Calorie Deficit / Surplus Calculator

Calorie Deficit vs Surplus — The Fundamentals

Body composition change comes down to energy balance. Eat fewer calories than you burn (deficit) and your body taps stored energy — mostly fat, some muscle. Eat more than you burn (surplus) and your body stores the excess — mostly as fat, some as muscle if you're training.

The math is straightforward: 1 kilogram of body fat stores approximately 7,700 calories (3,500 per pound). A daily deficit of 550 calories produces roughly 0.5kg of fat loss per week. A surplus of 275 calories per day adds about 0.25kg per week.

But the numbers alone don't tell the whole story. How fast you change your weight matters as much as the direction.

How the Calculator Works

This calculator combines your BMR (Mifflin-St Jeor equation, 1990), your activity level, and your chosen rate of change to produce a daily calorie target.

1. Step 1: Calculate your BMR — the calories your body burns at complete rest
2. Step 2: Multiply by activity level to get your TDEE (maintenance calories)
3. Step 3: Subtract (for deficit) or add (for surplus) based on your weekly rate goal
4. Step 4: If you enter a goal weight, project the timeline based on your rate

Recommended Rates of Change

For Fat Loss

Research by Helms, Aragon, and Fitschen (2014) established evidence-based guidelines for resistance-trained individuals:

• 0.5% bodyweight/week: Optimal for lean individuals (<15% body fat males, <25% females). Maximum muscle preservation.
• 1% bodyweight/week: Acceptable for higher body fat percentages. More aggressive but still sustainable for 8-12 weeks.
• Beyond 1%/week: Not recommended for anyone concerned about muscle retention. Significant metabolic adaptation and performance decrements.

A practical rule: the leaner you are, the slower you should cut. Someone at 25% body fat can safely lose 0.75-1kg/week. Someone at 12% should aim for 0.25-0.5kg/week.

For Muscle Gain

A 2019 meta-analysis in Sports Medicine (Slater et al.) found that surpluses beyond 500 kcal/day primarily increase fat gain, not muscle gain. The rate of muscle growth is capped by biology:

• Beginners (year 1): 0.5-1kg muscle/month possible. Surplus of 300-500 kcal works.
• Intermediate (year 2-3): 0.25-0.5kg/month. Surplus of 200-350 kcal is ideal.
• Advanced (year 4+): 0.1-0.25kg/month. Keep surplus to 150-250 kcal to limit fat gain.

Going higher than these rates just means more fat alongside the muscle — which you'll have to cut later.

Why Your BMR Sets the Floor

Our calculator warns you when your target drops below your BMR. Here's why: your BMR represents the energy your organs need to function. Consistently eating below this level triggers metabolic adaptation — your body downregulates thyroid output, reduces NEAT (non-exercise activity thermogenesis), and increases cortisol.

Trexler et al. (2014) documented that prolonged severe deficits can reduce metabolic rate by 15-20% beyond what weight loss alone would predict. This "metabolic damage" takes weeks to months to reverse through a maintenance phase.

If the calculator shows your target below BMR, select a slower rate. You'll lose fat at the same overall pace because you won't need as many diet breaks to recover.

The 7,700 Calorie Rule — And Why It's Approximate

The "7,700 kcal per kg of fat" rule (3,500 per pound) was derived by Max Wishnofsky (1958) and has been the basis of weight loss projections for decades. It's a reasonable approximation, but three factors make real-world results deviate:

• Body composition: You don't lose pure fat. A deficit also draws from glycogen (which holds water) and some muscle tissue. Early weight loss is faster than predicted because of glycogen/water depletion.
• Metabolic adaptation: Your BMR decreases as you lose weight. A 5kg loss reduces daily burn by ~50-70 kcal. The calculator doesn't auto-adjust for this — recalculate every 4-6 weeks.
• NEAT reduction: In a deficit, most people unconsciously move less — less fidgeting, slower walking, fewer spontaneous movements. This can reduce daily expenditure by 200-300 kcal without you noticing.

Practical Strategy: Cut and Bulk Cycles

Most intermediate to advanced lifters alternate between cutting (deficit) and bulking (surplus) phases:

• Cutting phase: 8-16 weeks at a moderate deficit. Goal: lose fat while preserving muscle. Keep protein at 2.0-2.4g/kg. Maintain training intensity.
• Maintenance phase: 2-4 weeks at TDEE after cutting. Allows metabolic recovery and psychological reset. Critical for long-term success.
• Bulking phase: 12-20 weeks at a lean surplus. Goal: build muscle with minimal fat gain. Protein at 1.6-2.0g/kg. Progressive overload on all lifts.

RepStack tracks your Strength Score across these phases so you can see that your strength is preserved during cuts and increasing during bulks. The 1RM Calculator helps you set training percentages for each phase.

Tips for Staying on Track

• Track weight trends, not daily weigh-ins. Water weight can swing 1-2kg day to day. Look at the weekly average. RepStack computes this automatically.
• Front-load protein. Hit your protein target first every day — it's the macro that preserves muscle and keeps you full. Use our Macro Calculator to dial this in.
• Don't slash calories on rest days. Muscle repair happens on rest days. Keep your deficit consistent 7 days a week rather than eating less when you don't train.
• Recalculate every 4-6 weeks. As your weight changes, so does your TDEE. A 5kg loss means ~50-70 fewer calories burned per day.
• If progress stalls for 2+ weeks, reduce calories by another 100-150 kcal/day or add 10-15 minutes of low-intensity cardio rather than slashing intake dramatically.