BMR vs TDEE - where your calories actually go
BMR is what you burn lying still, 60-70% of your total. TDEE adds movement and food digestion. You can't change BMR easily, but you can grow the activity slice.
People throw around BMR and TDEE like they mean the same thing. They don't. BMR is what your body burns just to stay alive, heart beating, lungs breathing, brain thinking, while you lie perfectly still. TDEE is the whole day's bill: BMR plus every step you take plus the energy your body spends digesting your food. Knowing the difference shows you exactly where your calories go, and which slice you can actually move.
What your body burns at rest (BMR)
BMR is the biggest chunk of your day, about 60 to 70 percent of everything you burn. It keeps your heart beating, your lungs breathing, your liver filtering, your brain running. You can estimate it with the Mifflin-St Jeor formula: for a man, 10 x weight(kg) + 6.25 x height(cm) - 5 x age + 5; for a woman, the same but minus 161. The number is mostly set by your size, your muscle, your age, and your sex. You can't flip a switch to raise it much, and crash-dieting can even push it down a little as your body fights back.
Assumptions: BMR via Mifflin-St Jeor for a 60 kg, 160 cm, 30-year-old woman: 10x60 + 6.25x160 - 5x30 - 161 = 1289 kcal, rounded to 1300. The 60-70% share of TDEE is from the National Academies DRI 2023. For a man of the same size the formula gives ~1451 kcal. Individual BMR varies with muscle mass, age, and sex; a severe calorie cut can lower it further through adaptive thermogenesis.
The slice you can actually grow: add a 30-minute walk
BMR is mostly fixed. The food you log controls the intake side. The slice you can move is activity. A 30-minute brisk walk for a 60 kg adult burns roughly 115 kcal, and it grows the activity slice of your TDEE without touching BMR. Pair that with your food deficit and both levers work, less in, a little more out. Burnie logs your food and shows your deficit; the walk simply adds to the burn side.
Assumptions: A 60 kg (~132 lb) adult walking briskly at 3.5 mph for 30 min burns roughly 115 kcal, interpolated from the Harvard Health chart (107 kcal at 125 lb, 133 kcal at 155 lb). This sits inside the physical-activity slice of TDEE, which ranges from ~15% for sedentary people up to ~50% for very active people per the National Academies DRI. BMR itself is unchanged by the walk.
How to grow the activity slice without a gym
- Take a 30-minute walk after lunch or dinner. It is the simplest way to add to your daily burn.
- Stand up and move for a minute every hour if you sit at a desk. These small breaks add up as daily movement.
- Take the stairs for one or two floors instead of the lift. Short climbs burn more than flat walking.
- Add two short strength sessions a week, squats, push-ups, or carrying groceries. Muscle helps your BMR stay higher over time.
- Do not chase a metabolism hack. The deficit from your food does the heavy lifting; movement adds to it.
The bottom line
Your calories split three ways: BMR takes the biggest share at 60 to 70 percent, activity takes 15 to 30 percent or more if you move a lot, and food digestion takes about 10 percent. You can't easily change BMR. You can grow the activity slice with daily movement, and you can control intake with the food you log. Burnie shows your food and your deficit, that is the engine. The walk is a quiet bonus on top.