Period calculator: predict your next period dates

Next period expected, from a last period of Thu, Jan 1, 2026worked example

Thu, Jan 29, 2026

Expected to run until: Mon, Feb 2, 20265 days of bleeding
Cycle length used: 28 days
Fertile window in this cycle: Sat, Feb 7, 2026 - Fri, Feb 13, 2026

This adds your cycle length to the first day of your last period, and then does it five more times. In the 612,613 cycles Bull and colleagues analysed, the mean was 29.3 days and only 13% were exactly 28 - so treat these as expected dates, not scheduled ones.

First day of your last periodFull bleeding, not spotting.Cycle length (days)The NHS describes 21 to 35 days as common.Period length (days)Only used to shade the end date.

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

The next six cycles
Cycle	Period starts	Period ends	Fertile window
1	Thu, Jan 29, 2026	Mon, Feb 2, 2026	Sat, Feb 7, 2026 - Fri, Feb 13, 2026
2	Thu, Feb 26, 2026	Mon, Mar 2, 2026	Sat, Mar 7, 2026 - Fri, Mar 13, 2026
3	Thu, Mar 26, 2026	Mon, Mar 30, 2026	Sat, Apr 4, 2026 - Fri, Apr 10, 2026
4	Thu, Apr 23, 2026	Mon, Apr 27, 2026	Sat, May 2, 2026 - Fri, May 8, 2026
5	Thu, May 21, 2026	Mon, May 25, 2026	Sat, May 30, 2026 - Fri, Jun 5, 2026
6	Thu, Jun 18, 2026	Mon, Jun 22, 2026	Sat, Jun 27, 2026 - Fri, Jul 3, 2026

Bull et al. 2019 measured a mean cycle of 29.3 days across 612,613 cycles from 124,648 people, with real variation between cycles in the same person. These dates are arithmetic on one number you entered.

The sources this defers to

NHS: periods — cycles commonly range from every 21 days to every 35 days
NHS: periods and fertility in the menstrual cycle — what the phases of the cycle are, and when ovulation happens within them

The same two inputs also give your fertile window with its uncertainty band. If a period does not arrive, the due date calculator shows what the 280-day convention points at.

A period calculator projects your next period dates from the first day of your last one and your average cycle length. It lists the next six cycles with the fertile window in each, and shows how wide the prediction really is: in 612,613 tracked cycles, only 13 percent were exactly 28 days long.

Estimated ovulation day, anchored to the luteal phaseconvention — ovulation day of cycle = cycle length - 14. Verified 2026-07-30 against ACOG, Fertility Awareness-Based Methods of Family Planning (patient FAQ) (Counted back from the next expected period rather than forward from the last one, because the follicular phase is what varies. The NHS puts ovulation at 10 to 16 days before the next period, and Bull 2019 measured a mean luteal phase of 12.4 days with a 95% interval of 7 to 17 days, so this is a midpoint of a week-wide window.)
Fertile window (ACOG patient guidance) — fertile window = ovulation - 5 days through ovulation + 1 day. Verified 2026-07-30 against ACOG, Fertility Awareness-Based Methods of Family Planning (patient FAQ) (The window is drawn around an estimated ovulation date, so it is only as good as that estimate. Widening it is the honest response to an uncertain ovulation date; narrowing it is not.)
Real-world cycle and luteal phase variation — mean cycle 29.3 days; 13% of cycles exactly 28 days; mean luteal phase 12.4 days (95% CI 7-17). Verified 2026-07-30 against Bull et al., npj Digital Medicine 2019 (612,613 cycles from 124,648 users of a fertility app, so the population self-selects towards people tracking closely. The direction of the finding -- that the textbook 28-day cycle is a minority -- is not in doubt.)

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

Frequently asked questions

How do you calculate when your next period is due?

Add your average cycle length to the first day of your last period. A 30-day cycle starting on the 3rd puts the next start on the 2nd of the following month. Day 1 is the first day of full bleeding, not spotting.

How accurate is a period prediction?

Less accurate than most apps imply. Bull and colleagues analysed 612,613 cycles in 2019 and found only 13 percent were exactly 28 days, with a mean of 29.3 days and real variation between cycles in the same person. A prediction six cycles out compounds that variation six times.

What counts as a normal cycle length?

The NHS describes cycles ranging from every 21 days to every 35 days, and says it is common for periods to be more or less frequent than 28 days. Cycles that are consistently outside that range, or that change markedly, are worth raising with a clinician.

Why does my period arrive on a different day each month?

Calendar months have different lengths, so a steady 28-day cycle drifts earlier through the year. On top of that the follicular phase - the first half of the cycle - genuinely varies; Bull 2019 put its 95 percent interval at 10 to 30 days.

Can this tool tell me why my period is late?

No. It only does arithmetic on the dates you enter. A late period has many possible explanations and this page is not able to distinguish between them; that is a conversation for a clinician or a test, not a calculator.

Does anything I type here get stored?

No. Every calculation runs in your browser, there is no account, and nothing is sent to a server. Cycle data is about as sensitive as data gets, so this wing of the site is built to never hold any.

Get a quarterly nudge to re-check your numbers

Four emails a year: a reminder to re-take your numbers, and what changed in the guidance behind these tools. No spam, unsubscribe any time.

Double opt-in: you will get one confirmation email and nothing else until you click it.

The next number

No body metric means much on its own. Here is what this one feeds into.

Ovulation calculatorThe same cycle length gives your fertile window - counted back from the next period, not forward from the last one.Due date calculatorIf a period does not arrive, this is the date the 280-day convention points at.

Related calculators

Ovulation calculatorCycle and ovulation Implantation calculatorEarly pregnancy Conception date calculatorEarly pregnancy

Every formula on this page is listed with its source on the methodology page, and changes to any reference value are logged in the data changelog. Canonical URL: https://myhealthmywellness.com/period-calculator