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Fasting windows & calories

Intermittent Fasting Calculator: Schedule, Eating Window, and Calories

This calculator maps 16:8, 18:6, 20:4, OMAD, or 5:2 onto a daily clock from your first-meal time, then estimates calories with Mifflin-St Jeor BMR and an activity multiplier. Fat loss targets 20% below TDEE, maintenance matches TDEE, and muscle gain adds 10%. It shows fasting and eating hours, macro grams, and a timeline—not medical advice for pregnancy, eating disorders, or medication timing.

By Jeff Beem

Updated

01

Profile

Units
Biometrics
Height
Activity (TDEE)

Feeds Mifflin–St Jeor TDEE.

Goal
Protocol
Timing

Eating window start

02

Targets

Target about 1,941 kilocalories per day. Goal: fat loss, roughly 20 percent below estimated maintenance. Protocol: 16:8 (Standard). Estimated basal metabolic rate 1,765 kilocalories; total daily energy expenditure about 2,426 kilocalories.

Macros
Protein169.8g (35%)
Carbs145.6g (30%)
Fats75.5g (35%)
Per meal1,941 kcal
Fasting timeline (model)

Illustrates modeled metabolic phases over 24 hours after your eating window. For 5:2, scheduling is day-based rather than hourly. Hover reveals phase labels for sighted users only.

Schedule
Fast starts8:01 PM
Ketosis band (est.)10:01 AM
Late fast (est.)10:01 AM - 12:01 PM
First meal12:00 PM
~1h after first meal1:00 PM
Last meal8:00 PM
Plan notes

For fat loss, the fasting window is only part of the plan. The bigger win usually comes from having a structure that makes calories easier to control without feeling like every evening turns into catch-up eating.

What breaks a fast?

  • Usually fine: black coffee, unsweetened tea, water, sparkling water.
  • Often breaks fast: cream, milk, sugar, some sweeteners, bone broth, MCT oil.

Reading your schedule and calorie panel

The dark results card shows BMR, TDEE, your goal-adjusted target, macro grams, and a plain-language summary. Below it, the timeline colors fasting versus eating hours from your protocol and first-meal time.

Example: male 30, 5′10″, 176 lb, 16:8 from noon

By default: lightly active (×1.375), fat loss (−20%). BMR ≈ 1,749 kcal, TDEE ≈ 2,405 kcal, target ≈ 1,924 kcal/day. Eating window 12:00 PM–8:00 PM; fasting roughly 8:00 PM–12:00 PM next day. Macros at that target (35% protein / 30% carbs / 35% fat): about 168 g protein, 144 g carbs, 75 g fat.

Protocol sets the clock, not the calories

Switching 16:8 to 18:6 shortens the eating window but leaves TDEE math the same. Shorter windows mean larger meals if you still hit the same calorie target—helpful for some people, miserable for others.

Goal slider is a percentage of TDEE

Fat loss multiplies TDEE by 0.8 (−20%). Muscle gain uses 1.1 (+10%). Maintenance holds TDEE. That is a simple structure, not a guarantee of a specific weekly scale change.

5:2 is a different layout

On 5:2, five days use the full target above; two “fasting” days typically land near 500–600 kcal in the widget copy. Plan those low days so the other five are not reactive overeating.

Intermittent fasting calculator: schedule, eating window, and calories

Plan 16:8, 18:6, 20:4, OMAD, or 5:2 with a clock-based eating window and a Mifflin-St Jeor calorie target for fat loss, maintenance, or muscle gain. General fitness planning only—not medical advice.

