A calorie deficit is the gap between the calories you burn every day and the calories you actually eat. Lose weight, and that gap is the whole game — no matter which diet you follow, no matter which app you log food in, the scale only moves when your weekly energy output exceeds your weekly energy intake. This Smekys calculator takes your two numbers — your TDEE (maintenance calories) and your planned daily intake — and turns them into a clear daily deficit, a deficit percentage, and an honest projection of how much weight you can expect to lose per week, per month, and per year.
Type a number into either box and every figure on the card updates instantly. You do not need to know your BMR, your body-fat percentage, or your macros to use it — those are useful refinements, but the deficit itself is just subtraction.
How to Use
- Find your TDEE — your total daily energy expenditure. This is the number of calories you burn on an average day at your current weight and activity level. If you do not know it, run your age, height, weight, and activity level through our TDEE calculator first, then paste the result back here.
- Enter that number in the Your TDEE (maintenance calories) box. The default of 2,643 kcal/day is a reasonable starting point for an average adult male; adjust it to your own figure.
- Type your planned daily calorie intake into the second box. This is what you actually intend to eat — not what you think you should eat, and not a number you picked from a magazine. Be honest. A deficit that looks perfect on paper but that you cannot sustain for twelve weeks is worth nothing.
- Optionally enter additional structured exercise per week. If your TDEE already reflects your normal daily movement, leave this at 0. If you plan to add walking, lifting, or cardio on top of that baseline, type the extra calories you expect to burn per week (a 5-day walking week is roughly 1,000–1,500 kcal). The exercise calories are spread evenly across the week and added to your effective daily deficit.
- Read the result card. The Daily Calorie Deficit is the headline diet-only gap; the Weekly Deficit line includes both diet and exercise. The Deficit as % of TDEE tells you how aggressive the diet gap is — under 20% is gentle, 20–30% is moderate, and above 30% is aggressive and hard to sustain.
- Look at the Estimated Weekly Weight Loss. The classic rule of thumb is that one pound of body fat stores roughly 3,500 kcal, so a 500 kcal/day deficit predicts about one pound lost per week. A 1,000 kcal/day deficit predicts about two pounds per week.
- Use the monthly and yearly projections to set realistic expectations. A 1.69 lb/week deficit compounds to roughly 7.3 lb per month and about 88 lb per year — but real-world loss always slows as your weight drops, because a lighter body burns fewer calories and your TDEE falls.
The Math Behind the Deficit
The calculation is deliberately simple. Start with two inputs:
- TDEE — calories burned per day.
- Target intake — calories eaten per day.
Then:
Daily deficit = TDEE − target intake
Weekly weight loss (lb) = daily deficit × 7 ÷ 3,500
The 3,500 kcal-per-pound figure comes from Max Wishnofsky's 1958 paper in the American Journal of Clinical Nutrition. It is an approximation — modern research shows real weight loss is nonlinear, because metabolism adapts and body composition changes — but it remains the standard planning tool used by dietitians, coaches, and apps because it is easy to reason about and close enough for week-to-week decisions.
Worked example on the defaults: TDEE 2,643 kcal/day, planned intake 1,800 kcal/day. The daily deficit is 843 kcal. Multiply by 7 to get a weekly deficit of 5,901 kcal, divide by 3,500, and you get 1.69 lb per week. Scale that to a 30.4-day month and you are looking at about 7.3 lb per month; over a full 365-day year the projection is roughly 88 lb. The deficit is 31.9% of TDEE — on the aggressive side, which is fine for a short cut but hard to hold for twelve months.
How Big Should Your Deficit Be?
There is no single perfect number, but there are well-worn guardrails:
- 0–20% below TDEE — a gentle deficit. Expect 0.25–0.5 lb per week. Sustainable, easy to train on, good for people who only have 10–15 lb to lose.
- 20–30% below TDEE — a moderate deficit. Expect 0.5–1.5 lb per week. This is the sweet spot for most adults: fast enough to stay motivated, gentle enough to keep your energy and strength.
- 30%+ below TDEE — an aggressive deficit. Expect 1.5–2+ lb per week. Useful for short, supervised cuts or for people with a lot of weight to lose, but hard to sustain, can blunt strength training, and often leads to rebound eating.
Most health authorities recommend losing no more than 1–2 lb per week. Faster loss is usually water and glycogen, not fat, and tends to come back just as fast.
