Which activity level should I choose?

Probably one below your instinct. The categories describe your whole week including work, not just deliberate exercise, and a desk job with three gym sessions is closer to light than to active. Given a step is worth 300 calories, erring low and adjusting upwards is the safer direction.

Why do men and women get different numbers for the same body?

The equation ends in +5 for men and −161 for women, a fixed gap of 166 calories. It is an average adjustment for typical differences in body composition at the same height and weight, not a measurement of any individual.

Calorie Calculator

Calculate your daily caloric needs (BMR and TDEE).

Guide to Caloric Needs

BMR - Basal Metabolic Rate

BMR is the minimum calories your body needs to maintain vital functions at rest - heart beating, breathing, maintaining body temperature. It accounts for about 60-70% of total energy expenditure. We calculate it using the Mifflin-St Jeor formula, which considers gender, weight, height, and age.

TDEE - Total Daily Energy Expenditure

TDEE is your total daily caloric needs, including BMR plus physical activity. Multiply BMR by activity multiplier: sedentary ×1.2, light ×1.375, moderate ×1.55, active ×1.725, very active ×1.9. This is the value to maintain weight.

Macronutrients - proportions

Protein - 1.6-2.2g per kg bodyweight, builds muscle
Fats - 0.8-1g per kg, for hormones and brain
Carbs - rest of calories, main energy source

Healthy weight loss

Safe caloric deficit is 300-500 kcal daily (0.5kg/week). Avoid drastic diets below BMR - body enters starvation mode. Stick to calculated TDEE for 2-3 weeks before adjustment. Remember about hydration and sleep.

The activity dropdown moves your answer by 1,229 calories — more than the equation ever could

For a 35-year-old man of 80 kg and 180 cm, the Mifflin-St Jeor equation gives a BMR of 1,755. Multiply by the sedentary factor of 1.2 and you get 2,106; by the very active factor of 1.9 and you get 3,335. One dropdown, a 58% spread, and 1,229 calories between two answers that both look authoritative.

How it works

Estimates basal metabolic rate from weight, height, age and sex using Mifflin-St Jeor.
Multiplies that by an activity factor between 1.2 and 1.9 to give total daily energy expenditure.
Applies a 500-calorie adjustment to show deficit and surplus targets around that figure.

men:   BMR = 10 × kg + 6.25 × cm − 5 × age + 5
women: BMR = 10 × kg + 6.25 × cm − 5 × age − 161

TDEE = BMR × activity factor
  sedentary 1.2 · light 1.375 · moderate 1.55 · active 1.725 · very active 1.9

Worked example

A man of 35, 80 kg, 180 cm.

BMR = 10 × 80 + 6.25 × 180 − 5 × 35 + 5 = 1,755
sedentary: 1,755 × 1.2 = 2,106
moderate: 1,755 × 1.55 = 2,720
very active: 1,755 × 1.9 = 3,335
one step from moderate to active adds 307 calories, or 11.3%

2,720 calories at a moderate activity level. The number after the decimal point in the equation matters far less than which line of the dropdown you picked.

Reading the result

Activity level is the input you can judge least well and that moves the answer most. Most people place themselves a category higher than their week actually justifies, and a single step up is worth about 300 calories a day — roughly a meal.
Mifflin-St Jeor is a population regression, so it describes the average person with your measurements rather than you. Individual metabolic rates vary around the estimate by roughly ten per cent in either direction, and it does not know your body composition: muscle and fat are counted the same by the scale.
The 500-calorie adjustment corresponds to about 3,500 calories a week, or roughly 0.45 kg of fat. That is arithmetic on stored energy, not a promise — real weight change is slower and noisier than the sum suggests, and water shifts dominate the first fortnight.
This is a starting estimate to be corrected by evidence. Track intake and weight for two or three weeks and adjust the number to what your own body did; that measurement beats any equation. Anyone pregnant, breastfeeding, under a clinician's care, or with a history of disordered eating should get their targets from a professional rather than a calculator.

Common questions

Which activity level should I choose?: Probably one below your instinct. The categories describe your whole week including work, not just deliberate exercise, and a desk job with three gym sessions is closer to light than to active. Given a step is worth 300 calories, erring low and adjusting upwards is the safer direction.
Why do men and women get different numbers for the same body?: The equation ends in +5 for men and −161 for women, a fixed gap of 166 calories. It is an average adjustment for typical differences in body composition at the same height and weight, not a measurement of any individual.

Sources

NHS — Calorie counting