Set your calorie goal
How large a daily calorie deficit do you want to create through fasting?
Based on ~3,500 kcal = 1 lb fat. Consult a doctor before starting any fasting plan.
From goal to fasting plan in two steps
No guesswork, no spreadsheets. Set your deficit, enter your body stats, done.
Set your calorie goal
Choose a preset deficit, 250, 500, 750, or 1,000 kcal, or enter a custom number. 500 kcal/day = ~1 lb/week.
Enter body stats
Sex, age, weight, and height power the Mifflin-St Jeor BMR calculation, the most accurate resting metabolic rate formula for non-athletes.
Get your fasting plan
See your exact hours to fast, the closest IF protocol, hourly burn rate, and estimated weekly fat loss, all in one screen.
Which deficit is right for you?
The right deficit balances fat loss speed with muscle preservation and energy levels.
Frequently Asked Questions
Not significantly in the short term. Fasting creates a calorie deficit by extending the period without food, your body continues burning calories at roughly your resting metabolic rate. Short-term fasting (under 24 hours) may modestly increase metabolic rate by 3–4%, but this calculator uses your baseline BMR for conservative, realistic estimates.
The Mifflin-St Jeor equation is the most validated BMR formula for general adults, with typical accuracy within ±10% of lab-measured values. Results are most accurate for sedentary to lightly active adults. Athletes with high muscle mass may have a BMR 5–15% higher than calculated.
This means your target calorie deficit exceeds what can be safely achieved in a single-day fast. Either lower your daily deficit target (500 kcal is the standard recommended maximum for sustainable weight loss) or combine moderate fasting with slightly reduced calories during your eating window.
Yes, both directly affect your BMR. Taller people burn more calories per hour (more body mass to maintain). BMR also decreases roughly 2–3% per decade after age 30. A 30-year-old woman (5'6", 145 lbs) burns about 57 kcal/hr vs. about 52 kcal/hr at age 50, meaning she needs roughly 10% longer to hit the same deficit.
For most healthy adults, yes. A 500 kcal/day deficit creates approximately a 1 lb/week fat loss rate, the upper bound most registered dietitians recommend for sustainable, muscle-preserving weight loss. Deficits above 750 kcal/day increase risk of muscle loss and metabolic adaptation. Always consult your doctor before starting any fasting regimen, especially if you take medications or have a medical condition.
The standard fasting calculator answers "when does my fast end?" given a chosen protocol. This calorie calculator works in reverse: you set a calorie deficit goal and get told how long to fast. Use this one when your primary goal is weight loss and you want to back-calculate from a target deficit rather than follow a named protocol.
How We Calculate
This calculator uses the Mifflin-St Jeor equation, the most clinically validated formula for estimating resting energy expenditure in non-athletic adults (Frankenfield et al., 2005).
BMR Formula:
- Men: BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age) + 5
- Women: BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age) − 161
Fasting duration:
Hourly calorie burn = BMR ÷ 24
Fasting hours needed = Target deficit ÷ Hourly burn
Weekly fat loss:
Based on the widely accepted estimate that ~3,500 kcal of deficit corresponds to approximately 1 lb of body fat (Hall & Kahan, 2018). Individual results vary by body composition, hormones, and activity level.
Note: This calculator estimates the deficit created by not consuming calories during a fast, not a metabolic acceleration. Fasting does not significantly raise calorie burn in short windows. It creates a deficit by extending the period without caloric intake.
Sources & References
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults. J Am Diet Assoc. 2005;105(5):775-789.
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247.
- Hall KD, Kahan S. Maintenance of Lost Weight and Long-Term Management of Obesity. Med Clin North Am. 2018;102(1):183-197.
- Tinsley GM, La Bounty PM. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutr Rev. 2015;73(10):661-674.
Data last verified:
What Is a Fasting Calorie Calculator?
A fasting calorie calculator works in reverse to a standard IF timer. Instead of asking "when does my fast end?", it asks "how long do I need to fast to hit a specific calorie deficit?" You set a target, say, 500 kcal, and the calculator tells you exactly how many hours of fasting that requires given your unique metabolic rate.
The key number underneath every result is your BMR, Basal Metabolic Rate. This is how many calories your body burns per day at complete rest, just keeping organs functioning. Divide by 24 and you get your hourly burn rate. Multiply by fasting hours and you get your deficit. The Mifflin-St Jeor equation is used to compute BMR, as it outperforms the older Harris-Benedict formula for general adults (Frankenfield et al., 2005).
This tool pairs directly with our intermittent fasting calculator, use this one to find your target fasting duration, then use the fasting calculator to plan your exact schedule and see your eating window.
Who Should Use This Calculator?
People with a specific weight loss goal use it to back-calculate the fasting duration they need, rather than picking a protocol and hoping the calories work out. If you want to lose exactly 1 lb/week, this calculator tells you the minimum daily fasting hours required for your body.
Experienced IF practitioners use it when switching protocols. Moving from 16:8 to 18:6? Plug in your body stats to see how much additional deficit those two hours create per week, the answer often motivates the upgrade.
People tracking calories with apps use the BMR and hourly burn figures as a baseline for their deficit calculations. The BMR alone, updated for your current weight, is useful even outside fasting contexts.
Always consult a healthcare provider before starting a fasting regimen if you take medications that require food, are pregnant, have a history of eating disorders, or have a metabolic condition such as diabetes. This tool is for planning purposes and does not constitute medical advice.
Know your number. Own your fast.
Enter your deficit target and body stats above, get your exact fasting hours in seconds. Then use the fasting calculator to lock in your schedule.
Plan your fasting scheduleFastingTimeCalculator.com Team
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