Examples
Example 1 — The default cut. TDEE 2,643 kcal/day, planned intake 1,800 kcal/day. Daily deficit 843 kcal, deficit 31.9% of TDEE, projected weekly loss 1.69 lb, monthly loss 7.3 lb, yearly projection 88 lb. This is an aggressive but common starting point for someone who has 40+ lb to lose.
Example 2 — A gentle deficit. A woman with a TDEE of 1,900 kcal/day eats 1,600 kcal/day. The daily deficit is 300 kcal, about 15.8% of TDEE. Projected weekly loss is 0.6 lb, monthly loss about 2.6 lb. Slow, but easy to stick to and compatible with strength training.
Example 3 — Same diet plus a walking routine. Take the default cut (TDEE 2,643, intake 1,800) and add 1,500 kcal/week of extra exercise — say five 30-minute brisk walks. That adds 1,500 ÷ 7 ≈ 214 kcal/day to the effective deficit, lifting the weekly loss from 1.69 lb to about 2.11 lb (monthly 9.2 lb, yearly 110 lb). The diet-only daily deficit still reads 843 kcal; the exercise is reflected in the weekly-weight-loss line, not the headline deficit.
Example 4 — A large deficit stress test. Enter a TDEE of 2,000,000 kcal/day (a sanity check for the engine) and an intake of 1,000,000 kcal/day. The daily deficit is 1,000,000 kcal and the projected weekly loss is 2,000 lb — the calculator handles magnitudes this large without overflow, even though no real human burns that many calories.
Example 5 — Zero boundary. If both TDEE and intake are 0, the deficit is 0 and the projected weekly loss is 0 lb. The engine treats this as a valid no-op rather than dividing by zero or crashing.
FAQ
What is a calorie deficit?
A calorie deficit is simply the gap between the calories you burn each day (your TDEE) and the calories you eat. When you eat less than you burn, your body makes up the difference by burning stored body fat, and the scale goes down. The deficit is the single variable that drives weight loss — every diet works by creating one.
How big a deficit do I need to lose 1 pound a week?
Using the 3,500 kcal-per-pound rule, you need a 500 kcal-per-day deficit. That multiplies to 3,500 kcal per week, which equals about one pound of fat. Enter a TDEE of, say, 2,600 kcal/day and an intake of 2,100 kcal/day and the calculator will show roughly 1.0 lb per week.
What is a safe calorie deficit per day?
Most experts recommend a deficit of 300–750 kcal per day, or about 20–25% below your TDEE. That typically produces 0.5–1.5 lb of weight loss per week. Deficits above 1,000 kcal/day are aggressive, can cost muscle mass, and are hard to sustain without medical supervision.
Why does my projected weight loss slow down over time?
The 3,500 kcal rule is a linear approximation, but real metabolism is not. As you lose weight, your TDEE drops because a lighter body burns fewer calories at rest. A 200 lb person with a 2,643 kcal TDEE may see their TDEE fall to 2,300 kcal after losing 20 lb, so the same 1,800 kcal diet produces a smaller deficit. Re-enter your updated TDEE every 4–6 weeks.
Can I build muscle while in a calorie deficit?
Yes, especially when you are new to resistance training, are returning after a layoff, or are carrying extra body fat. A moderate deficit (20–25% below TDEE) combined with progressive strength training and adequate protein (roughly 0.8–1.0 g per pound of body weight) lets most people retain or even add some muscle while losing fat. Very large deficits make this much harder.
What happens if my intake is higher than my TDEE?
You are in a calorie surplus, not a deficit. The calculator will show a negative daily deficit, meaning you are projected to gain weight. To cut fat, your planned intake must be below your TDEE.
How do I find my TDEE?
Your TDEE is your BMR (basal metabolic rate) multiplied by an activity factor (1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active, 1.9 extra active). Our TDEE calculator computes this from your age, height, weight, sex, and activity level. For best results, weigh yourself daily for two weeks while eating at that level and adjust up or down based on the trend.
Disclaimer
This calorie deficit calculator is provided by Smekys for informational purposes only and is not medical advice. The 3,500 kcal-per-pound rule is a planning approximation; real weight loss varies with age, sex, genetics, hormones, medications, body composition, sleep, and adherence. Do not start a diet, especially an aggressive one, without consulting a registered dietitian or your physician, particularly if you have diabetes, a history of disordered eating, are pregnant or breastfeeding, or take medications that affect blood sugar or weight. Always verify your numbers with a qualified healthcare professional before making changes to your eating or exercise habits.