Most people burn between 1,200 and 1,800 calories a day doing absolutely nothing. This resting calorie burn, often called your resting metabolic rate (RMR), accounts for 60% to 70% of all the energy your body uses daily. It fuels everything from breathing and circulating blood to maintaining body temperature and keeping your brain running.
What Your Body Spends Energy On at Rest
Even when you’re lying on the couch, your organs are working hard. Your heart and kidneys are the most energy-hungry tissues in your body, burning roughly 440 calories per kilogram of tissue per day. Your brain comes next at about 240 calories per kilogram, followed closely by your liver at 200. These organs are small relative to your total body weight, but together they account for most of your resting calorie burn.
Skeletal muscle, by contrast, burns only about 13 calories per kilogram per day at rest. Fat tissue burns even less: around 4.5 calories per kilogram. The reason muscle still matters for your metabolism is simple math. You carry far more muscle than brain or kidney tissue, so even at a low per-kilogram rate, your muscles collectively contribute a meaningful share of your resting burn. This is why people with more muscle mass tend to have a higher resting metabolic rate, and why losing muscle (through crash dieting or aging) can lower it.
How to Estimate Your Number
The most widely used formula for estimating resting calories is the Mifflin-St Jeor equation. For women, it looks like this:
(9.99 × weight in kg) + (6.25 × height in cm) – (4.92 × age in years) – 161
For men, the formula uses the same structure but swaps the final constant to + 5 instead of – 161.
To put that in real terms: a 35-year-old woman who weighs 150 pounds (68 kg) and stands 5’5″ (165 cm) would get a resting metabolic rate of roughly 1,390 calories per day. A 35-year-old man at 180 pounds (82 kg) and 5’10” (178 cm) would land around 1,775 calories.
If you know your body fat percentage, the Katch-McArdle formula may give a more personalized result because it’s built on lean body mass rather than total weight: 370 + (21.6 × lean body mass in kg). This makes it particularly useful for people who carry significantly more muscle or significantly more fat than average, since the standard formulas can miss the mark for those groups.
Why Your Number Differs From Someone Else’s
Several factors push your resting calorie burn higher or lower:
- Body size and composition. A larger body requires more energy to maintain. Within two people of the same weight, the one with more muscle will burn more at rest.
- Age. Metabolism holds relatively steady through middle age, then begins a gradual decline. Research published in Science found that resting metabolic rate starts dropping meaningfully around age 46, with total daily energy expenditure declining from about age 63 onward, at a rate of roughly 0.7% per year.
- Sex. Men typically have higher resting metabolic rates than women, largely because they tend to carry more muscle mass.
- Thyroid function. Your thyroid hormones, particularly T3, directly regulate how much oxygen your cells consume and how much heat they produce. An underactive thyroid slows this process down, lowering your resting burn. An overactive thyroid speeds it up.
- Genetics. Individual variation in organ size, hormone levels, and cellular efficiency means two people of identical height, weight, age, and sex can still differ by a few hundred calories per day.
How Accurate Are These Formulas?
For healthy adults at a normal weight, predictive equations are reasonably reliable. They’ll typically land within 10% of your true resting metabolic rate. But accuracy drops sharply at the extremes. People with a BMI below 16 or above 40 can see much larger errors, and chronic conditions like kidney disease, liver cirrhosis, or COPD can shift resting metabolism in ways no formula accounts for.
The gold standard for measuring your actual number is indirect calorimetry. During this test, you breathe into a mask or canopy hood for 15 to 30 minutes while a device measures the oxygen you consume and the carbon dioxide you produce. From those two numbers, a precise calorie figure is calculated. Some gyms, sports performance clinics, and dietitian offices offer this test, and handheld consumer devices like the MedGem and Breezing have made at-home versions more accessible, though less precise than clinical-grade equipment.
Resting Calories vs. Total Daily Calories
Your resting metabolic rate is the biggest piece of your daily calorie burn, but it’s not the whole picture. Total daily energy expenditure includes three components: your resting metabolism (60% to 70%), the energy your body uses to digest food (about 10%), and the calories burned through movement and exercise (the remaining 20% to 30% for a moderately active person).
This means a person whose resting burn is 1,500 calories might actually use 2,000 to 2,500 calories in a full day, depending on activity level. If you’re trying to understand your calorie needs for weight management, your resting rate is the foundation, but you’ll want to multiply it by an activity factor (typically 1.2 for sedentary up to 1.9 for very active) to get a realistic daily total.
Can You Raise Your Resting Burn?
The most effective lever you have is building or preserving muscle. Because muscle tissue burns roughly three times as many calories per kilogram as fat tissue at rest, shifting your body composition toward more lean mass raises your baseline. Strength training is the most direct way to do this, and it becomes especially important after your 40s when muscle loss naturally accelerates.
Extreme calorie restriction works against you here. When you eat well below your resting metabolic rate for extended periods, your body adapts by lowering its energy output, a process sometimes called metabolic adaptation. Your resting burn can drop beyond what weight loss alone would predict, making further fat loss harder and weight regain easier. Moderate calorie deficits paired with resistance training help preserve muscle and keep your resting metabolism from falling unnecessarily.