Your RMR, or resting metabolic rate, is the number of calories your body burns just to keep you alive while you’re at rest. It fuels breathing, circulation, cell repair, and brain function, and it accounts for the largest portion of the calories you burn each day. For most people, RMR falls somewhere between 1,200 and 2,000 calories per day, though it varies widely based on body size, age, sex, and muscle mass.
RMR vs. BMR
You’ll often see RMR and BMR (basal metabolic rate) used interchangeably, and they measure nearly the same thing. The difference is technical: BMR is measured under stricter conditions, typically first thing in the morning after an overnight fast, 24 hours without exercise, in a completely rested and stress-free state. RMR allows slightly more flexibility. It can be measured in a sitting or lying position after at least 15 minutes of rest, with no exercise in the prior 12 hours. Because of those looser conditions, RMR tends to run a few percent higher than BMR, but the two numbers are close enough that most calculators and nutrition guides treat them as the same.
How to Estimate Your RMR
The most widely recommended formula is the Mifflin-St Jeor equation. It uses your weight in kilograms, height in centimeters, age in years, and sex:
RMR = (9.99 × weight) + (6.25 × height) – (4.92 × age) + 166 for men, or – 161 for women.
So a 35-year-old woman who weighs 68 kg (150 lbs) and is 165 cm (5’5″) tall would calculate: (9.99 × 68) + (6.25 × 165) – (4.92 × 35) – 161 = roughly 1,400 calories per day. A man of the same size and age would get about 1,570, since the equation adds 166 instead of subtracting 161.
This is an estimate. The gold standard for measuring RMR is indirect calorimetry, a clinical test where you breathe into a mask or canopy for 15 to 30 minutes while a machine analyzes how much oxygen you consume and carbon dioxide you produce. Even the best lab equipment shows day-to-day variation of about 3 to 5%, which translates to roughly 50 to 75 calories. Prediction equations can be off by more than that, especially for people with very high or very low body fat.
What Determines Your RMR
Lean body mass is the single biggest factor. Muscle tissue is metabolically active even at rest, requiring energy to maintain itself. This is why two people of the same weight can have noticeably different RMRs: the one with more muscle burns more calories doing nothing.
Thyroid hormones play a central regulatory role. Your thyroid produces hormones that control how quickly your cells produce energy and generate heat. When thyroid function is elevated (hyperthyroidism), resting energy expenditure increases, often causing weight loss. When thyroid function is low (hypothyroidism), metabolism slows, leading to weight gain and fatigue. The mechanism involves how efficiently your cells use oxygen to produce energy. Thyroid hormones increase the “leak” of energy as heat in your cells, meaning your body burns more fuel just to maintain normal function.
Sex matters because men typically carry more muscle mass relative to body fat. Age matters too, though not as dramatically as people assume during midlife.
How RMR Changes With Age
A large study analyzing thousands of people across the lifespan found that metabolism stays remarkably stable throughout adulthood, only beginning to decline after age 60. The drop is gradual: about 0.7% per year. That means a person in their 90s needs roughly 26% fewer calories per day than someone in midlife, but a 45-year-old and a 55-year-old have nearly identical metabolic rates when you account for body composition. The midlife weight gain many people experience is more likely driven by changes in activity level, diet, and muscle loss than by a crashing metabolism.
Metabolic Adaptation During Weight Loss
When you eat significantly fewer calories than your body needs, your RMR drops. This isn’t just because you weigh less and have less tissue to fuel. Your body also actively slows its metabolism beyond what the tissue loss alone would explain. Researchers call this metabolic adaptation, or adaptive thermogenesis.
In one study, participants who lost an average of 7.3 kg (about 16 lbs) saw their RMR drop by roughly 100 calories per day. About 60% of that reduction came from simply having less body tissue to maintain. The remaining 40%, around 40 calories per day, was the body’s adaptive response, essentially becoming more fuel-efficient in response to the calorie deficit. In people who lose very large amounts of weight, this adaptive component can reach 200 to 400 calories per day.
This is one reason weight loss often stalls and why maintaining weight loss can feel harder than losing it in the first place. Your body is burning fewer calories than a same-sized person who was never in a deficit.
How Temperature Affects Your RMR
Your body burns more calories in cold environments because it works harder to maintain core temperature. Research comparing energy expenditure at different room temperatures found that people burned about 96 more calories per day at 18°C (64°F) compared to a comfortable 28°C (82°F). Even a mildly cool room at 22°C (72°F) increased expenditure by about 73 calories. Interestingly, heat exposure at 38°C (100°F) did not significantly raise or lower RMR compared to a comfortable temperature. Cold makes your body work harder; heat does not.
From RMR to Total Daily Calories
Your RMR tells you what your body burns at rest, but you also burn calories through physical activity and digesting food. To estimate your total daily energy expenditure (TDEE), multiply your RMR by an activity factor:
- Sedentary (desk job, little exercise): RMR × 1.2
- Lightly active (exercise 1 to 3 days per week): RMR × 1.375
- Moderately active (exercise 3 to 5 days per week): RMR × 1.55
- Very active (hard exercise 6 to 7 days per week): RMR × 1.725
- Extremely active (intense training plus physical job): RMR × 1.9
Using the earlier example of a 35-year-old woman with an RMR of 1,400: if she exercises moderately three to five days a week, her estimated TDEE would be about 2,170 calories per day. That’s the number she’d use as a starting point for maintaining her current weight.
The Role of What You Eat
Digesting food itself burns calories, a process called the thermic effect of food. Not all macronutrients cost the same amount of energy to process. Protein has the highest thermic effect by far: your body uses 20 to 30% of the calories in protein just to digest and absorb it. Carbohydrates cost 5 to 10%, and fat costs 0 to 3%. This is one reason higher-protein diets tend to support slightly greater overall energy expenditure. If you eat 200 calories of protein, your body might spend 40 to 60 of those calories on digestion alone, compared to just 0 to 6 calories for the same amount of fat.
What You Can Actually Change
Most of the factors that determine your RMR, like age, sex, and genetics, are outside your control. But the biggest modifiable factor is lean body mass. Resistance training that builds or preserves muscle will keep your RMR higher over time, which is especially relevant during weight loss, when the body tends to shed both fat and muscle. Eating adequate protein supports this by both preserving muscle and costing more energy to digest.
Crash dieting works against you on both fronts. Severe calorie restriction accelerates muscle loss and triggers greater metabolic adaptation, leaving you with a lower RMR than a more moderate approach would. A slower rate of weight loss, combined with strength training and sufficient protein, minimizes the metabolic penalty.