Daily Calorie Deficit
Time to Goal
Estimated Goal Date
Table of Contents
How weight loss is calculated
One kilogram of body fat stores approximately 7,700 calories of energy. To lose a kilogram you need a cumulative calorie deficit of that size — spread however you like across days or weeks. The calculator works backwards from your chosen weekly rate to find the daily deficit required:
Daily Deficit = (Weekly Rate × 7,700) ÷ 7
The timeline is then total weight to lose divided by weekly pace:
Weeks to Goal = (Current Weight − Goal Weight) ÷ Weekly Rate
Worked example using the calculator defaults (90 kg current, 80 kg goal, 0.5 kg/week):
- Weight to lose: 90 − 80 = 10 kg
- Daily deficit: (0.5 × 7,700) ÷ 7 = 3,850 ÷ 7 ≈ 550 kcal/day
- Timeline: 10 ÷ 0.5 = 20 weeks (about 4.6 months)
So a 90 kg person aiming for 80 kg at a 0.5 kg/week pace needs to run a daily calorie deficit of roughly 550 kcal for about 20 weeks. Note this is a mathematical projection — real losses tend to come in slightly under this in months 2–6 because of metabolic adaptation (see Limitations below).
Choosing a weekly pace
The right pace depends on how much you have to lose and how aggressive you can sustain. The table below shows the daily calorie deficit required for each rate, and the practical trade-offs.
| Weekly rate | Daily deficit | Best for | Trade-off |
|---|---|---|---|
| 0.25 kg/week (gradual) | ~275 kcal | Close to goal weight, athletes, body recomposition | Slow — 10 kg takes 40 weeks |
| 0.5 kg/week (steady) | ~550 kcal | Most people most of the time | Manageable hunger; sustainable for months |
| 0.75 kg/week (moderate) | ~825 kcal | Significant weight to lose, motivated start | Needs higher protein & strength training to spare muscle |
| 1.0 kg/week (fast) | ~1,100 kcal | Medically supervised programs, large amounts to lose | Hunger, fatigue, muscle loss risk; hard to sustain |
A reasonable heuristic: aim for a deficit of about 1% of body weight per week. For an 80 kg person that's roughly 0.8 kg/week as a maximum; smaller people should use a smaller absolute pace to avoid disproportionate muscle loss.
Creating the deficit: diet vs exercise
A calorie deficit can come from eating less, moving more, or both. Diet is the heavier lever because it's much easier to skip 500 kcal than to burn an extra 500 kcal — a 30-minute run for an 80 kg person burns roughly 300 kcal, and most people compensate by eating more afterwards.
- Diet changes (~70–80% of the deficit for most people): reduce portion sizes, swap high-calorie drinks for water, prioritise protein and fibre to manage hunger. A simple plate template — half vegetables, a quarter protein, a quarter starch — gets most people 80% of the way there without tracking.
- Strength training: doesn't burn many calories but preserves muscle during a deficit, which keeps your metabolic rate higher and helps shape body composition.
- Cardio: contributes calories burned, supports cardiovascular health, and improves appetite regulation in many people. 150–300 minutes per week of moderate activity is the standard recommendation.
Use the Calorie Calculator to estimate your daily maintenance calories, then subtract the daily deficit above to find your target intake.
Limitations
- Water weight dominates week 1. A typical deficit produces 1–3 kg of scale loss in the first week, most of it water and glycogen, not fat. By week 3 the rate settles to roughly what the math predicts.
- Metabolic adaptation. Hall et al. (2011) demonstrated that the 7,700 kcal/kg model overestimates long-term loss because resting metabolic rate drops as you lose weight. Expect the actual loss to be 10–25% less than projected over 6–12 months unless you keep recalculating maintenance.
- NEAT compensation. Non-exercise activity thermogenesis — fidgeting, posture, walking around — falls in a deficit without you noticing. Studies have measured drops of 100–300 kcal/day in NEAT alone.
- Tracking error. Self-reported food intake typically understates real intake by 20–50% in free-living adults. Food label tolerance is up to 20% in the US. If your projected deficit isn't producing the expected loss, audit intake before assuming your metabolism is the problem.
- Hormonal and medical factors. Thyroid, PCOS, certain medications (steroids, antipsychotics, some antidepressants), sleep deprivation, and chronic stress all affect both metabolic rate and water retention. If progress is much slower than expected over 8–12 weeks with consistent intake, talk to a doctor.
- It's not all fat. Even in a well-managed deficit, 15–25% of the weight lost is typically lean tissue (muscle plus water). Strength training and adequate protein (1.6–2.2 g/kg body weight per day) reduce that share.
Setting a realistic goal weight
This calculator only works if the goal weight is sensible. Check it against the BMI Calculator — aim for a goal BMI in the 18.5–24.9 range unless you have a reason to be outside it (athletes with high muscle mass often land above 25 healthily). For muscular individuals, the body fat percentage and FFMI calculators give a better target than scale weight alone.
Once you reach goal weight, maintenance is its own challenge. The CDC's National Weight Control Registry tracks long-term weight-loss maintainers; the common patterns are eating breakfast, weighing weekly, ~60 minutes of activity per day, and keeping the diet relatively consistent across weekdays and weekends.
Sources & references
- NIH NIDDK — Adult Overweight & Obesity Treatment — US government clinical guidance on safe weight-loss rates and approaches.
- Hall KD, Sacks G, Chandramohan D, et al. (2011). "Quantification of the effect of energy imbalance on bodyweight." The Lancet 378(9793): 826–837 — the paper showing the simple 7,700-kcal-per-kg rule overestimates long-term loss; introduces a more accurate dynamic model.
- CDC — Losing Weight — US public health guidance on sustainable weight loss.
- NHS — Start Losing Weight — UK National Health Service patient guidance.
- National Weight Control Registry — long-running study of people who have maintained significant weight loss for >1 year.
FAQs
One kilogram of body fat stores roughly 7,700 calories. To lose a kilogram per week you need a 7,700-calorie deficit spread over 7 days, or about 1,100 calories per day. The calculator multiplies your chosen weekly rate by 7,700 and divides by 7 to give the daily target. At 0.5 kg/week that's ~550 kcal/day; at 0.25 kg/week it's ~275 kcal/day.
It's a useful approximation, not a physical constant. Hall et al. (2011) in The Lancet showed that the simple 3,500-calorie-per-pound (7,700 kcal/kg) model overestimates long-term weight loss because it ignores metabolic adaptation — as you lose weight, your basal metabolic rate drops, so the same deficit produces less loss over time. For the first 1–3 months the figure works well; over a year, expect actual loss to be 10–25% less than the rule predicts unless you keep recalculating your maintenance calories.
The CDC and NHS both recommend 0.5–1 kg (1–2 lbs) per week as sustainable. Faster loss is possible but usually means a higher share of muscle and water lost alongside fat — and a higher rebound risk. Very low calorie diets (below 1,200 kcal/day for women, 1,500 kcal/day for men) should only be done under medical supervision.
The first week of a deficit always drops more on the scale than the deficit alone predicts — you're shedding water, glycogen (which holds about 3 g of water per gram), and gut contents. By weeks 2–3 those one-off losses are done and the rate settles to roughly what the calorie deficit predicts. This is normal and not a sign anything is broken. Trust the four-week average, not the daily scale.
Yes — about every 5–10 kg of loss. A lighter body burns fewer calories at rest. If you started at 90 kg with a maintenance of ~2,400 kcal and you're now 80 kg, your maintenance is closer to 2,250 kcal, so the same intake now produces a smaller deficit. Recalculate using the Calorie Calculator and adjust intake or activity to keep the deficit constant.