What a calorie calculator for weight loss actually estimates
It estimates two things: how many calories your body burns in a day, and how far below that number you would need to eat to lose weight at a chosen pace. The first step is your basal metabolic rate (BMR), the energy you use at rest, calculated with the Mifflin-St Jeor equation. The second multiplies BMR by an activity factor to get maintenance calories, also called TDEE. The last subtracts a daily deficit.
Women: BMR = 10 × kg + 6.25 × cm − 5 × age − 161
Maintenance = BMR × activity factor (1.2 to 1.9)
Daily deficit = lb per week × 3,500 ÷ 7
Target = maintenance − deficit
How many calories should you eat to lose weight?
It depends on your size, age, sex and activity, so any single number is a guess. A worked example: a 40-year-old man, 178 cm and 95 kg, has a BMR of about 1,868 kcal. At 1.375 (light exercise) his maintenance is about 2,570 kcal. A 500 kcal deficit puts his starting target near 2,070 kcal a day, which is the calorie target for roughly a pound of loss per week on paper. The same plan for a 62 kg woman of the same age and height lands far lower, which is why generic "1,500 calories" advice fits few people.
Why the projection slows down
The old rule says a 3,500-calorie deficit equals one pound of loss. It is a useful rule of thumb, but it assumes your energy needs never change. They do: a lighter body burns less. In the example above, losing 10 kg lowers his BMR by about 100 kcal and his maintenance by about 137 kcal, so a fixed 2,070 kcal target becomes a smaller deficit. The chart in the tool reflects that by recalculating maintenance at each new weight. It does not model muscle loss, hormones or appetite changes, so treat the timeline as a range of plausible, not a promise. Research from NIH scientists on dynamic weight models (Hall et al., The Lancet, 2011) makes the same point about the 3,500 rule overstating long-term loss.
Protein, fat and carbs: a starting split
Calories decide the direction of weight change; the split affects how you feel on them. Higher protein tends to be more filling and helps protect muscle while you eat less. The tool starts at 1.6 g of protein per kg of body weight, fat at a quarter of calories and carbohydrate for the rest. If you have kidney disease or another medical condition, ask a clinician before changing protein intake.
When the scale stops moving
Weigh yourself at the same time several mornings a week and look at the weekly average, since water can mask fat loss for days. If the average has not moved for three to four weeks while you have followed the plan, recalculate at your current weight, check portions and drinks, and consider a small change of 100 to 200 calories rather than a large cut. Recalculate every 5 to 10 kg lost.
Calories you should not go below
The tool will not show a very low target. If your result falls under its threshold, a gradual pace, a more accurate activity level or a conversation with a doctor or registered dietitian is a better route than cutting further. Pregnancy, a history of disordered eating, medical conditions and some medications all call for individual advice.
Your calorie target is only as good as your food estimates
Studies of self-reported intake repeatedly find people underestimate what they eat, and the gap usually hides in things the plate does not show: a tablespoon of cooking oil is roughly 120 kcal, dressings can outweigh the salad, and drinks add up. The Food Analyzer Quiz asks six quick questions about how you eat and track, then shows where hidden calories are most likely to catch you out, and includes a free scanner to try on a real meal.
- Find the meals most likely to be heavier than they look
- See where oils, sauces and drinks add up for you
- Check your plan against what you actually eat
Frequently asked questions
How many calories should I eat a day to lose weight?
Subtract a daily deficit from your maintenance calories. A 500 kcal deficit is a common starting point for about a pound a week, but the right number depends on your body and activity.
Is the calculator's number exact?
No. It uses an equation built on averages, so your real maintenance can differ by a few hundred calories. Track your weekly average weight and adjust.
Why does my weight loss slow down over time?
A lighter body burns fewer calories, and adherence, water and activity shift. The projection chart shows this slowdown.
How fast should I try to lose weight?
Many people use about 0.5 to 1 lb per week as a sustainable range. Faster paces leave less room for error and are harder to maintain.
Do I need to count every calorie?
Not necessarily. Many people log carefully for a few weeks to learn portions, then check in periodically.
Can I use this if I have a medical condition?
Use it only as a rough guide and talk to a healthcare professional first, especially if you are pregnant, have a history of disordered eating, or take medication that affects weight or appetite.
Sources and methodology
BMR uses the Mifflin-St Jeor equation (Mifflin et al., American Journal of Clinical Nutrition, 1990). Activity factors are standard multipliers. The 3,500 kcal per pound conversion is an approximation; see Hall et al., The Lancet, 2011, and the NIDDK Body Weight Planner for a fuller dynamic model. For general guidance see the CDC and NIH. Educational use only; not medical advice.