Ovulation calculator
Your fertile window and next periods, cycle by cycle.
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This tool runs entirely in your browser. What you paste is processed on your device and is never sent to a server, so it is safe to use with tokens, keys and unpublished text.
Example
Example: a 31-day cycle puts ovulation around day 18, not day 14
Her last period started on Monday 2 March 2026 and her cycles run about 31 days. She picks that date, types 31 as the cycle length, leaves the luteal phase at 14 and shows the next 3 cycles.
- The next period is expected 31 days on, Thursday 2 April. Ovulation is taken as 14 days before it: Thursday 19 March, day 18 of the cycle.
- The fertile window is the five days before ovulation and the day itself, 14 to 19 March; the most fertile days are 17 and 18 March.
- With a 28-day cycle the same start date would put ovulation on 16 March, day 15 — three days earlier.
| Period starts | Fertile window | Ovulation |
|---|---|---|
| 2 Mar 2026 | 14 Mar – 19 Mar 2026 | 19 Mar 2026 |
| 2 Apr 2026 | 14 Apr – 19 Apr 2026 | 19 Apr 2026 |
| 3 May 2026 | 15 May – 20 May 2026 | 20 May 2026 |
An estimate for a regular 31-day cycle: ovulation around 19 March, 19 April and 20 May. Any cycle can run shorter or longer, so ovulation tests or temperature charting show the actual day, and it is not contraception — ask your doctor about anything beyond timing.
How it works
- Pick the first day of your last period and type your usual cycle length, from one period to the next.
- Leave the luteal phase at 14 days unless you know yours.
- The next ovulation and fertile window appear first, then a table of the coming cycles.
Frequently asked questions
- How is ovulation estimated?
- It is taken as 14 days before the next period — the luteal phase, which varies less than the rest of the cycle. In a 28-day cycle that is 14 days after the first day of your period — day 15, counting that first day as day 1; in a 32-day cycle, day 19.
- What is the fertile window?
- The five days before ovulation and the day itself: sperm can live up to five days, the egg about a day. The two days before ovulation are the most fertile.
- How reliable is it?
- For regular cycles it is a fair guide, but ovulation can shift in any cycle. Ovulation tests or temperature charting show it more precisely.
- Can I use it to avoid pregnancy?
- No. A calendar estimate is not contraception: ovulation can come earlier or later than calculated.
- What if my luteal phase is shorter than 14 days?
- Type yours if you know it, from 10 to 16 days. With 12 days, the same 31-day cycle from 2 March puts ovulation on 21 March (day 20) instead of 19 March, and the fertile window moves to 16–21 March. A short luteal phase is worth asking your doctor about.
- Why does the next period move from the 2nd to the 3rd?
- Because months differ in length and the calculator counts days, not dates. A 31-day cycle from 2 March ends on 2 April, as March has 31 days, but from 2 April it ends on 3 May, as April has 30.
- Is anything uploaded?
- No. It is worked out in your browser, and nothing you type leaves your computer.
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