A calorie deficit is the gap between the calories your body burns and the calories you eat, sustained over weeks to produce fat loss. This calculator estimates a daily intake target using your sex, age, weight, height, activity level, and target deficit applied to the Mifflin-St Jeor BMR equation, expressed as target calories per day plus projected weekly and monthly pounds lost.
The reference heuristic is roughly 3,500 calories per pound of fat (per the National Institutes of Health Body Weight Planner, 2024), so a 500-calorie daily deficit projects to about 1 pound per week of weight loss; a 250-calorie deficit projects to about a half pound, and the upper safe limit for most adults is around 1,000 calories per day, or 2 pounds per week. The Mifflin-St Jeor equation, the most accurate predictive BMR formula validated against indirect calorimetry, sets your maintenance baseline (per Frankenfield et al., Journal of the American Dietetic Association, 2005).
Reality runs slightly behind the math. Water-weight shifts dominate the first one to two weeks, and metabolic adaptation slows fat loss after four to six weeks of sustained restriction, so the linear projection typically overstates the trend by 15 to 25 percent over a 12-week cut. Treat the weekly and monthly outputs as directional targets, weigh in two to three times per week at the same time of day, and adjust the deficit only after a two-to-three-week trend, not after a single weigh-in.
Key takeaway
The standard 3,500 calorie ≈ 1 lb fat conversion is a useful starting heuristic but breaks down at extremes. Real weight loss isn't linear, water-weight shifts dominate the first 1-2 weeks; metabolic adaptation slows fat loss after 4-6 weeks. A 500 cal/day deficit produces ~1 lb/week on paper; in practice, expect closer to 0.7-0.8 lb/week averaged over several months once water and adaptation are factored in.
How it's calculated
Two pieces of math:
Step 1, TDEE (Mifflin-St Jeor):
- Men:
BMR = 10 × kg + 6.25 × cm − 5 × age + 5 - Women:
BMR = 10 × kg + 6.25 × cm − 5 × age − 161 TDEE = BMR × activity factor(1.2 sedentary → 1.9 very active)
Step 2, apply the deficit:
target intake = TDEE − daily deficitweekly loss (lb) = (deficit × 7) ÷ 3500monthly loss (lb) = weekly loss × 4.345(avg weeks/month)
The 3,500 cal/lb constant is a rough average, it varies with body composition (lean people lose proportionally more muscle, denser people more fat) and the size of the deficit. Treat the projection as directional; the scale (averaged over 2-3 weeks) is the real feedback.
Source: Mifflin-St Jeor BMR + 3,500 cal/lb fat-loss heuristic
Examples
30-year-old man, 75 kg, 175 cm, moderate activity, 500 cal/day deficit
- Sex Male
- Age 30 years
- Weight (kg) 75 kg
- Height (cm) 175 cm
- Activity level Moderate (3–5 days/week)
- Daily deficit 500 kcal/day
TDEE of about 2,633 minus a 500-cal/day deficit puts the target intake near 2,133. Math says ~1 lb/week loss, ~4.3 lb/month. Comfortable deficit for someone with a structured training schedule, small enough to sustain for 8-12 weeks without big drops in performance.
45-year-old woman, 80 kg, 165 cm, light activity, 750 cal/day deficit
- Sex Female
- Age 45 years
- Weight (kg) 80 kg
- Height (cm) 165 cm
- Activity level Light (1–3 days/week)
- Daily deficit 750 kcal/day
TDEE around 1,987 with a 750-cal deficit gives a target of about 1,237, projected
1.5 lb/week, ~6.5 lb/month. Note that 1,237 sits below her BMR (1,445), a sign the deficit is too aggressive for sustained adherence at this body size. A 500-cal deficit (target ≈ 1,487, just above BMR) would be the safer choice.
Frequently asked questions
How big a calorie deficit should I aim for?
Most healthy adults do well with 300-500 cal/day, fast enough to see weekly progress, slow enough to keep training and energy levels stable. People with substantial weight to lose can go higher (up to 1,000 cal/day) under medical supervision. People near their goal weight should run smaller deficits (200-300 cal) to preserve muscle. Avoid deficits that drop intake below your BMR for more than a few days at a time, sustained sub-BMR eating triggers metabolic adaptation and nutrient deficiencies.
Why isn't my weight loss matching the math?
Three reasons. Water weight dominates the first 1-2 weeks; people often lose 3-5 lb fast then slow dramatically. Metabolic adaptation kicks in after 3-6 weeks, your body reduces non-exercise activity thermogenesis (NEAT, the fidgeting and walking you do unconsciously) so actual TDEE drops. Tracking inaccuracy is the third, most people underestimate intake by 15-25%. If the scale stalls for >2 weeks, audit portion sizes with a food scale before reducing intake further.
Is the 3,500 calories per pound rule actually accurate?
It's a rough average that holds up reasonably well at moderate deficits over short periods. Pure fat is closer to 3,500 cal/lb; muscle is much less. The mix of fat-vs-muscle loss depends on protein intake, resistance training, and starting body fat. Lean dieters lose proportionally more muscle (so the scale moves faster than fat is actually leaving). The 3,500 rule also doesn't account for the metabolic-adaptation effect that slows real-world loss over time.
How do I know if my deficit is too aggressive?
Warning signs: persistent fatigue, drop in workout performance, chronic hunger that interrupts sleep, hair thinning, irregular periods (women), low libido, mood swings, or cold extremities. Any of these for more than a week or two means the deficit is too steep, increase intake by 200-300 cal and reassess. A sustainable cut should feel like a manageable level of hunger between meals, not a constant crisis.