What this tool does
A protein-sparing modified fast (PSMF) is a very-low-calorie, high-protein diet used in clinics for rapid fat loss. This free calculator shows your protein target (up to 2.4 g/kg), 30 g carb and fat caps, and holds calories at a 1,200/1,500 kcal educational floor — it will not output sub-floor crash targets.
How it works
Enter sex, age, height, weight, and activity. Protein uses the high end of an ISSN-aligned deficit range (about 2.4 g/kg body weight). Carbs and fat are capped at 30 g each as planning minimums. Total kcal = protein×4 + carbs×4 + fat×9. If that sum falls below 1,200 kcal (women) or 1,500 kcal (men), the display holds at the floor.
Formula and methodology
Protein (g) ≈ 2.4 × body weight (kg). Carbs = 30 g cap. Fat = 30 g minimum. Displayed kcal = max(protein×4 + carbs×4 + fat×9, 1,200 or 1,500 by sex). Classic clinic PSMFs often target 800–1,200 kcal from lean protein alone — that range is not something a public calculator should print.
How to interpret the result
Treat this as an educational sketch of PSMF macro logic, not permission to start an aggressive cut. Compared with a normal 15–20% calorie deficit (moderate protein, flexible carbs/fat) or keto (high fat at maintenance), a PSMF is the most protein-heavy and lowest-calorie pattern — usually short-term and medically supervised. If you searched for an 800 kcal meal plan, use the weight-loss calculator instead.
Example
Comparison sketch — Normal deficit: TDEE minus ~15–20%, protein ~1.6–2.2 g/kg, flexible carbs/fat, no fixed floor. Keto: carbs very low, fat high, calories often at maintenance. PSMF (this tool): protein ~2.4 g/kg, carbs and fat capped at 30 g each, calories held at 1,200 kcal (women) or 1,500 kcal (men). A 70 kg person gets about 168 g protein (672 kcal), plus 30 g carbs (120 kcal) and 30 g fat (270 kcal) = ~1,062 kcal from macros — the floor raises the displayed total.
Common mistakes
- Running a forum PSMF for weeks without medical oversight or blood work.
- Stacking extra cardio on top of an already tiny intake.
- Skipping electrolytes, vitamins, or a refeed plan on longer cuts.
- Treating this calculator as approval — it holds a safety floor by design.
When this estimate may be off
- Pregnancy, eating-disorder history, type 1 diabetes, kidney disease, or any prescribed medical diet.
- You wanted a normal 15–20% deficit — use the calorie calculator or weight-loss calculator.
- You need LBM-based protein per pound — this tool uses total body weight, not lean mass.