Use this independent UK ovulation calculator to estimate your likely ovulation date, fertile window and next period from the first day of your last period and your usual cycle length. It is a calendar estimate only: ovulation can move from cycle to cycle and this tool must not be used as contraception.

Ovulation and fertile window calculator

How this ovulation calculator works

The calculator predicts the next period from your last-period date and usual cycle length, then uses 14 days before that date as a central ovulation estimate. NHS information says ovulation is difficult to pinpoint and, for most women, occurs around 10 to 16 days before the next period. This is why the result should be treated as an estimate rather than an exact date.

What is the fertile window?

The fertile window is the part of the cycle around ovulation when pregnancy is possible. The calculator highlights the five days before the estimated ovulation date through the following day as a practical planning window. NHS guidance notes that sperm can survive for up to 7 days, so pregnancy can occur from sex several days before ovulation.

Worked example

If the first day of a period is 1 September and the usual cycle is 28 days, the next period is estimated around 29 September. Counting back 14 days gives a central ovulation estimate around 15 September. Actual ovulation may occur earlier or later.

Trying to get pregnant

The NHS advises that having sex without contraception every 2 to 3 days gives a good chance of sperm being present around ovulation. Calendar timing is not necessary for everyone and should not create pressure.

Irregular cycles and limitations

  • Calendar estimates are less useful when cycle length varies substantially.
  • Stress, illness, hormonal conditions and other factors can change ovulation timing.
  • This calculator cannot confirm that ovulation occurred.
  • Do not use the fertile-window result to avoid pregnancy. Use an appropriate contraceptive method.
  • Speak to a GP if your periods are irregular and you are struggling to get pregnant, or if you have other fertility concerns.

Authoritative sources

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Frequently asked questions

Is this an official NHS ovulation calculator?

No. It is an independent UK calendar tool that references NHS fertility guidance.

Does ovulation always happen on day 14?

No. Day 14 is only a common example for a 28-day cycle. NHS guidance says ovulation is generally around 10 to 16 days before the next period.

Can I use this calculator as contraception?

No. Ovulation timing varies and a calendar estimate is not reliable contraception.

What if my periods are irregular?

A calendar prediction becomes less reliable. NHS advice says irregular periods can make pregnancy harder because ovulation may not happen regularly; speak to a GP if you are concerned.

Independent informational tool only. It does not diagnose ovulation, fertility or pregnancy and is not affiliated with or endorsed by the NHS.

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