Understanding the difference between resting metabolic rate (RMR) and basal metabolic rate (BMR) is like comparing a simmer to a boil—both measure how your body burns energy at rest, but under slightly different conditions. This guide breaks down the science, how each is measured, and why the numbers matter for your daily calorie needs. Whether you’re tracking macros or just curious, you’ll leave with a clear picture of your body’s energy engine.
Ingredients
- 1 basal metabolic rate (BMR) measurement (e.g., from a morning fasted test)
- 1 resting metabolic rate (RMR) measurement (e.g., from a relaxed, non-fasted state)
- 1 body weight scale (accurate to 0.1 kg or 0.2 lb)
- 1 body composition analyzer (optional, for lean mass)
- 1 stopwatch or timer (for tracking measurement duration)
- 1 comfortable chair or bed (for RMR test)
- 1 fasting period of 8–12 hours (for BMR test)
- 1 quiet, dimly lit room (for both tests)
Instructions
- Understand the definitions. BMR is the energy your body needs to maintain vital functions (breathing, circulation, cell production) while completely at rest, measured after 8–12 hours of fasting and 8 hours of sleep, in a supine position, in a thermoneutral environment. RMR is similar but measured under less strict conditions—you can be seated or reclined, and fasting is only required for 3–4 hours. RMR is typically 10–20% higher than BMR because it includes the energy cost of recent food intake and slight muscle activity.
- Prepare for a BMR measurement. Schedule your test for early morning, after a full night’s sleep (at least 8 hours) and an overnight fast of 8–12 hours. Avoid caffeine, nicotine, and exercise for at least 12 hours prior. Wear light clothing and arrive at the lab without rushing.
- Prepare for an RMR measurement. For RMR, you only need to avoid heavy exercise for 24 hours and fast for 3–4 hours. You can be seated or reclined, but you must remain still and awake. No need to sleep overnight in a lab.
- Perform the measurement (if using indirect calorimetry). You’ll breathe into a mask or mouthpiece connected to a metabolic cart that measures oxygen consumption and carbon dioxide production. For BMR, lie supine in a dark, quiet room for 30 minutes, with the last 20 minutes used for data collection. For RMR, sit or recline comfortably for 20–30 minutes, with the last 10–15 minutes for data. The machine calculates your metabolic rate in calories per day.
- Record your numbers. Write down your BMR and RMR values. For example, a typical adult might have a BMR of 1,500 kcal/day and an RMR of 1,650 kcal/day. These numbers are your baseline for calorie planning.
- Calculate your total daily energy expenditure (TDEE). Multiply your RMR by an activity factor (e.g., 1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for very active). This gives you the calories needed to maintain your current weight. Use BMR only if you’re calculating a strict bed-rest scenario.
Tips
- Substitutions: If you don’t have access to a metabolic cart, use predictive equations like the Mifflin-St Jeor formula for BMR (men: 10×weight in kg + 6.25×height in cm – 5×age + 5; women: –161). For RMR, add ~10% to the BMR estimate. These are approximations, not exact measurements.
- Common mistake: Confusing the two terms in conversation or on fitness apps. Many devices report RMR but label it as BMR. Always check the measurement conditions before trusting the number.
- Practical tip: For consistent tracking, measure RMR at the same time of day, under the same conditions (e.g., 4 hours after a light meal, in the afternoon). This reduces variability.
- Safety note: These measurements are non-invasive and safe. However, if you have a respiratory or cardiovascular condition, consult your doctor before undergoing indirect calorimetry.
Serving Suggestions
Use your RMR as the foundation for a balanced meal plan. For weight maintenance, pair your TDEE with a diet of whole foods: lean proteins (chicken breast, tofu), complex carbs (quinoa, sweet potatoes), and healthy fats (avocado, olive oil). For weight loss, create a deficit of 300–500 calories per day, but never eat below your BMR without medical supervision. For muscle gain, add 200–300 calories above TDEE with extra protein.
Variations
- BMR via direct calorimetry: A research-only method that measures heat produced in a sealed chamber. Not practical for home use but gives the gold-standard BMR.
- RMR with body composition: Some devices estimate RMR using bioelectrical impedance, which factors in lean mass. This can be more accurate than age-based formulas for athletes.
- Sleep metabolic rate (SMR): A subset of BMR measured during sleep, often used in sleep studies. It’s slightly lower than BMR due to reduced muscle tone.
Storage and Reheating
Your metabolic rate numbers are not perishable, but they do change with your body. Recalculate every 4–6 weeks or after a significant weight change (5–10 lb / 2–5 kg). Store your measurements in a journal or app to track trends. There’s no reheating involved—just re-measure when needed. For accurate comparisons, always use the same method (e.g., same formula or same lab).
Frequently Asked Questions
Which is higher, BMR or RMR? RMR is usually higher because it includes the thermic effect of food and slight activity. In practice, RMR is about 10–20% above BMR. For example, if your BMR is 1,400 kcal, your RMR might be 1,540–1,680 kcal.
Can I measure BMR at home? Not accurately without a metabolic cart. Home devices like smart scales estimate BMR using formulas, which can be off by 10–15%. For most people, RMR from a fitness tracker is sufficient for planning.
Why does my fitness tracker show a different number? Trackers typically estimate RMR using algorithms based on your age, weight, and height, but they don’t account for muscle mass or recent meals. That’s why they can differ from lab measurements.
How often should I recalculate my RMR? Recalculate after any weight change of 5–10 lb (2–5 kg) or if you change your activity level significantly. For weight loss, recalculate every 4–6 weeks to adjust your calorie goals.
Does fasting affect RMR? Yes, fasting lowers RMR because the body reduces energy expenditure to conserve fuel. That’s why BMR is measured after a longer fast, while RMR is measured after a shorter one. Eating a meal can temporarily raise RMR by 5–10% for a few hours.
Is it safe to eat below my BMR? For most people, eating below BMR for extended periods can lead to muscle loss, nutrient deficiencies, and a slowed metabolism. Unless under medical supervision, aim to eat at least your BMR in calories.
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