What This Calculator Does

This intermittent fasting calculator answers two practical questions: when should I eat, and about how many calories fit my goal? Choose a protocol, set the time your eating window opens, enter age, sex, height, weight, and activity, then pick fat loss, maintenance, or muscle gain. Outputs include fasting and eating times, BMR, TDEE, a goal-adjusted calorie target, macro grams, and a color-coded day timeline.
  • Who it helps:
    Anyone sketching a time-restricted routine who wants a realistic window and a calorie number tied to body size—not a one-size-fits-all 1,500 kcal guess.
  • What it outputs:
    Clock times for fasting and eating, daily calorie target, macro split, BMR/TDEE breakdown, and protocol-specific guidance text in the results panel.
  • Limitations:
    Estimates use population equations and self-reported activity. They do not model thyroid issues, medications, pregnancy, eating-disorder history, or individual metabolic adaptation to fasting.

How the Math Works

Basal metabolic rate (BMR) uses Mifflin-St Jeor:
BMRmale=10w+6.25h−5a+5\text{BMR}_{\text{male}} = 10w + 6.25h - 5a + 5
BMRfemale=10w+6.25h−5a−161\text{BMR}_{\text{female}} = 10w + 6.25h - 5a - 161
where w is weight in kg, h is height in cm, and a is age. Total daily energy expenditure (TDEE) multiplies BMR by an activity factor (1.2 sedentary through 1.725 very active; default 1.375 lightly active). Goal adjustment: fat loss = TDEE × 0.8 (−20%), maintenance = TDEE, muscle gain = TDEE × 1.1 (+10%).
  • Worked example (defaults):
    30-year-old male, 80 kg, 175 cm, lightly active (1.375), fat loss: BMR ≈ 1,749 kcal, TDEE ≈ 2,405 kcal, target ≈ 1,924 kcal/day. With 16:8 and first meal at noon, eating runs about 12:00 PM–8:00 PM.
  • Macros:
    Protein, carb, and fat grams come from goal-specific percentages of the calorie target (e.g., fat loss defaults to 35% / 30% / 35% protein / carbs / fat by calories).
  • 5:2 variation:
    Five days use the computed target; two fasting days are described as roughly 500–600 kcal in the UI. Weekly average intake still depends on how you eat on non-fasting days.

How to Use This Calculator

Change one input at a time when tuning: if the schedule fits life but weight does not move after several weeks of honest logging, adjust calories or activity—not necessarily the fasting length.
  • Protocol:
    16:8 is the default—16 hours fasting, 8 eating. 18:6 and 20:4 shorten the eating side; OMAD compresses to about one hour; 5:2 alternates low-calorie days.
  • First meal time:
    Sets when the eating window opens. A noon start on 16:8 closes eating around 8 PM, which suits people who skip breakfast.
  • Body stats & activity:
    US or metric units. Activity multipliers match common TDEE tables; pick the level that describes a typical week, not your best week.
  • Calorie target:
    Fat loss (−20%), maintenance, or muscle gain (+10%). Read BMR and TDEE in the breakdown rows to sanity-check the multiplier.
  • Meal frequency (optional):
    When shown, split the daily target across two or three meals. OMAD protocol forces one meal; 5:2 hides per-meal splits on fasting days.

Common Intermittent Fasting Schedules

Protocol comparison

16:8

Difficulty
Beginner
Primary benefit
Fits lunch and dinner without shifting dinner early

18:6

Difficulty
Intermediate
Primary benefit
Shorter eating window, still room for two meals

20:4

Difficulty
Advanced
Primary benefit
Very short eating block; needs planning

OMAD

Difficulty
Advanced
Primary benefit
One main meal; simple rules, easy to under-fuel

5:2

Difficulty
Intermediate
Primary benefit
Weekly structure instead of daily hours

Calories, drinks, and what breaks a fast

Using the calorie target inside the window

Intermittent fasting does not override energy balance. The schedule reduces grazing hours; the calorie number still drives weight change.
  • Fat loss:
    The −20% target creates a moderate deficit. Pair it with protein-forward meals so the eating window does not end in rebound snacking.
  • Maintenance:
    Hold TDEE while you test whether time restriction alone improves adherence without changing scale weight.
  • Muscle gain:
    +10% above TDEE is modest. Shorter fasting windows usually make it easier to spread protein across meals.

