Period Cycle Calculator
Predict your next period dates, cycle days, and PMS window based on your last period and cycle length.
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How to use this calculator
- 1
Enter the first day of your most recent period in YYYY-MM-DD format.
- 2
Input your average cycle length (count from the first day of one period to the first day of the next).
- 3
Enter how long your period usually lasts.
- 4
Choose how many months ahead you want predictions.
- 5
Your upcoming period dates, PMS windows, and ovulation dates are all shown.
Frequently asked questions
How are the next period dates calculated?
Each predicted start is the date you entered plus a whole multiple of your cycle length. A 28-day cycle that began on 5 January 2026 gives starts on 2 February, 2 March and 30 March. The end date is the start plus your period duration minus one day, so a 5-day period starting on 2 February ends on 6 February.
How does the calculator estimate ovulation?
It places ovulation at cycle length minus 14 days after each period start, which assumes a fixed 14-day luteal phase. For a 28-day cycle that is 14 days after the first day of bleeding. It is an estimate from the calendar only. If you want to set the luteal length yourself, the ovulation calculator on this site has that input.
What is the “PMS may start” date?
It is simply seven days before each predicted period start. The number seven is a fixed setting in the tool and is not personalised, and the output only lists it for the next three periods.
How many cycles will it list?
It rounds up the number of cycles that fit into the chosen horizon, treating a month as 30 days, and then adds one. With a 28-day cycle, 3 months lists five rows, counting the period you entered as row 1. With a 35-day cycle, 3 months lists four.
What input is rejected?
A date that is not written as YYYY-MM-DD, or that does not exist, such as 2026-02-30, returns a message asking for a valid date. Cycle length must be between 15 and 365 days, and the period duration must be between 1 day and the cycle length. A blank cycle length or duration falls back to 28 and 5.
How accurate are predictions for later months?
They assume every cycle is exactly the length you typed, so any variation in real cycles accumulates the further ahead you look. Treat the next one or two periods as the useful part and re-enter your real start date whenever a period begins.
Can I use this for contraception or to diagnose a problem?
No. These are estimates derived from the cycle length you enter. They are not a method of contraception and not a diagnosis. If your cycles are very irregular, absent, unusually painful or heavy, speak to a doctor. This is general information, not medical advice.
Period cycle calculator — predicted start dates, end dates and PMS reminders
Date arithmetic from a single start date
The calculator takes the first day of your last period, your usual cycle length and the number of days your period lasts. From those it lists repeated cycles by adding the cycle length each time. For every cycle it prints the start date, the end date, and an estimated ovulation date placed 14 days before the next period would start. For the following three periods it also prints a reminder date seven days earlier. Nothing about your body is measured; the output depends entirely on the numbers entered.
Worked examples
The first example is a 28-day cycle with a 5-day period, last starting on 5 January 2026, for 3 months. The second is a 35-day cycle with a 4-day period from the same date. Dates are as the calculator prints them.
| Row | 28-day cycle, 5-day period | Est. ovulation | 35-day cycle, 4-day period | Est. ovulation |
|---|---|---|---|---|
| 1 (entered) | 5 Jan – 9 Jan | 19 Jan | 5 Jan – 8 Jan | 26 Jan |
| 2 | 2 Feb – 6 Feb | 16 Feb | 9 Feb – 12 Feb | 2 Mar |
| 3 | 2 Mar – 6 Mar | 16 Mar | 16 Mar – 19 Mar | 6 Apr |
| 4 | 30 Mar – 3 Apr | 13 Apr | 20 Apr – 23 Apr | 11 May |
| 5 | 27 Apr – 1 May | 11 May | not listed | not listed |
Reading the PMS reminder and the next-three list
Below the table the calculator repeats the next three periods in full. In the 28-day example those are starting 2 February, 2 March and 30 March, with reminder dates of 26 January, 23 February and 23 March. In the 35-day example they are 9 February, 16 March and 20 April, with reminders on 2 February, 9 March and 13 April. The reminder is always exactly seven days before the predicted start. It is a fixed offset used as a prompt to plan ahead, not a prediction of symptoms.
What counts as a valid entry
Dates must be written year-month-day, for example 2026-01-05, and must exist on the calendar, so 30 February is rejected. A cycle length from 15 to 365 days and a period duration of at least one day and no longer than the cycle are accepted. Entering nothing in the cycle or duration boxes uses 28 and 5 days. Each cycle advances by exactly the same number of days, whatever the month.
Limits worth knowing
A calendar projection is only as good as the regularity of the cycle behind it. Real cycles can lengthen or shorten, and the ovulation column is a calculation from the cycle length, not an observation. These estimates are not a method of contraception and not a diagnosis, and they cannot tell you why a period is late. Anyone with consistently irregular, missed, very heavy or painful periods should talk to a doctor, who can look at the whole picture.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →