Calorie Deficit Calculator: How Many Calories Should You Eat to Lose Weight?

A calorie deficit is the foundation of every successful weight-loss program. Eat fewer calories than your body burns, and you lose weight. Eat more, and you gain. The hard part isn't the principle — it's figuring out the specific number of calories you should eat based on your body and activity level.

This calculator uses the Mifflin-St Jeor equation — currently considered the most accurate BMR formula for the general population — plus standard activity multipliers to estimate your maintenance calories (TDEE). Then it applies your chosen weight-loss rate to give you a daily target.

How the calculator works

Behind the scenes the calculator runs three calculations:

1. BMR (Basal Metabolic Rate) — the calories your body burns at complete rest, just keeping you alive. Calculated via Mifflin-St Jeor:
   Male: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
   Female: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
2. TDEE (Total Daily Energy Expenditure) — your maintenance calories. BMR multiplied by an activity multiplier:
   • Sedentary (1.2): desk job, no exercise
   • Lightly active (1.375): 1-3 exercise days per week
   • Moderately active (1.55): 3-5 exercise days per week
   • Very active (1.725): 6-7 exercise days per week
   • Extremely active (1.9): twice-daily training or physical job
3. Your target — TDEE minus your chosen daily deficit (250 kcal for 0.5 lb/week, 500 kcal for 1 lb/week, 1000 kcal for 2 lb/week). The calculator enforces a minimum floor of 1,200 kcal regardless of inputs.

Why the 3,500-calorie rule isn't exactly right

You've probably heard “a pound of fat = 3,500 calories.” This is the basis for most deficit math: a 500-calorie daily deficit × 7 days = 3,500 calories = 1 pound lost per week.

The rule is a useful approximation but not exact. The actual energy density of body fat is closer to 7,700 calories per kilogram (~3,500 per pound). However, weight loss involves more than just fat — water, glycogen, and lean tissue all contribute, especially in the first 2-4 weeks. So your actual scale movement may be faster or slower than the formula predicts.

Most people find their real weight loss is 70-90% of what calculators predict over a 12-week period. The math is directionally right but expect some variance, especially as you get leaner.

Examples by body weight

Here are some quick reference points. These assume the Mifflin-St Jeor formula with the listed activity level.

How big should your deficit be?

The right deficit depends on three things: how much fat you have to lose, how much time you have, and how well your body tolerates aggressive cutting.

• 0.5 lb/week (250 cal deficit) — the gentle option. Best for people with less than 20 lb to lose, lean already, or who can't handle hunger well. Sustainable for years.
• 1 lb/week (500 cal deficit) — the standard recommendation. Aggressive enough to see results in the scale and mirror weekly. Sustainable for 12-20 weeks for most people.
• 2 lb/week (1000 cal deficit) — aggressive. Only appropriate for people with significant amounts to lose (50+ lb overweight). Not sustainable long-term; usually run in 6-8 week cycles with diet breaks.

Rule of thumb: your max sustainable deficit is roughly 25% of your TDEE. So if your TDEE is 2,000, don't go below 1,500 calories. If your TDEE is 3,000, you can sustain 2,250-ish. Going more aggressive triggers metabolic adaptation — your body downregulates calorie burn to compensate, and your scale stops moving.

The minimum calorie floors (don't ignore these)

• 1,200 kcal/day for women — generally the absolute minimum. Below this you risk nutrient deficiencies, fatigue, hair thinning, and amenorrhea.
• 1,500 kcal/day for men — similar reasoning. Most men have higher BMRs and shouldn't go below this.
• Active people add ~300-500 kcal to these floors regardless of weight.

Our calculator caps at 1,200 kcal — if your inputs would suggest going lower, you need to either reduce your weight-loss rate or increase your activity (so your TDEE rises).

What about protein?

Calorie deficit alone isn't enough — you also need adequate protein to preserve lean muscle while you lose fat. Most evidence-based recommendations:

• 0.7-1.0 g protein per pound of bodyweight for active people in a deficit
• 1.5-1.8 g protein per kg of bodyweight in metric

For a 180-lb person, that's 130-180g protein per day — which is a LOT. Most people undereat protein in a deficit and end up losing 25-30% of their weight loss as muscle instead of pure fat. See our guide on high-protein, low-calorie foods for practical sources.

Common mistakes that wreck your deficit

1. Underestimating calorie intake by 30-50%. Studies consistently show people undercount calories by 1/3 or more — sauces, oils, beverages, “just a taste” while cooking, weekend overshoots. Photo tracking apps like MyBiteIQ reduce this by automating estimation.
2. Overestimating calorie burn. Apple Watch / Fitbit / cardio machines often inflate calorie burn by 20-50%. Don't “earn back” calories you didn't actually burn.
3. Weekend reset. A 500-cal weekday deficit erased by a 2,000-cal weekend overshoot = net zero. Track weekends too.
4. Setting too aggressive a deficit too long. Your body adapts. 12 weeks at 1,500 kcal often produces less loss than 12 weeks at 1,800 kcal because the latter is sustainable enough to actually maintain.
5. Ignoring sleep, stress, and water. Calorie math is necessary but not sufficient. Sleep less than 6 hours, and your appetite hormones (ghrelin, leptin) drive overeating. Chronic stress raises cortisol and water retention. Drink your water and sleep 7-9 hours.

How to actually maintain your deficit day-to-day

Knowing your calorie target is step 1. Hitting it consistently is the hard part. A few practical strategies:

• Pre-decide your meals. The biggest cause of overeating is decision fatigue at the end of the day. Plan your meals the night before, or use an AI meal plan generator.
• Track at least the first 30 days. Calorie awareness compounds. After a month of tracking, you'll have a much better intuition for portions even when you stop logging.
• Don't restrict to perfection. Aim for 80-90% adherence. The other 10-20% buys you sustainability.
• Weigh in weekly, not daily. Daily scale fluctuations are mostly water and food weight. The weekly average is the real signal.

A note on individual variation

Calculators give you a starting estimate, not a guarantee. Real-world TDEE varies by ±10-15% from formula predictions due to NEAT (non-exercise activity thermogenesis), gut microbiome, genetics, and other factors. The right way to use this calculator:

1. Get your initial number
2. Eat at that target for 2 weeks while tracking
3. Weigh in weekly
4. Adjust by ±100-200 kcal based on actual results

If you're losing faster than expected, eat a bit more. If you're losing slower, drop another 100-200. Most people land at their true sustainable target after 4-6 weeks of small adjustments.