Calorie Deficit Weight Loss Calculator
A physiology-based model that estimates fat loss, lean mass changes, calorie deficits, and total weight change during sustained calorie restriction. It can be used for Intermittent Fasting, OMAD, time-restricted eating, or traditional calorie deficit diets. Adjust your inputs to see how your projected weight curve changes over time. For educational purposes only - estimates are based on simplified physiological assumptions and individual results may vary.
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Your Starting Point
Enter your baseline parameters.
Calculator Mode
Physiological Parameters
If you don’t know your Body Fat or Bone Mass, leave them blank and we’ll auto-estimate.
Daily Activity
Weight Loss Plan
Model Parameters
What Happens During a 16 to 24 Hour Fast
🍽️ Digestion (0–4h)
Your body is still living off the last meal. Glucose is the main fuel and insulin is elevated. The real shift is still ahead.
📉 Blood Sugar Falls (3–8h)
With the meal absorbed, blood glucose drifts down toward fasting levels. The dip is gentle and normal, and it triggers the hormone shift that comes next.
🔽 Insulin Falls (4–8h)
Insulin drops roughly 20–50% from its fed peak while glucagon rises, and your liver taps stored glycogen. This flip is the master switch of the whole fast.
🔥 Fat Oxidation Increases (8–16h)
Glycogen is spent faster than it is replaced, so your body leans more on stored fat. Often the hardest stretch: hunger waves and fatigue are common. The 2–5 lb drop here is mostly water, not fat.
🌱 Ketones Rising (16–24h)
Your liver starts turning fatty acids into ketones, climbing to around 0.1–0.5 mmol/L. Still low, but the first sign the metabolic switch is underway.
🪫 Glycogen Depletion (18–30h)
Liver glycogen is roughly 70–90% depleted, so fat and ketones carry more of the load. Sodium losses rise, which is why electrolytes start to matter here.
⚡ Moderate Ketosis (24–48h)
Ketones reach roughly 0.5–2.0 mmol/L and become real fuel for brain, heart and muscle. Many people notice a clear lift in mental clarity around here.
💪 HGH Surge (~24h)
Growth hormone surges around the one-day mark, rising several-fold in research. One of its jobs is protecting lean muscle while you run on fat.
Timing varies. Your last meal's carbohydrate content, activity level, and metabolic health can shift these ranges by hours.
A 16:8 window gets you to Ketones Rising, while OMAD reaches Moderate Ketosis and the HGH surge.
Frequently Asked Questions
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A deficit of roughly 500 cal per day produces about 1 lb of loss per week. Larger deficits work faster but increase muscle loss and are harder to sustain. This calculator shows the projected curve for whatever deficit you enter, so you can see the tradeoff.
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For fat loss specifically, total calories matter most. Controlled studies show similar results when calories are matched. IF helps many people because eating in a shorter window makes the deficit easier to maintain, not because the timing itself burns extra fat.
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Neither is inherently better for weight loss at the same calorie intake. OMAD produces a longer daily fast and deeper metabolic effects, but makes hitting protein and micronutrient targets harder. 16:8 is easier to sustain for most people.
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Two common reasons. Your metabolic rate drops as you lose weight, so the same intake becomes a smaller deficit. And day-to-day water shifts can mask real fat loss for weeks. Recalculate your target at every 10 lb lost and judge progress over 3 to 4 weeks, not day to day
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Some lean mass loss is normal in any deficit. Resistance training and adequate protein reduce it substantially. The calculator estimates lean mass change separately so you can see the split.
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It is a simplified physiological model, not a measurement. It uses your RMR, activity level, and body composition. Real results vary with adherence, metabolic adaptation, hydration, and how accurately you track intake.
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Most people do well with 8 to 16 week blocks followed by a period at maintenance. Extended aggressive restriction increases muscle loss, hormonal suppression, and dropout risk.
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Many people fast 16 hours daily without issue. Longer or more aggressive protocols are not appropriate during pregnancy, with a history of eating disorders, when underweight, or alongside certain medications and conditions. Talk to your doctor before starting.
Interested in extended or multi-day fasting? Check out my other calculator
Share Your Feedback and Fasting Results
Tried the calculator or hit your goal with IF or a calorie deficit? Share your results, observations, or questions.