What is a bulk calculator?
A bulk calculator adds a controlled calorie surplus on top of your TDEE (Total Daily Energy Expenditure) to determine the calorie and macro targets needed for muscle growth. It uses your body stats to compute BMR, multiplies by an activity factor, then adds a mode-specific surplus.
When you enter your body fat percentage, the calculator upgrades to the Katch-McArdle BMR formula (more accurate for trained individuals) and generates a personalised bulk vs cut recommendation based on where your body fat sits relative to evidence-based thresholds.
Lean bulk vs clean bulk vs aggressive bulk
The three bulk modes differ in surplus, weekly gain rate, and muscle:fat ratio (Barakat et al., 2020; Helms et al., 2014):
| Mode | Daily surplus | Weekly gain | Muscle ratio | Best for |
|---|---|---|---|---|
| Lean bulk | +200 kcal | ~0.1 kg | ~55% | Staying lean year-round, higher BF% |
| Clean bulk | +350 kcal | ~0.2 kg | ~45% | Intermediate lifters, best balance |
| Aggressive bulk | +600 kcal | ~0.35 kg | ~35% | Underweight beginners, very lean athletes |
Weekly gain estimated at 7,700 kcal/kg. Muscle ratios from Barakat et al. 2020.
Should you bulk or cut? Body fat thresholds
Your starting body fat is the biggest determinant of whether a bulk will be productive. High body fat worsens insulin sensitivity and shifts the muscle:fat gain ratio unfavourably. Thresholds are based on ACE body fat classification:
| Sex | Bulk (ideal) | Lean bulk only | Cut first |
|---|---|---|---|
| Men | 10–15% BF | 15–20% BF | >20% BF |
| Women | 18–25% BF | 25–30% BF | >30% BF |
Macro breakdown for bulking
- Protein — 2.0 g/kg: Maximises muscle protein synthesis during a caloric surplus (Morton et al., 2018). Calculated first. Provides 4 kcal/g.
- Fat — 25% of calories: Supports testosterone production, cell membrane integrity, and fat-soluble vitamin absorption. Provides 9 kcal/g.
- Carbohydrates — remaining: Maximises glycogen stores for training performance. Provides 4 kcal/g. Gets the largest allocation in a bulk to fuel high-intensity sessions.
How the bulk calorie calculation works
Two BMR formulas are used depending on available input:
- Mifflin-St Jeor (default): BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 (men) or −161 (women).
- Katch-McArdle (with BF%): BMR = 370 + 21.6 × lean body mass(kg). More accurate for trained individuals because it accounts for the higher metabolic activity of muscle tissue.
Bulk Target = (BMR × activity multiplier) + surplus
Frequently asked questions
How many calories should I eat to bulk?
Eat your TDEE plus a surplus: +200 kcal for a lean bulk (~0.1 kg/week), +350 kcal for a clean bulk (~0.2 kg/week), or +600 kcal for an aggressive bulk (~0.35 kg/week). Larger surpluses add more fat alongside muscle.
What is the difference between a lean bulk and a clean bulk?
A lean bulk uses +200 kcal/day to minimise fat gain at about 55% muscle ratio. A clean bulk uses +350 kcal/day for faster progress at ~45% muscle ratio. An aggressive bulk uses +600 kcal/day but drops to ~35% muscle:fat gain ratio.
Should I bulk or cut first?
Men above 20% body fat and women above 30% body fat should cut first. Bulking at high body fat worsens insulin sensitivity and shifts the muscle:fat gain ratio unfavourably. Cut to 10–15% (men) or 18–25% (women) before starting a productive bulk.
How much protein do I need when bulking?
Aim for 2.0 g per kg of bodyweight per day (~0.9 g/lb). This maximises muscle protein synthesis during a surplus. For an 80 kg person that is 160 g/day — roughly 640 kcal from protein.
How long should a bulk phase last?
Most effective bulks run 3–6 months, stopping when body fat reaches ~17% for men or ~27% for women. A 4–8 week mini-cut restores optimal body composition before the next bulk. Perpetual bulking at elevated body fat produces diminishing muscle returns.