Last updated: 2026-05-17

Intermittent fasting

Intermittent fasting (IF) is an eating pattern that alternates between defined periods of eating and fasting. Unlike conventional diets that focus on what to eat, IF focuses on when to eat. This calculator handles two of the most widely practiced approaches: Time-Restricted Eating (TRE), which sets a daily eating window by clock time (the 16:8, 18:6, 14:10, and 20:4 protocols), and the 5:2 diet, which eats normally for five days and restricts calories to roughly 25% of daily needs on two non-consecutive days.

Time-Restricted Eating mode

Time-Restricted Eating compresses all daily food intake into a continuous eating window. The window length is the day minus the fasting hours, and the window closes that many hours after it opens:

$\text{Eating window} = 24\text{ h} - \text{Fasting hours}$

$\text{Eating window closes} = \text{Eating start time} + \text{Eating window hours}$

A 16:8 protocol with an eating window that opens at noon closes at 8:00 PM:

$12{:}00 + (24 - 16)\text{ h} = 12{:}00 + 8\text{ h} = 20{:}00$

The four standard protocols differ only in the fasting-to-eating ratio:

The 14:10 starting protocol

Clinical guidance from Cleveland Clinic and research covered by Healthline recommends a 14-hour overnight fast — rather than jumping straight to 16:8 — as the starting point for women, because extended fasting windows can cause more pronounced hormonal disruption in women than in men. The 14:10 protocol is also a natural fit for an overnight fast: stopping eating at 8 PM and having breakfast at 10 AM already completes a 14-hour fast without any special effort.

5:2 mode

The 5:2 diet does not restrict eating time — it restricts how much is eaten on two designated fast days per week. The calorie target on fast days is set at 25% of Total Daily Energy Expenditure (TDEE):

$\text{Fast-day calories} = \text{TDEE} \times 0.25$

This 25% target was derived from clinical work by Johnson et al. (Free Radical Biology & Medicine, 2007) and validated in subsequent trials by Varady et al. (Nutrition Journal, 2013). For most adults, it works out to roughly 400–600 kcal on fast days.

TDEE is computed in two steps.

Step 1 — Basal Metabolic Rate (BMR) via the Mifflin-St Jeor equation (1990):

Step 2 — TDEE, multiplying BMR by an activity factor (1.2 sedentary → 1.9 twice-daily training):

$\text{TDEE} = \text{BMR} \times \text{activity factor}$

Worked example: a 35-year-old man, 75 kg and 170 cm165 lb and 5 ft 7 in (75 kg and 170 cm), doing light exercise (factor 1.375):

$\text{BMR} = 10(75) + 6.25(170) - 5(35) + 5 = 1{,}642.5 \text{ kcal/day}$

$\text{TDEE} = 1{,}642.5 \times 1.375 \approx 2{,}258 \text{ kcal/day}$

$\text{Fast-day target} = 2{,}258 \times 0.25 \approx 565 \text{ kcal on fast days}$

On the five feast days, eating at or near TDEE (≈2,258 kcal) maintains weight while the two fast days create the weekly calorie deficit.

Evidence

A 2025 network meta-analysis of randomised trials compared IF protocols against continuous caloric restriction:

• Modified alternate-day fasting showed the largest body weight reduction: −5.18 kg−11.4 lb on average.
• 5:2 was particularly effective for LDL cholesterol reduction.
• 16:8 TRE showed strong evidence for weight loss and improvement in fasting insulin.

The difference between IF and continuous caloric restriction was often modest — both reduce weight, and the most effective approach is the one a person can sustain. A 2024 study also raised questions about elevated cardiovascular risk with strict 8-hour TRE in people with existing heart disease, worth discussing with a physician before starting.

When to use this calculator, and when not to

The calculator is intended for adults aged 18 and over who are considering starting IF and want a concrete daily schedule before committing. It is not a substitute for a physician's guidance. Several groups — including pregnant women, people with eating disorder histories, and those on insulin — should not use IF without medical supervision (see the FAQ section below).