The formula
How to calculate intermittent fasting
Intermittent fasting methods are usually named by their split, like 16:8 for 16 hours fasting and 8 hours eating. This works out the actual clock times for your fasting and eating windows from the method you pick and the hour you start fasting.
The fasting window runs from your chosen start hour for the number of hours the method specifies, wrapping past midnight where needed. The eating window fills the remaining hours in the 24-hour day, starting the moment the fast ends.
Here is what each field means:
- Fasting method
- Fasting start hour — 24-hour clock, whole hours — e.g. 20 = 8pm
- Days per week
Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.
The order the fields are filled in makes no difference to the result — the calculator recomputes the whole formula from whatever is currently in every field, not step by step. That means it is safe to adjust one number, watch the result change, and adjust it back, without worrying about resetting anything first.
Why intermittent fasting matters
A calculation like this usually gets used at a decision point rather than out of curiosity — comparing two real options, checking a number a lender or adviser has quoted, or working out whether a plan that sounded fine in conversation still holds up once it is written down with actual figures. The maths itself is rarely complicated; what is hard is remembering which figures to use and in what order, which is exactly what a dedicated calculator is for.
This tends to come up when comparing two concrete alternatives — two lenders, two savings products, two ways of structuring the same decision — rather than in the abstract. Run both scenarios through the same calculator with the same assumptions and the comparison becomes fair, because the only thing changing between the two results is the number you are actually trying to test.
This kind of calculation rarely stands entirely alone. A intermittent fasting figure usually feeds into a wider decision — how it compares with a competing offer, whether it fits inside a monthly budget, what it does to a longer-term plan — and the value of having it as an exact number rather than a rough guess is that those follow-on comparisons stop being guesswork too. Once one figure in a decision is precise, it is worth making the effort to get the others precise as well, rather than mixing an exact calculation with several estimates and treating the result as equally reliable.
In practice, most people arrive at a page like this one having already tried a version of the calculation by hand or in a spreadsheet, and use the calculator here to confirm it rather than replace it. That is a reasonable way to use it — the two should agree to the last decimal place if the same inputs and the same formula are used, and if they do not, the formula shown above is the one to check your own working against first.
Worked example
Take the figures the calculator starts with:
- Fasting method: 16
- Fasting start hour: 20
- Days per week: 7
That gives:
- Fasting start (24h): 20 h
- Fasting end (24h): 0 h
- Eating window start (24h): 0 h
- Eating window end (24h): 0 h
- Fasting duration: 16 h
- Eating duration: 8 h
- Weekly fasting hours: 112 h/week
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
The clock times tell you when to eat, not what or how much. Fasting is a timing strategy — it restricts the hours food is allowed, but calories eaten during the window are still what determines weight change. Eating enough during an 8-hour window to offset a calorie deficit is entirely possible.
Where this goes wrong. This calculator deliberately leaves out any goal-based guidance — it only does the clock arithmetic for whichever method and start hour you enter. It also does not account for anything happening within a single hour; enter a whole-number start hour on the 24-hour clock.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
Not directly — fasting itself does not burn extra calories beyond normal metabolism. Any weight effect comes indirectly, usually because a shorter eating window makes it easier to eat less overall, not because fasting speeds up metabolism.
The calculator wraps the clock automatically — starting a 16-hour fast at 20:00 ends at 12:00 the next day, crossing midnight in between. The hours shown are always in 24-hour format for the day the window starts or ends.
The headline figure is fasting start (24h). With 20 fasting start hour, that comes to 20 h. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.