Estimate energy use over a fasting window using BMR, duration, activity, and broad phase assumptions. Includes an illustrative breakdown of total calories and fat-linked energy.
Your body continues to use energy while you are not eating, but the exact fuel mix varies by person and by fast length. This page uses BMR, activity, and a set of simplified phase assumptions to estimate total energy use during a fasting window.
It is best read as a planning and reference tool rather than a metabolic test. It does not measure ketones, autophagy, hormone levels, or actual tissue loss, and early scale changes during fasting often reflect water and glycogen shifts as much as fat loss.
The calculator shows total calories, an illustrative fat-linked share, and a phase-by-phase breakdown so you can compare different fasting durations under the same assumptions.
This page is useful for turning a fasting window into a rough energy-use estimate and for comparing short and longer fasts with the same assumptions. It is mainly an expectation-setting tool, not proof of a specific metabolic state.
BMR (Mifflin-St Jeor): - Men: (10 x weight_kg) + (6.25 x height_cm) - (5 x age) + 5 - Women: (10 x weight_kg) + (6.25 x height_cm) - (5 x age) - 161 Calories per Hour = (BMR x activity_factor x fasting_adjustment) / 24 Total Fast Calories = Calories per Hour x fasting_hours Activity factors used on this page: - Resting/sleeping: 1.0 - Light activity (desk work): 1.2 - Moderate activity: 1.4 - Active (exercise during fast): 1.6 Fasting adjustment used on this page: - <= 48 hours: 1.05 - > 48 hours: 1.00 Illustrative fat-linked energy assumptions used on this page: - Hours 0-4: 30% - Hours 4-12: 50% - Hours 12-18: 75% - Hours 18-36: 90% - Hours 36-72: 95% - Fat equivalent = fat-linked calories / 9
Result: ~1,647 kcal total / ~99 g fat equivalent
BMR = (10 x 80) + (6.25 x 178) - (5 x 35) + 5 = 1,742.5 kcal/day. With light activity (1.2x) and this page's short-fast adjustment (1.05x), the estimate is about 1,647 kcal over 18 hours. Using the page's internal phase assumptions, about 888 kcal are treated as fat-linked energy, or roughly 99 grams of fat equivalent. This is not a direct measurement of body-fat loss.
This page starts with your BMR, adjusts it for the activity level selected, and then applies a small short-fast adjustment used by this calculator. It then spreads those calories across the fasting window and applies a set of broad phase assumptions to estimate the fat-linked share. Those phase assumptions are built for comparison and planning; they are not measurements of what your body is doing minute to minute.
Fasting-related scale changes are not the same thing as direct fat loss. Short-term weight change often reflects shifts in glycogen, water, gut contents, and sodium balance. That is why this calculator reports a fat equivalent from the page's assumptions rather than claiming to measure exact tissue change.
The most useful way to read this page is comparatively: how does a 14-hour fast differ from an 18-hour or 24-hour fast under the same assumptions? It is less useful as a precise physiologic forecast, especially during longer fasts or when medication use, illness, heavy training, or unusual sleep patterns are in the picture.
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This worksheet estimates fasting energy use from a BMR equation, an activity factor, a small short-fast adjustment, and broad phase assumptions for fuel mix. It is an illustrative comparison tool, not a measure of ketones, autophagy, or exact fat loss.
Short fasting windows do not appear to cause a large immediate drop in energy expenditure for most people, but responses vary and the literature is not uniform. This page uses a small fixed adjustment for fasts up to 48 hours as a simplifying assumption, not a measurement of your personal metabolic rate.
The fat output on this page is a rough energy-share estimate based on built-in phase assumptions. It is useful for comparison, but it is not a direct measure of how much stored body fat was lost during that fast.
Some fat is used even in the fed state. During fasting, the share of energy coming from fat may rise over time, but there is no single clock time that applies to everyone. This page uses broad time windows to approximate that shift.
Short fasts do not imply dramatic lean-tissue loss for everyone, but protein turnover and tissue use vary with duration, diet quality, training, and health status. This page does not estimate muscle loss.
Tolerance varies. Some people handle light or moderate activity well while fasting, while others notice dizziness, lower performance, or a harder time recovering. This calculator does not assess exercise safety or performance readiness.
It is a simplified timeline used by this page to change the assumed fat-linked share of total energy use as the fast gets longer. It should not be read as a direct readout of ketosis, autophagy, or hormone levels.