Calculator
What the Intermittent Fasting Calculator does
Intermittent Fasting Calculator: calculates Eating Window, Fasting Hours and Eating Hours from Fasting Pattern and Start Eating At (24h). Example: fasting pattern 16:8 (fast 16h / eat 8h) and start eating at (24h) 12 gives eating window 12:00 - 20:00.
With Fasting Pattern and Start Eating At (24h) entered, the Intermittent Fasting Calculator works out Eating Window, Fasting Hours and Eating Hours. There is no submit step; the numbers move as you adjust them.
It is mostly used by readers checking a number from a clinic. Knowing how a figure is derived makes it much easier to interpret sensibly.
The page holds your values in memory and forgets them the moment you close it.
Field definitions, the method, and a worked example from the default values are all set out further down the page.
What do the Intermittent Fasting Calculator fields mean?
The Intermittent Fasting Calculator uses 2 inputs. Rather than an empty form you get a filled one, useful when you only want to test a single change.
| Field | What to enter | Default |
|---|---|---|
| Fasting Pattern | choose from 16:8 (fast 16h / eat 8h), 18:6, 20:4 or 14:10 | 16:8 (fast 16h / eat 8h) |
| Start Eating At (24h) | accepts 0 to 23 | 12 |
How does the Intermittent Fasting Calculator work?
Each output is derived from the inputs above in a single pass; there is no hidden state carried between runs, so the same inputs always give the same calculation.
Each field is verified before use; failures are reported explicitly rather than swallowed.
Worked example
These are the values the Intermittent Fasting Calculator loads by default, and the result it produces from them. The defaults produce the result shown here; change any of them and the answer moves.
| Fasting Pattern | 16:8 (fast 16h / eat 8h) |
|---|---|
| Start Eating At (24h) | 12 |
| Eating Window | 12:00 - 20:00 |
|---|---|
| Fasting Hours | 16 |
| Eating Hours | 8 |
| Plan | Eat between 12:00 and 20:00. Fast the remaining 16 hours. Stay hydrated; not for everyone (pregnancy, eating… |
How to use it
- Pick a value for Fasting Pattern: the options are 16:8 (fast 16h / eat 8h), 18:6, 20:4 or 14:10.
- Enter Start Eating At (24h) (0 to 23).
- The answer follows your cursor, in effect. Change a field and read the new figure.
- Look at Eating Window, Fasting Hours, Eating Hours and Plan in the output panel.
- If you need a record, Copy Link saves the inputs and Print produces a clean page.
What this tool does not do
- This is a screening estimate, not a diagnosis.
- Body composition, ethnicity and age all shift what a "normal" result means.
- Batch work is not supported; each run handles one set of values.
- Garbage in, garbage out: confirm each figure means what you think it does before trusting the result.
Frequently asked questions
Only Fasting Pattern and Start Eating At (24h). A starting value is in place for each, so the form is usable immediately, without filling anything in first.
Start Eating At (24h) accepts 0 to 23. Anything beyond those bounds is flagged rather than approximated.
With the values loaded when the page opens, eating window comes out as 12:00 - 20:00. Edit any input and that figure moves with it.
No. Your browser is doing the computing, so we could not see the values even if we wanted to.
No cost and no account. Use it as often as you like.
It is a screening estimate, not a diagnosis. The formulas are standard, but they describe averages and you are not an average. Use it as a starting point for a conversation with a professional.