TDEE for fat loss: how to calculate your deficit (without losing muscle)

Knowing your TDEE — total daily energy expenditure — is the easy part. The math is well-defined: BMR (resting calories) × activity multiplier (how much you move). The harder question is what to subtract from that number to lose fat without losing muscle.

This guide covers the actual deficit math, why "1,200 calories" is wrong for almost everyone, and the protein target that determines whether the weight you lose is fat or muscle.

How to calculate your TDEE

The Mifflin-St Jeor formula is the current standard:

• Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
• Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161

Then multiply BMR by your activity factor:

• Sedentary (desk job, no exercise): × 1.2
• Lightly active (1-3 sessions/week): × 1.375
• Moderately active (3-5 sessions/week): × 1.55
• Very active (6-7 sessions/week): × 1.725
• Extra active (athlete, manual labour): × 1.9

For a 32-year-old man, 178cm, 80kg, training 4x/week: BMR = 1,758 → TDEE = 2,725 kcal/day at maintenance.

The right size of deficit

The standard internet advice is "500-calorie deficit per day = 1lb per week." This is roughly correct (a pound of fat is about 3,500 kcal) but it ignores two important details:

1. The deficit shouldn't exceed 20-25% of TDEE

Cutting harder than that triggers compensatory adaptations: your body slows non-exercise movement (NEAT), drops thyroid output, and pulls protein from muscle. The scale moves but body composition gets worse, not better.

For our 2,725 kcal example: max sustainable deficit is around 550-680 kcal. So eat 2,050-2,175 kcal/day. That's 0.5-0.7 kg/week of fat loss — not dramatic, but sustainable for 12+ weeks without metabolic damage.

2. Faster cuts work for short windows

If you're under 12 weeks out from a specific event (a race, a trip, a wedding), an aggressive 25-30% deficit is fine for 6-8 weeks. Beyond that window, the body adapts and the cut stalls.

The protein target that protects muscle

This is where most cuts go wrong. In a calorie deficit, your body breaks down both fat and muscle for energy. Protein intake is the variable that determines the ratio.

The research-backed minimum for muscle preservation in a deficit:

• Untrained: 1.6 g/kg bodyweight
• Recreationally trained: 1.8-2.0 g/kg
• Trained athlete in a deficit: 2.2-2.4 g/kg

For an 80kg trained athlete: 160-180g of protein per day, every day. Not on average over the week. Every day.

What this looks like at 4 meals: 40-45g protein per meal. That's roughly a palm-sized portion of chicken/fish + a yogurt, OR a 2-egg omelette + protein shake. Hitting it consistently is harder than the math suggests.

Where the rest of the calories come from

Once protein is locked in (~1,300 kcal of the daily target for our example), the remaining ~750-870 kcal split between carbs and fat. Two general patterns:

Higher carb (better for training performance)

~25% fat, balance carbs. Suited to athletes doing hard sessions 4+ times per week. Glycogen stays full, training output stays high, recovery is faster.

Higher fat (better for satiety)

~40-45% fat, balance carbs. Suited to people who struggle with hunger on a deficit. Fat has higher per-gram satiety than carbs at equivalent calories.

Neither is "correct" — it depends on whether you're optimising for training quality or daily food adherence.

Common mistakes

• Cutting too aggressively. The 1,200-calorie advice you'll see online is suitable for very small, sedentary women. For most people it's a 40-50% deficit and unsustainable.
• Not adjusting after weight loss. When you weigh less, your TDEE drops. Recalculate every 4-6 kg lost.
• Eyeballing protein. Most people guess high. Actually weigh portions for one week to calibrate. Most discover they're 30-40g short.
• Counting "diet days only." If you eat at maintenance Saturdays + Sundays, the weekly average matters more than the weekday number. Don't blow your weekly target on weekends.

Frequently asked questions

How long should a cut last?

8-16 weeks at 15-20% deficit is the standard sustainable range. Beyond 16 weeks, schedule a 2-week diet break at maintenance to reset hormones, then resume.

What if my weight stalls for 2-3 weeks?

First check that you're actually in deficit (food tracking gets sloppy after a few weeks). If yes, drop another 100 kcal — but not more. A 2-week stall is normal physiological adaptation, not a metabolic emergency.

Can I do a recomp instead of a cut?

Recomp (gain muscle while losing fat at maintenance) only works for two groups: complete beginners in their first 6-12 months of training, and people returning from a long break. Everyone else needs a cut + a separate building phase.

To get your specific TDEE, deficit, and per-meal targets in 60 seconds: the TDEE & Macro Planner calculates everything from your stats and goal. The AI coach reviews your numbers and tells you exactly what to adjust if your weight stalls.