How Do You Calculate Intermittent Fasting Hours?
Calculate intermittent fasting hours from your last meal. See a 16:8 example, compare Blimbi's five schedules and learn what its estimated phases mean.
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In 30 seconds
To calculate a fast, add your fasting hours to the time you last ate. If you finish dinner at 8 p.m. and choose 16:8, your fast ends at noon the next day. Blimbi shows that planned end time for you; its phase guide estimates from time, rather than measuring your body.
- Add your fasting hours to the time you last ate to find your planned end.
- With an 8 p.m. last meal, a 16:8 fast ends at noon the next day.
- Blimbi offers 12:12, 14:10, 16:8, 18:6 and 20:4; choose a rhythm you can sustain.
- Its five phase stops are time-based educational estimates, not body measurements.
How do you calculate fasting hours?
Use the time you actually finished eating. Add the fasting hours to find when you can next eat; the rest of the day is your eating window.
| Protocol | Fast ends | Eating window |
|---|---|---|
| 12:12 | 8 a.m. | 8 a.m. to 8 p.m. |
| 14:10 | 10 a.m. | 10 a.m. to 8 p.m. |
| 16:8 | Noon | Noon to 8 p.m. |
| 18:6 | 2 p.m. | 2 p.m. to 8 p.m. |
| 20:4 | 4 p.m. | 4 p.m. to 8 p.m. |
Blimbi does the calculation when you enter your last meal time. The PLANNED END card shows when the target you chose finishes.
Which fasting schedule should you choose?
You choose a usual rhythm in Choose your rhythm during onboarding and can change it under Profile, Settings, Protocol and reference. The reference time is a guide; Blimbi never starts or ends a fast automatically. Gentle · 7 days builds from 12:12 to 16:8; Basic · 28 days uses 16:8 on weekdays and weekends off. Trial syntheses find broadly similar average effects on weight and many cardiometabolic outcomes across common schedules, though people differ.
For the evidence behind each schedule, see intermittent fasting schedules compared, and for everyday use of the timer, how to use the Blimbi timer.
Randomized-trial syntheses find that common intermittent-fasting schedules produce broadly similar average effects on body weight and many cardiometabolic outcomes, although individuals may respond differently.Source 1
| Program | What it does |
|---|---|
| Gentle · 7 days | Moves from 12:12 to 16:8 in small steps |
| Basic · 28 days | 16:8 on weekdays, with weekends off |
| Long fast · 24–48 h | Asks you to accept a supervision notice for 36 hours or more; talk to a professional before any extended fast |
What do Blimbi’s fasting phases mean?
Blimbi's five time-based stops
0–4 h
Digestion
The first hours after your last recorded meal.
4–12 h
Glycogen running down
A time-based label, not a reading of your stores.
12–24 h
Metabolic switch
The real timing varies from person to person.Source 2
24–36 h
Ketosis
Blimbi does not measure ketones.
36 h or more
Deep fasting
Shown with a supervision notice; extended fasts need professional supervision.
Are the phase times exact?
Mixed evidence
Why phase times vary
The phase times are estimates. Reviews describe a gradual shift from glucose toward fat and ketone use, with no fixed hour that fits everyone. Blimbi calculates its stops from recorded time and presents them as an educational guide, not a measurement.Source 2
You do not need to extend a fast to reach a phase. The timing of the metabolic switch varies with prior meals, activity and energy stores.
There is no single hour when the metabolic switch begins for everyone; timing depends on prior meals, activity and individual energy stores.Source 2
Who should check before fasting?
Blimbi’s Long fast · 24–48 h program asks you to accept a supervision notice at 36 hours or more. Extended fasts need professional supervision.
Talk to a qualified health professional before fasting if you are pregnant or breastfeeding, under 18, living with or recovering from an eating disorder, have diabetes, take insulin or a medicine affected by meal timing, are an older adult, or are acutely ill. Do not change the dose or timing of any medicine without your clinician. See Blimbi’s safety guide.
Frequently asked questions
How do you calculate intermittent fasting hours?
Add your fasting hours to the time you last ate. If dinner ends at 8 p.m. on 16:8, your fast ends at noon the next day. Trial syntheses find broadly similar average effects across common schedules, so choose one you can sustain.Source 1
Are Blimbi's phase times accurate for my body?
They are not measurements. Blimbi calculates each stop only from recorded time, and reviews do not support a single hour when the metabolic switch begins for everyone.Source 2
Sources
Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trialsZhila Semnani-Azad et al. BMJ, June 18, 2025.DOI: 10.1136/bmj-2024-082007 · PMID: 40533200Systematic review and meta-analysis
Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of FastingStephen D. Anton et al. Wiley, February 1, 2018.DOI: 10.1002/oby.22065 · PMID: 29086496Narrative or mechanistic review
Fasting Safely with DiabetesMartin M. Grajower. National Institute of Diabetes and Digestive and Kidney Diseases, August 26, 2020.Official guidance
Calorie restriction and fasting diets: What do we know?National Institute on Aging. National Institute on Aging, August 14, 2018.Official guidance