A calorie deficit is one of the most searched — and most misunderstood — ideas in nutrition. Tap a topic to understand it clearly and safely.
Use the Calorie Calculator to estimate your maintenance calories and a gentle target, or the TDEE Calculator to see your daily energy expenditure. For a plan tailored to you, a registered dietitian or doctor is the best guide.
General education, not medical or nutrition advice. If thoughts about food, calories or weight feel distressing or hard to control, you deserve support — a doctor or a national eating-disorder helpline can help.
How This Calorie Calculator Guide Works
This page breaks down the energy-balance logic behind any calorie calculator: how calories in compare to calories out, and what that gap actually does to your weight over time. It’s built as an explainer you can tap through, rather than a form that spits out a personal target.
Formula: Weight change follows simple energy balance: Daily energy gap = Calories consumed − TDEE. A negative gap (eating below TDEE) is a deficit that supports loss; a positive gap is a surplus that supports gain; a gap near zero keeps weight roughly stable. Using the same rough rule of thumb this guide cites — about 3,500 kcal ≈ 1 lb of body fat — Estimated monthly weight change (lb) ≈ (Daily gap × 30) ÷ 3,500.
- Calorie intake — total calories consumed from food and drink each day
- TDEE — total daily energy expenditure: BMR plus activity, i.e. calories out
- Daily energy gap — intake minus TDEE; negative is a deficit, positive is a surplus
- ~3,500 kcal ≈ 1 lb — the rule-of-thumb constant used to project weight change from an energy gap
- 30-day window — the period this guide’s example projections use to turn a daily gap into a monthly estimate
Example Scenarios
| Daily energy gap | Monthly total (30 days) | Estimated monthly change (~3,500 kcal/lb) |
|---|---|---|
| −300 kcal (deficit) | −9,000 kcal | −2.57 lb |
| −500 kcal (deficit) | −15,000 kcal | −4.29 lb |
| −750 kcal (deficit) | −22,500 kcal | −6.43 lb |
| −1,000 kcal (deficit) | −30,000 kcal | −8.57 lb |
| +500 kcal (surplus) | +15,000 kcal | +4.29 lb |
Calorie Calculator FAQ
What’s the difference between calories in and calories out?
Calories in is everything you eat and drink; calories out (TDEE) is your total daily energy expenditure — resting metabolism (BMR) plus all movement and exercise. Weight change is driven by the ongoing gap between the two, not by any single meal or day.What is TDEE and why does it matter here?
TDEE (Total Daily Energy Expenditure) is roughly BMR plus activity — the number of calories your body uses in a day. Comparing your intake to your TDEE tells you whether you’re in a deficit, a surplus, or roughly at maintenance.Why does eating at my TDEE keep my weight stable?
When intake and TDEE are roughly equal, there’s no meaningful energy gap for your body to store as fat or draw from stored energy, so weight tends to hold steady over time, allowing for normal day-to-day fluctuation from water and food volume.How is the monthly weight-change estimate calculated?
It multiplies your daily energy gap by 30 days, then divides by the rough rule-of-thumb constant of about 3,500 kcal per pound of body fat. It’s a projection based on a simplified rule, not a guaranteed outcome.Why might my actual weight change differ from the projected estimate?
Your TDEE isn’t perfectly constant — it shifts as your weight changes, and water retention, hormones, sleep, and consistency all affect the scale. Treat the projection as a rough directional estimate rather than an exact prediction.Does the same logic work for gaining weight, not just losing it?
Yes — the energy balance equation works in both directions. A sustained positive gap (eating above TDEE) is a surplus that generally supports gradual weight gain, using the same rough kcal-to-pound conversion in reverse.Related Calculators
To put your own numbers into this energy-balance logic, try the BMR calculator for your resting energy use, the macro calculator to split a target intake into protein, carbs and fat, or the weight loss timeline calculator to see a gradual projection over weeks and months.