RMR / BMR Calculator (Mifflin-St Jeor)
Resting Metabolic Rate (RMR/BMR) calculator. Mifflin-St Jeor + Harris-Benedict + Katch-McArdle side-by-side. Plus TDEE at 5 activity levels.
RMR Calculator
3 equations compared
TDEE at activity levels
| Sedentary (×1.2) | — |
| Lightly active (×1.375) | — |
| Moderately active (×1.55) | — |
| Very active (×1.725) | — |
| Extra active (×1.9) | — |
How to use the RMR calculator
Enter sex, age, weight, height
Standard inputs. Weight in kg (lbs × 0.4536). Height in cm (inches × 2.54). All three Mifflin/Harris-Benedict/Katch-McArdle equations use these.
Enter body fat % (optional)
Only used by Katch-McArdle (which uses lean body mass instead of total weight). If unknown, estimate from BMI: 22 BMI ≈ 15% bf (male) or 22% (female); 30 BMI ≈ 25% (male) or 32% (female). Leaner = higher RMR/kg.
Read RMR (kcal/day at rest)
Mifflin-St Jeor (1990): current scientific gold standard. Harris-Benedict (revised 1984): classic; tends to overestimate by 5-10%. Katch-McArdle: best for athletes + lean individuals because it uses fat-free mass. Typical adult RMR: 1,200-1,800 kcal/day for women, 1,500-2,200 for men.
Apply activity multiplier for TDEE
Total Daily Energy Expenditure (TDEE) = RMR × activity multiplier. Sedentary (1.2): desk job, no formal exercise. Lightly active (1.375): 1-3 days/week light exercise. Moderately active (1.55): 3-5 days. Very active (1.725): 6-7 days hard. Extra active (1.9): athlete + physical job. Most adults overestimate their activity level — choose conservatively.
Use TDEE for weight management
Maintenance: eat ~TDEE calories. Cut (lose weight): TDEE − 500 kcal/day ≈ 0.5 kg/week loss. Bulk (gain weight): TDEE + 250-500 kcal/day. Re-calculate every 5-10 kg of weight change since RMR scales with body mass.
RMR — what your body burns just to exist
Resting Metabolic Rate (RMR) — sometimes used interchangeably with Basal Metabolic Rate (BMR) though technically slightly different — is the energy your body burns at complete rest, performing only essential functions: breathing, circulation, cellular maintenance, brain activity, organ function. RMR accounts for ~60-75% of total daily energy expenditure in most sedentary adults. The remaining 25-40% comes from physical activity + the thermic effect of food (TEF, ~10% of calories burned digesting). Knowing your RMR is the foundation of any structured nutrition plan — for weight loss, weight gain, or athletic performance. Estimating it accurately matters more than people realise.
Why Mifflin-St Jeor replaced Harris-Benedict
Harris-Benedict (originally 1919, revised 1984) was the de facto standard for 70+ years. Mifflin-St Jeor 1990 specifically targeted the era\'s changing body composition + measurement accuracy issues. Multiple validation studies (e.g. Frankenfield 2003) showed Mifflin-St Jeor has the smallest mean prediction error (~10% under-estimate average, vs Harris-Benedict\'s ~5-10% over-estimate). The American Dietetic Association adopted Mifflin-St Jeor as the recommended formula. Most modern fitness apps, RDs + sports nutritionists use it. Harris-Benedict still appears in older textbooks + some gyms. The difference is usually 50-150 kcal/day — small in isolation, meaningful over weeks.
Mifflin-St Jeor is the most accurate population-level RMR equation. Individual RMR variation is ±10-15% around prediction. For precise personalised values, get an indirect calorimetry measurement at a sports performance center.
The Katch-McArdle advantage for lean individuals
The Mifflin + Harris-Benedict equations use total body weight. This systematically underestimates RMR for very lean / muscular individuals + overestimates for high body-fat individuals — because adipose tissue burns far less per kg than muscle. Katch-McArdle uses fat-free mass (FFM = total weight × (1 − bodyFat%)): RMR = 370 + 21.6 × FFM. For a 90 kg athlete with 10% body fat (FFM = 81 kg), Katch-McArdle predicts 2,120 kcal while Mifflin might predict 1,950 kcal — a meaningful 170 kcal/day gap that adds up to 14,000 kcal over 12 weeks (~2 kg fat loss equivalent). For lean athletes, Katch-McArdle is the better choice; for general population without known body fat %, Mifflin is reasonable.
Activity multiplier reality check
Most people overestimate their activity level dramatically. Self-reported "moderately active" is usually closer to "lightly active" by objective accelerometer data. Office workers who train 3-4 times/week often genuinely fall into "lightly active" (×1.375) — the desk-job baseline is sedentary. "Very active" should be reserved for: 6+ days/week structured training PLUS active occupation (construction, nursing, parenting toddlers). Athletes during peak training: extra active (×1.9) genuine. Be honest with yourself; weight management failures often trace back to overstated activity assumptions.
