Contents
Calorie Deficit Calculator
Calculator
A calorie deficit means eating less energy than you spend. Work out what you burn in a day, then subtract 250–500 calories: that produces roughly 0.25–0.5 kg (0.5–1 lb) of weight loss per week. Larger deficits are not proportionally better and cost more lean muscle.
Every weight-loss plan is a calorie deficit wearing different clothing. The useful question is not whether you need one, but how big it should be, and that answer depends on what your body actually spends in a day. This calculator estimates that figure, subtracts a sensible deficit, and refuses to hand you a target low enough to be a bad idea.
Deficit size and weekly loss
What each daily deficit produces, and how long it would take to lose 5 kg (11 lb).
| Daily deficit (kcal) | Per week (kg) | Per week (lb) | Weeks to lose 5 kg |
|---|---|---|---|
| 250 | 0.23 | 0.50 | 22 |
| 500 | 0.45 | 1.00 | 11 |
| 750 | 0.68 | 1.50 | 7 |
| 1,000 | 0.91 | 2.00 | 6 |
Based on roughly 7,700 kcal per kilogram of body mass. Real-world loss is slower than this straight-line maths suggests, because your daily burn falls as you get lighter.
How it is calculated
Three steps, each building on the last.
1. Resting burn (BMR). This is what your body spends doing nothing at all: breathing, circulating blood, maintaining tissue. It is typically 60–70% of your total. The calculator uses the Mifflin-St Jeor equation, published in 1990 and still the most accurate general-purpose predictor for adults:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5 for men, or − 161 for women
2. Daily burn (TDEE). Your BMR multiplied by an activity factor covering movement, exercise and the energy cost of digestion:
| Activity level | Multiplier |
|---|---|
| Sedentary: desk job, little exercise | 1.2 |
| Lightly active: exercise 1–3 days a week | 1.375 |
| Moderately active: exercise 3–5 days a week | 1.55 |
| Active: exercise 6–7 days a week | 1.725 |
| Very active: physical job or twice-daily training | 1.9 |
3. Target. Subtract your chosen deficit from your TDEE. Worked example: a 30-year-old man, 80 kg and 180 cm, exercising 3–5 days a week. His BMR is 1,780 kcal and his TDEE is 2,759. A 500 kcal deficit gives a target of 2,259 kcal a day, and an expected loss of about 0.45 kg (1 lb) a week.
Why there are safety limits
This calculator will not return an arbitrarily low number, and that is deliberate. Two limits apply.
The deficit is capped at 20% of your daily burn. A fixed 750 kcal cut is a reasonable dent in a 3,000 kcal budget and a severe one in a 1,500 kcal budget. Below roughly 80% of maintenance, an increasing share of the weight lost comes from muscle rather than fat, and appetite and fatigue both rise sharply, which is why aggressive deficits so often end in abandonment rather than results.
The target never drops below 1,500 kcal for men or 1,200 kcal for women. Below those levels it becomes difficult to meet basic vitamin and mineral needs from food. Very-low-calorie diets do have legitimate clinical uses, but they belong under medical supervision with monitoring, not behind a web form.
If the calculator has clamped your target, the honest conclusion is usually that your daily burn is low relative to your goal, and the fix is raising the burn side rather than cutting the intake side further.
Making the number work
Every input here is an estimate. Mifflin-St Jeor predicts resting burn within about 10% for most people, which is roughly ±180 kcal on a 1,800 kcal BMR, and the activity multipliers are coarser still: most people place themselves one bracket too high.
So treat the target as a starting hypothesis, not a prescription. Eat at it for two to three weeks, weigh yourself under consistent conditions, and adjust based on what actually happens. If weight has not moved after three weeks, lower the target by 100–200 kcal. If you are losing faster than about 1% of body weight a week, raise it.
Two things worth protecting while in a deficit: adequate protein, which limits how much of the loss comes from muscle, and resistance training, which does the same job from the other direction. Weight loss without either tends to remove more muscle than most people want.
FitnessLi can help with the tracking side. Step counting, weight logging and BMI are free in the app with no trial or payment card. Daily calorie and nutrition tracking is a paid subscription feature.
References
FAQ
What is a calorie deficit?
Eating less energy than your body spends over the same period. Your body makes up the shortfall from stored energy, which is what produces weight loss.
How big should my calorie deficit be?
For most people, 250–500 calories a day. That produces roughly 0.25–0.5 kg (0.5–1 lb) of loss per week, slow enough to preserve muscle and stay liveable, fast enough to see progress.
How many calories is 1 kg of body weight?
Around 7,700 kcal, or about 3,500 kcal per pound. It is a useful rule of thumb but not a precise law: as you get lighter your daily burn falls, so a fixed deficit yields less loss over time.
Why has my weight loss stalled?
Usually because your daily burn has fallen with your body weight, so the same intake is no longer a deficit. Recalculate at your current weight. Underreported intake and overestimated exercise burn are the other two common causes.
Is 1,200 calories a day safe?
It is the lowest figure this calculator will suggest, and only for women. At that level meeting vitamin and mineral requirements from food becomes difficult. If your target has been clamped to the floor, raising your activity is a better route than cutting further.
Does FitnessLi track calories for free?
No. Step counting, weight logging and BMI are free in the app, with no trial or payment card. Calorie and nutrition tracking, workouts and recipes are part of the paid subscription.
