TDEE is the total number of calories you burn in a day, including everything you do. Eat at this number and your weight holds steady. It's the number every calorie goal is built from.
TDEE starts with your BMR and multiplies it by an activity factor that reflects how much you move: 1.2 for sedentary, up to 1.9 for very active physical jobs. The result is your maintenance level — the calories that keep your weight stable.
To lose weight, eat below your TDEE; a deficit of about 500 kcal/day targets roughly half a kilo (one pound) of loss per week. To gain, eat above it. The calculator above shows lose / maintain / gain targets automatically, and caps the weight-loss number so it never falls below your BMR.
One thing most calculators miss: your TDEE falls as you lose weight, because a lighter body burns fewer calories. A fixed target slowly stops producing a deficit. The goal planner handles this by recomputing your TDEE every week.
Be honest and slightly conservative. Most people with desk jobs who exercise a few times a week are 'light' or 'moderate', not 'active'. Overestimating activity is the most common reason a target doesn't produce results.
It's a starting point, not a measurement. Use it for 2–3 weeks while tracking your weight, then adjust based on what actually happens on the scale.
BMR is rest-only energy. TDEE is BMR plus activity and digestion. TDEE is the number you use for calorie goals.