Inputs
Formula Used
- BMR (Mifflin–St Jeor): 10×kg + 6.25×cm − 5×age + s, where s = +5 (male) or −161 (female).
- TDEE: BMR × activity_factor.
- Target daily shift: ±(rate_per_week × 7700) / 7 kcal/day (loss is negative, gain is positive).
- Feast calories are solved so the schedule average matches the target average.
- Weekly deficit: (TDEE − average_intake) × 7.
- Projected change: weekly_deficit / 7700 (approx kg/week).
- Protein calories: 4 × protein_g. Fat calories: 9 × fat_g. Carbs fill remaining calories: (kcal − protein_kcal − fat_kcal)/4.
- Fast-day macros are scaled by the fast/feast calorie ratio.
7700 kcal per kilogram is an approximation. Real outcomes vary with water shifts, adherence, training load, and metabolic adaptation.
How to Use This Calculator
- Choose your unit system, sex, age, height, weight, and activity level.
- Select your goal and a realistic weekly change target.
- Pick a fast-day intensity (25%, fixed 500 kcal, custom, or water fast).
- Set a start date, whether the first day is fast or feast, and the number of weeks.
- Optionally refine macro targets using protein and fat per kilogram.
- Press Calculate to view BMR, TDEE, calorie targets, macros, and your schedule.
- Use the CSV/PDF buttons to save or share the plan.
Example Data Table
| Input | Example |
|---|---|
| Unit system | Metric |
| Sex | Male |
| Age | 30 years |
| Height | 175 cm |
| Weight | 80 kg |
| Activity | Moderate (1.55) |
| Goal | Lose 0.5 kg/week |
| Fast day | 25% of maintenance |
| Weeks | 4 |
| First four days | Type | Calories |
|---|---|---|
| Day 1 | Fast | ~25% of maintenance |
| Day 2 | Feast | Solved to hit weekly target |
| Day 3 | Fast | Same as Day 1 |
| Day 4 | Feast | Same as Day 2 |
Alternate-day structure and energy balance
Alternate-day fasting alternates low-intake days and higher-intake days. The weekly outcome depends on the average calories across the full cycle, not a single day. This calculator displays fast-day calories, feast-day calories, and the resulting average so you can compare the plan against maintenance.
How the calculator estimates maintenance needs
Maintenance calories are estimated from basal metabolic rate and an activity multiplier. Basal metabolic rate uses the Mifflin–St Jeor equation with weight, height, age, and sex. Activity factors offered range from 1.2 to 1.9, covering sedentary through very active lifestyles. Maintenance equals BMR × activity.
Setting fast-day calories with guardrails
Fast-day options include 25% of maintenance, a fixed 500 kcal, a custom value, or a zero-calorie day. To avoid extreme settings, fast-day calories are capped at 75% of maintenance. The macro panel scales fast-day targets from feast-day targets, prioritizing protein first, then fat, then carbohydrates.
Solving feast-day targets for your weekly goal
When you choose a weekly change target, the calculator converts it into a daily energy shift using 7700 kcal per kilogram. It then solves the feast-day calorie target so that the schedule average matches your goal while holding the fast-day value constant. Conservative minimums are applied: 1500 kcal for males and 1200 kcal for females.
Macro planning for protein, fat, and carbs
Feast-day protein defaults to 1.6 g per kilogram (or 1.8 for gain), while fat defaults to 0.8 g per kilogram. Protein is counted at 4 kcal per gram and fat at 9 kcal per gram. Carbohydrates fill the remaining calories, producing a complete macro target you can use for meal planning.
Interpreting the schedule and the chart
The schedule table lists each date with its day type, calories, and macro targets. The chart visualizes calorie swings, making it easier to spot overly aggressive gaps between fast and feast days. If the projected trend is faster than desired, reduce fast-day intensity, lower the weekly target, or shorten the planning window.
Most people run a simple 4-week block, review energy and hunger, then adjust. Because water and glycogen shift, scale changes can look uneven; use the weekly trend figure and the calorie average to judge progress over time too.
FAQs
1) What is alternate-day fasting in practical terms?
You alternate a lower-calorie day with a higher-calorie day. The goal is a controlled weekly calorie average while still allowing satisfying feast days. Many people start with 25% fast days rather than zero intake.
2) Should I choose 25% of maintenance or 500 kcal for fast days?
Use 25% if you want fast days that scale with body size and activity. Use 500 kcal if you prefer a simple fixed cap. If either option feels too hard, raise fast-day calories or shorten the plan.
3) How are feast-day calories calculated here?
Feast-day calories are solved so the average intake across all planned days matches your goal-adjusted target. The calculator holds your chosen fast-day calories constant, then balances feast days. It may apply a conservative minimum to avoid very low feast targets.
4) Do I need to change workouts on fast days?
Many people keep easy walks or light strength sessions on fast days, and schedule harder training on feast days. If performance drops, increase fast-day calories, add electrolytes, or move intense sessions to feast days.
5) Why does the projected rate differ from my scale changes?
The projection uses an energy model and an approximate 7700 kcal per kilogram conversion. Water, glycogen, sodium, sleep, and digestion can shift weight quickly. Use weekly averages and several weeks of data before changing your settings.
6) Who should avoid fasting or get medical advice first?
Avoid fasting during pregnancy, breastfeeding, or if you are under 18. Get medical advice if you have diabetes, take glucose-lowering or blood-pressure medicines, have kidney or heart disease, or have a history of disordered eating.