Your Fasting Day Calorie Breakdown
How to Use This Tool
Follow these simple steps to get your personalized fasting day calorie targets:
- Enter your age, biological sex, weight, and height using the input fields. Select the correct units for weight (kg/lbs) and height (cm/inches).
- Choose your activity level on fasting days from the dropdown menu to reflect how much you move during fasting periods.
- Select your fasting duration (e.g., 16:8, 18:6) to match your current fasting schedule.
- Click the Calculate button to generate your detailed calorie breakdown.
- Use the Reset button to clear all inputs and start over, or the Copy Results button to save your targets to your clipboard.
Formula and Logic
This calculator uses the medically validated Harris-Benedict equation to calculate your Basal Metabolic Rate (BMR), adjusted for biological sex. It then applies standard activity level multipliers to find your Total Daily Energy Expenditure (TDEE) for non-fasting days.
For fasting days, your calorie target is calculated by multiplying your TDEE by a fasting duration factor, which reduces calories based on how long you fast:
- 16:8 fast: 25% of TDEE
- 18:6 fast: 20% of TDEE
- 20:4 fast: 15% of TDEE
- 24-hour fast: 10% of TDEE
- 36-hour fast: 5% of TDEE
- 48-hour fast: 2% of TDEE
Protein targets are set at 1.2g per kg of body weight to preserve muscle mass during fasting. Water intake recommendations follow the standard 35ml per kg of body weight guideline.
Practical Notes
Keep these lifestyle-specific tips in mind when using your results:
- Calorie targets are estimates only. Adjust based on how your body feels during fasts—reduce calories further if you feel sluggish, or increase slightly if you feel overly hungry.
- On fasting days, prioritize nutrient-dense, high-fiber foods to stay full longer on fewer calories.
- Drink the recommended water intake to avoid dehydration, which is common during fasting periods.
- If you have underlying health conditions, consult a healthcare provider before starting a fasting routine.
- Track your progress weekly and adjust your fasting duration or calorie target as needed to meet your goals.
Why This Tool Is Useful
This calculator removes the guesswork from planning fasting day meals, helping you stay consistent with your routine without overeating or under-eating. It accounts for your unique body metrics and activity level, so targets are personalized rather than generic. The detailed breakdown includes protein and water targets to support overall health during fasting, not just calorie counts. It’s free, works entirely in your browser, and requires no sign-up or personal data storage.
Frequently Asked Questions
Is this calculator suitable for all fasting types?
Yes, it supports common intermittent fasting schedules (16:8, 18:6, 20:4) as well as longer fasts (24, 36, 48 hours). Select the option that matches your current routine for accurate results.
Can I use this if I exercise on fasting days?
Absolutely—select your activity level based on how much you move on fasting days, including any light exercise. The calculator adjusts your TDEE to reflect this activity for more accurate targets.
Do I need to count calories exactly on fasting days?
No, these are guidelines to help you plan. Focus on eating whole, nutrient-dense foods within your target range rather than tracking every calorie perfectly.
Additional Guidance
If you are new to fasting, start with a 16:8 schedule and gradually increase fasting duration as your body adjusts. Pair your fasting routine with a balanced non-fasting day diet to avoid nutrient deficiencies. Re-calculate your targets every 4-6 weeks as your weight changes to keep your plan effective. Avoid fasting if you are pregnant, breastfeeding, or have a history of eating disorders—consult a professional first.