10 Things to Know About RMR
Mifflin-St Jeor 1990: current scientific standard. Replaced Harris-Benedict in clinical + research practice.
RMR ≈ 60-75% of total daily energy expenditure. Activity is the variable portion.
Muscle tissue burns ~3× more per kg than fat tissue. Body composition matters more than total weight.
RMR declines ~1-3% per decade after age 30 — mostly due to muscle loss, not direct aging.
Katch-McArdle uses fat-free mass — best for lean / muscular individuals.
Indirect calorimetry (lab measurement): ±5% accurate. Equations: ±10-15%. Individual variation is real.
Thermic effect of food: protein 20-30% TEF, carbs 5-10%, fat 0-3%. High-protein diets slightly boost TDEE.
"Starvation mode" is overstated. Severe caloric restriction reduces RMR 10-15%, not 30-50% as popular claims.
NEAT (Non-Exercise Activity Thermogenesis) — fidgeting, posture, etc. — varies 200-800 kcal/day between people.
Resistance training preserves RMR during fat loss; cardio alone reduces RMR more. Body composition strategy matters.
Frequently asked questions
BMR is measured immediately after waking, in a thermoneutral environment, in the post-absorptive state (12+ hours fasted). RMR is measured at rest under less strict conditions — typically 4-5% higher than BMR. In practice the terms are used interchangeably; this calculator gives RMR-equivalent values.
Two reasons: (1) Less body mass = less RMR (smaller body burns less). (2) "Adaptive thermogenesis" — RMR drops slightly more than mass loss alone predicts (~10% additional reduction), as your body conserves energy. Recalculate RMR every 5-10 kg change.
Yes, modestly. Build muscle (resistance training): each kg of muscle adds ~15 kcal/day to RMR. Adequate protein (1.6-2.2 g/kg/day): supports muscle + has higher TEF. Don\'t crash diet: severe restriction reduces RMR. Sleep + manage stress: chronic cortisol elevation can suppress thyroid function + RMR.
Different equations + different activity-multiplier conventions. Apple Health uses Mifflin-St Jeor + step-based activity. Garmin uses proprietary HR + acceleration. Whoop uses Strain Score. None are precisely accurate to your individual physiology. Use any one consistently; don\'t mix sources.
Hypothyroidism can reduce RMR 15-30% vs equation predictions. Hyperthyroidism can elevate 20-40%. If you suspect thyroid issues (fatigue, weight changes despite consistent diet, hair changes, temperature dysregulation), get TSH + Free T4 testing. Equation-based RMR doesn\'t apply if thyroid is dysregulated.
The phrase is dramatic; the underlying phenomenon (adaptive thermogenesis) is real but modest. Severe caloric restriction reduces RMR 10-15% beyond what mass loss predicts. After a "diet break" + return to maintenance, RMR rebounds substantially over 4-12 weeks. Sustainable rate of loss (0.5-1 kg/week) avoids most of this issue.
Track results over 2-3 weeks. If you\'re losing weight on stated TDEE − 500, the multiplier is right. If maintaining weight on stated TDEE − 500, the multiplier is overestimating. Adjust down a tier (×1.55 → ×1.375) and retest. Calorie tracking accuracy + label inaccuracy can also contribute.
No. All inputs stay in your browser.
Yes, materially. Females typically have higher body fat % and lower muscle mass at the same weight as males, leading to lower RMR. Equations adjust ~150-200 kcal/day. For transgender individuals on hormone therapy, body composition shifts over months — use the equation that matches your current body composition / metabolic state.
Mifflin MD. Am J Clin Nutr 1990;51:241. Frankenfield D. J Am Diet Assoc 2003 (validation study). Eric Helms\' "The Muscle and Strength Pyramid" for evidence-based nutrition. Lyle McDonald\'s website for technical metabolism content.
Related News
You may be interested in these recent stories from our newsroom.
-
The FDA wants clinical trials to report in real time, with AI doing the watching
The US Food and Drug Administration has started two pilot trials that send their results to the agency as the data comes in, rather than in...
-
Bayesian Health Wins First FDA Clearance for Continuous AI Sepsis Monitor, Study Shows 18% Mortality Drop
Bayesian Health's TREWS system became the first AI device cleared by the FDA to continuously monitor all hospitalised patients for sepsis —...
-
Commure Raises US$70M at US$7B Valuation to Deploy AI Agents Across Hospital Administration
On 19 May 2026, Commure closed a US$70 million round at a US$7 billion post-money valuation, led by General Catalyst. Its AI agents already...
75 more free tools
Calculators, converters, security tools — no signup.