Fasting Day Calorie Calculator

How to Use This Tool

Start by selecting your gender from the dropdown menu. Enter your age in years, weight (with unit selection for kg or lbs), and height (with unit selection for cm or inches).

Choose your typical activity level from the provided options, then select your desired calorie deficit percentage for fasting days. Click the Calculate Calories button to generate your personalized breakdown.

Use the Reset button to clear all inputs and results if you need to run a new calculation.

Formula and Logic

This calculator uses the Mifflin-St Jeor formula to calculate your Basal Metabolic Rate (BMR), the number of calories your body burns at rest. The formula adjusts for gender, age, weight, and height:

• Male BMR: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5
• Female BMR: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161

Maintenance calories are calculated by multiplying BMR by an activity multiplier based on your selected activity level. Fasting day target calories are derived by applying your chosen deficit percentage to your maintenance calories.

Protein recommendations use a standard 1.8 grams per kilogram of body weight. Carbohydrate and fat splits are set to 40% and 30% of remaining calories after protein, respectively.

Practical Notes

All unit conversions are handled automatically: weight in lbs is converted to kg (1 lb = 0.4536 kg), and height in inches is converted to cm (1 inch = 2.54 cm).

Never drop below 1200 calories per day for women or 1500 calories per day for men, even on aggressive fasting deficits. Consult a healthcare provider before starting any new fasting or calorie restriction plan.

Prep fasting day meals in advance to avoid impulse eating, and track your intake with a simple food diary to stay on target. Budget-friendly fasting day staples include eggs, lentils, frozen vegetables, and canned fish.

• Common conversions: 70 kg = 154 lbs, 170 cm = 67 inches
• Adjust your activity level based on actual daily movement, not intended exercise
• Recalculate every 4-6 weeks as your weight changes to keep targets accurate

Why This Tool Is Useful

Generic fasting calorie guides often ignore personal metrics like age, gender, and activity level, leading to inaccurate targets. This tool provides a personalized breakdown tailored to your body and lifestyle.

Detailed outputs include BMR, maintenance calories, fasting targets, and macronutrient splits, so you can plan balanced meals instead of guessing total calories. The visual progress bar helps you quickly see your chosen deficit level.

Whether you follow 16:8 intermittent fasting, periodic 24-hour fasts, or another schedule, this tool adapts to your needs.

Frequently Asked Questions

Can I use this calculator if I follow a 16:8 fasting schedule?

Yes, this tool calculates calorie needs for any fasting day regardless of your fasting window length. Enter your activity level for the day to get accurate targets, even if you are fasting for 16 hours or more.

What if my weight is in pounds?

Use the weight unit dropdown to select lbs, and the tool will automatically convert to kilograms for all calculations. No manual conversion is needed on your end.

Is it safe to follow a 50% calorie deficit on fasting days?

A 50% deficit may be too low for individuals with active lifestyles or specific health conditions. We recommend consulting a healthcare provider before following aggressive deficits, and never dropping below 1200 calories/day (women) or 1500 calories/day (men).

Additional Guidance

Stay hydrated on fasting days by drinking 3-4 liters of water daily, and include fiber-rich foods like leafy greens and whole grains to stay full longer. Adjust your deficit based on how you feel: if you experience fatigue or dizziness, increase your calorie target by 10-15%.

Combine fasting with strength training 2-3 times per week to preserve muscle mass while in a calorie deficit. Revisit this calculator every month to adjust your targets as your weight and activity levels change.