How Many Calories Do You Need

Daily calorie needs come from four parts: resting metabolism, the energy used to digest food, planned exercise, and every bit of movement in between. A 30-year-old woman at 65 kg and 165 cm who walks 8,000 steps and trains twice a week maintains on roughly 2,100 kcal. This is general information, not medical advice. If you have a health condition, are pregnant, or are under 18, ask a doctor or dietitian for a target that fits you.

The four parts of daily energy use

Resting metabolic rate supplies 60 to 70 percent of daily burn and is the cost of running organs at complete rest. Digesting food, called the thermic effect, takes about 10 percent overall: protein costs 20 to 30 percent of its own calories to process, carbohydrate 5 to 10 percent, fat 0 to 3 percent. Planned exercise and all other movement make up the remaining 20 to 30 percent, and that share varies most between two people of the same height and weight.

Movement outside the gym, grouped as non-exercise activity thermogenesis, shifts 300 to 800 kcal a day between adults. Fidgeting, standing on trains, walking to the shop and pacing during phone calls all count. A desk worker sitting 10 hours and a nurse on her feet 10 hours can share a height and weight and differ by 1,000 kcal of output, which is why one maintenance number never fits two people.

Estimating with the Mifflin-St Jeor equation

For men, resting rate is 10 x weight in kg, plus 6.25 x height in cm, minus 5 x age, plus 5. For women it is the same calculation with minus 161 at the end. Worked example one: a 34-year-old man at 82 kg and 180 cm gives 820 + 1,125 - 170 + 5 = 1,780 kcal at rest. Worked example two: a 29-year-old woman at 64 kg and 166 cm gives 640 + 1,037.5 - 145 - 161 = 1,371 kcal at rest.

Multiply the resting figure by an activity factor to reach maintenance. The Mifflin equation and the revised Harris-Benedict formula land within about 5 percent of each other for most adults, and both can sit 100 to 200 kcal away from a measured indirect calorimetry test. Indirect calorimetry, where you breathe into a mask for 20 minutes, is the only way to measure a personal number rather than estimate it.

Activity factors you can apply

Use the multiplier that matches your real week rather than your best week. All four figures below apply to the 1,780 kcal man from the previous section.

These multipliers come from doubly labelled water studies that tracked total energy expenditure in free-living adults. Pick the middle option, hold intake steady for 14 days, weigh daily at the same time, then move the target by 100 to 150 kcal if the 7-day average drifts up or down.

• 1.2 sedentary - Desk work, under 5,000 steps, no planned training: 1,780 x 1.2 = 2,136 kcal.
• 1.375 light - One to three training days or 5,000 to 7,500 steps: 1,780 x 1.375 = 2,448 kcal.
• 1.55 moderate - Three to five training days or 7,500 to 10,000 steps: 1,780 x 1.55 = 2,759 kcal.
• 1.725 very active - Six or seven hard sessions or manual work all day: 1,780 x 1.725 = 3,071 kcal.

Why any estimate drifts

Food logs understate intake by 20 to 50 percent in controlled studies, through missed snacks, cooking oil, dressings and portions larger than the label assumes. Weighing food for 3 to 4 days and logging every drink gives a far closer picture than estimating by eye.

Metabolism adapts to a calorie cut as well. During a 10 to 20 percent reduction, resting rate can fall 5 to 15 percent beyond what the lost tissue explains, through lower thyroid output and fewer spontaneous movements. Recalculate every 5 kg of weight change, because the intake that held 90 kg will slowly add weight at 80 kg.

Setting a target without going too low

A deficit of 250 to 500 kcal below maintenance supports about 0.25 to 0.5 kg of loss a week, a pace most people can hold for months. Dipping below roughly 1,200 kcal a day for women or 1,500 for men makes it hard to fit in protein, iron, calcium and essential fats, and belongs under medical supervision only.

Watch for hunger that never settles, broken sleep, hair shedding, cold hands or a menstrual cycle that stops. Each is a reason to eat at maintenance for a few weeks and talk to a dietitian. Disordered eating is a real risk when calorie counting turns into rigid rules; in the US you can call or text 988 for crisis support, and in Australia the Butterfly Foundation helpline is 1800 33 4673.