What breaks a fast in practice

  • Usually fine:
    Water, sparkling water, black coffee, unsweetened tea, plain electrolytes without calories.
  • Usually breaks the fast:
    Cream, milk, sugar, juice, bone broth with calories, or supplements that carry meaningful energy.
  • When in doubt:
    Keep fasting hours boring—water and plain coffee—so you are not negotiating every splash of milk.

Strict definitions vary by study design; this page uses the practical rule most people follow at home.

Intermittent Fasting Calculator FAQ

What is intermittent fasting?

Intermittent fasting is a schedule for when you eat, not a mandatory food list. Most people pick a named routine—16:8, 18:6, 20:4, OMAD, or 5:2—and build meals around work, training, and appetite. This page maps that window onto a clock and pairs it with a calorie target from your stats and goal.

Which intermittent fasting schedule should a beginner start with?

16:8 is the usual starting point: a 16-hour fast and an 8-hour eating window. At defaults here (eating opens at noon), that means roughly 12:00 PM–8:00 PM for food and the rest for fasting. It is easier to sustain than OMAD or 20:4 while you learn hunger patterns.

How many calories should I eat with 16:8?

Calories depend on your body and goal, not the protocol name. On defaults (30-year-old male, 5′10″, 176 lb, lightly active, fat loss): BMR ≈ 1,749 kcal, TDEE ≈ 2,405 kcal, target ≈ 1,924 kcal/day (20% below TDEE). Maintenance keeps TDEE; muscle gain adds about 10%. The fasting window does not change that math—it only sets when those calories fit.

Can I drink coffee or tea during the fasting window?

Plain water, black coffee, and unsweetened tea are the standard fasting-window drinks. Cream, milk, sugar, juice, or calorie-containing supplements count as food for most people trying to keep a clean fast.

What breaks a fast?

Anything with meaningful calories breaks a practical fast. Water, sparkling water, black coffee, unsweetened tea, and plain electrolytes without calories are the usual exceptions. If you are debating whether half-and-half counts, treat it as intake.

Will intermittent fasting make me lose muscle?

Not automatically. Muscle loss gets more likely when calories stay too low for too long, protein is short, or resistance training drops off. On a cut, hit protein during the eating window and keep lifting. The macro row on the results panel is a starting split, not a medical prescription.

Can I exercise while fasting?

Yes, but timing matters. Walking and easy cardio are usually fine fasted; hard intervals or heavy lifting often feel better after the first meal. If training quality crashes, move the session closer to your eating window rather than forcing fasted PRs.

Is intermittent fasting safe for everyone?

No. Skip or get clinician guidance if you are pregnant, breastfeeding, underweight, recovering from an eating disorder, on medications that require food, or managing conditions where fasting is risky. This tool estimates calories; it does not screen you medically.

Sources & citations

References used for the calculation method and definitions. Links open in a new tab when available.

[1]
Mifflin MD et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247

Mifflin-St Jeor equation for basal metabolic rate (BMR), the default formula in this calculator.

[2]
Intermittent fasting for weight management and metabolic health (umbrella review, 2024)

PubMed record of a 2024 umbrella review of meta-analyses on time-restricted eating, 5:2, and alternate-day fasting for weight and metabolic outcomes in adults.

Fitness Reference Note

Informational Use: These calculations (BMI, Calories, etc.) are based on standard statistical formulas and are intended for general reference and goal-setting purposes only.

Consult Experts: This tool does not provide medical advice, diagnosis, or treatment. Results may not be accurate for athletes, pregnant individuals, or those with underlying health conditions.

Health Safety: Always consult with a healthcare professional or qualified trainer before beginning any new diet or intensive exercise program.

Privacy First: All calculations are performed locally in your browser. No health data is stored or transmitted to any server.

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