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7-day fast

7 Day Fast Weight Loss Calculator — Free

A seven-day water fast produces a larger cumulative calorie deficit than shorter fasts. After day 3, additional scale change is driven mainly by fat burned at your TDEE rather than another large glycogen water flush.

The 7-day preset is pre-selected. Week-long fasts require medical clearance for many people—especially if you take medications or have chronic health conditions.

Use this page to model expectations, not as a target. Supervised medical fasting exists in clinical settings; unsupervised week-long fasts carry significant risks.

Largest deficit

Seven days of TDEE burn adds up.

Water taper

Glycogen water is mostly gone by day 3; later loss skews toward fat.

Fill in your details and click Calculate Weight Loss to see your estimated breakdown.

What to know before you start

What a 7-day estimate really shows

Week-long water fasts appear in some clinical and wellness contexts, but they are not a casual weight-loss shortcut. Our 7-day preset compounds seven days of estimated fat loss from your TDEE while modeling early glycogen water separately. The total can look impressive on paper; in practice, muscle preservation, electrolyte balance, and safe refeeding matter far more than the headline number.

Older adults and lighter individuals often see smaller fat-loss projections than younger, heavier users—even over the same seven days—because BMR declines with age and lower body mass. That is why age belongs in the equation, and why two people following identical fast lengths can experience very different outcomes.

When a 7-day fast is not appropriate

Pregnancy, breastfeeding, underweight status, active eating disorders, type 1 diabetes, kidney disease, and many cardiovascular conditions are contraindications for unsupervised prolonged fasting. Medications for blood pressure, blood sugar, and mental health may need adjustment or make fasting unsafe altogether.

If you are exploring a 7-day fast for health reasons, work with a clinician who can monitor vitals, labs, and refeeding. Our printable 7-Day Fast Planner includes educational checklists, but it does not replace professional supervision.

Water Fasting Weight Loss Reference Table

Sample estimates for a 35-year-old male, 175 cm, 80 kg. Your results will differ—use the calculator for personalized numbers.

Fast lengthWater weightFat lossTotal estimate
3 days1.6 kg0.9 kg2.5 kg
4 days1.7 kg1.2 kg3.0 kg
5 days1.9 kg1.5 kg3.4 kg
7 days2.2 kg2.1 kg4.3 kg

What happens day by day

General patterns during a water fast—useful alongside reference breakdowns.

PhaseHoursWater trendFat trendNotes
Day 10–24hLow–moderateMinimalGlycogen begins depleting; mild scale drop
Day 224–48hModerate–highLowWater loss accelerates as glycogen falls
Day 348–72hPeakModerateMost glycogen gone; largest water-weight window
Days 4–572–120hTaperingIncreasing shareFat oxidation becomes a larger part of total loss
Days 6–7120–168hLowDominantExtended fast; medical oversight often advised

Estimates use Mifflin-St Jeor BMR, TDEE × 1.35, 3,500 cal/lb fat conversion, and glycogen water modeling. Not medical advice.

Common questions

Answers for this fast length—see the main calculator FAQ for the full FAQ.

How much weight can you lose on a 7-day water fast?
A seven-day water fast can produce a visibly larger total estimate because fat loss compounds daily while early water weight has largely already shifted. After roughly 72 hours, day-to-day scale changes often look smaller even when fat loss continues steadily. Our 7 Days preset models that pattern; it is not a target or a safe default for everyone. Week-long water-only fasting can affect blood pressure, glucose, and electrolytes—get medical clearance first, especially if you take prescriptions, are pregnant, or have a history of eating disorders.
How much weight will I lose while water fasting?
Total scale change is the sum of two very different processes: fat burned for energy and water released as glycogen stores empty. Fat loss scales with your metabolism (sex, age, height, weight) and how many days you fast; water weight can move quickly in the first 72 hours and then slow. This calculator uses the Mifflin-St Jeor BMR equation, a sedentary–lightly active TDEE multiplier (×1.35), and a glycogen water model so you can see what may be temporary versus adipose tissue—not a single “pounds per day” guess.
How much weight do you lose per day while water fasting?
There is no fixed “X lbs per day” that applies to everyone. Days 1–3 frequently show the largest daily scale drops because glycogen depletion releases water; days 4+ often look flatter on the scale while fat loss from your TDEE continues in the background. A 180-lb active male and a 140-lb older female can diverge sharply even on the same calendar. This calculator models that curve—water taper plus steady fat from your personal BMR/TDEE—rather than repeating a generic blog number. Weigh yourself at the same time of day if you track, and remember that hydration shifts the scale independently of fat.
Is this water fasting calculator free to use?
Yes. Water Fasting Calc is free, requires no account, and runs calculations locally in your browser—your inputs are not stored on our servers. It supports metric and imperial units, separates estimated fat loss from glycogen-linked water weight, and offers presets for common fast lengths plus custom days. Results are grounded in published BMR equations and standard calorie-to-fat conversions, with clear disclaimers that outputs are estimates, not medical advice. Use it to understand mechanisms and ballpark ranges; confirm any real fasting plan with a qualified professional.

More water fasting calculators

Each tool uses the same science with a preset fast length—or browse our main calculator for all options, guides, and FAQs.

Also see: reference table, trusted resources, FAQ

Medical disclaimer · Estimates only—not medical advice.