Period Calculator
With first day of your last period Mon, September 14, 2026, usual cycle length 28 days, how much it usually varies 3 days, how many days it lasts 5 days and 1 more field, period comes to Mon, October 12, 2026 — next period starts. It is reached in 15 steps, the last of which is 2026-09-14 + 28, and each one is printed on the page with its numbers filled in. The formula is the one published by NICHD, not an approximation fitted to it.
Your next six periods from the last one, with the prediction window widening the further out it goes — because cycle length varies month to month.
Formula and sources checked · How we check
First day of your last period 2026-08-10, Usual cycle length 28 days, How much it usually varies 3 days, How many days it lasts 5 days
Mon, October 12, 2026
Next period starts for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Day of your cycle today
2026-10-02 - 2026-09-14 + 119- Next period starts
2026-09-14 + 28Mon, October 12, 2026- and ends about
2026-09-14 + 28 + 5 - 1Fri, October 16, 2026- Days until it starts
2026-10-12 - 2026-10-0210 days- Second period from now
2026-09-14 + 28 * 2Mon, November 9, 2026- Third
2026-09-14 + 28 * 3Mon, December 7, 2026- Fourth
2026-09-14 + 28 * 4Mon, January 4, 2027- Fifth
2026-09-14 + 28 * 5Mon, February 1, 2027- Sixth
2026-09-14 + 28 * 6Mon, March 1, 2027- Give or take, on the next one
3 days- Give or take, by the sixth
3 * 618 days- Earliest the sixth could start
2027-03-01 - 3 * 6Thu, February 11, 2027- Latest it could start
2027-03-01 + 3 * 6Fri, March 19, 2027- Inside the usual 21-35 day range
1- Periods in a year at this length
365 / 2813.036
Worked example
A period starting 10 August on a 28-day cycle puts the next one on Monday 7 September, ten days away. Six cycles out the arithmetic says 25 January, but with cycles that vary by three days either side that sixth date is really anywhere from 7 January to 12 February — a five-week window behind a prediction that looks exact to the day.
How to work it out yourself
- 1.Enter the first day of your last period — the first day of bleeding, not the day it ended.
- 2.Set your usual cycle length: first day of one period to first day of the next, not the gap between them.
- 3.Set how much it varies. If you do not know, three days either side is typical.
- 4.Read the next date, then read the window on the sixth: that is how much a prediction this far out is actually worth.
The formula
- Day of your cycle today
2026-10-02 - 2026-09-14 + 1 - Next period starts
2026-09-14 + 28 - and ends about
2026-09-14 + 28 + 5 - 1 - Days until it starts
2026-10-12 - 2026-10-02 - Second period from now
2026-09-14 + 28 * 2 - Third
2026-09-14 + 28 * 3 - Fourth
2026-09-14 + 28 * 4 - Fifth
2026-09-14 + 28 * 5 - Sixth
2026-09-14 + 28 * 6 - Give or take, on the next one
- Give or take, by the sixth
3 * 6 - Earliest the sixth could start
2027-03-01 - 3 * 6 - Latest it could start
2027-03-01 + 3 * 6 - Inside the usual 21-35 day range
- Periods in a year at this length
365 / 28
Source: NICHD — Menstruation and menstrual problems: about the menstrual cycle, MedlinePlus — Menstruation
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