over 4–6 weeks.