Ovulation Calculator: When Do I Ovulate?
Enter the first day of your last period and your usual cycle length to estimate the day you ovulate. It runs in your browser, so nothing is sent or stored.
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Calculated in your browser. Nothing you enter is sent or stored. Estimates only, and not a method of contraception.
How the ovulation calculator works
Ovulation is when an ovary releases an egg. For most people it happens about 14 days before the next period starts, not 14 days after the last one began. That difference matters, because it means the day of ovulation depends on how long your cycle is. The calculator first works out when your next period is due, then counts back 14 days.
The formula
- Next period = first day of last period + cycle length
- Ovulation day = next period date minus 14 days
- Fertile window = 5 days before ovulation up to 1 day after
A worked example: your last period started on 1 June and your cycles are 30 days long. Your next period is due on 1 July, so ovulation is estimated for 17 June, and the fertile window is 12 to 18 June. If your cycle were 26 days instead, ovulation would be estimated for 13 June.
| Cycle length | Estimated ovulation (cycle day) |
|---|---|
| 24 days | Day 10 |
| 28 days | Day 14 |
| 32 days | Day 18 |
| 35 days | Day 21 |
Signs that can go with ovulation
Some people notice clear, stretchy cervical mucus, mild one-sided pain in the lower belly, light spotting, a slightly higher sex drive, or a small rise in resting body temperature after ovulation. These signs vary a lot from person to person, and many people notice nothing. A calculator and body signs together are a better guide than either on its own, but neither is exact.
If your cycles are irregular
The 14-day rule assumes that the time between ovulation and your next period is steady, which is usually true, and that you know how long this cycle will be, which is the hard part when cycles vary. If your cycle changes length from month to month, ovulation moves with it, and the calculated day can be off by a week or more. Enter your shortest and longest cycle in the irregular cycles box to see a range instead of a single day.
Cycles that regularly vary a lot, or that are missed, can be a sign of PCOS, thyroid problems or other conditions worth checking with a doctor. See irregular cycles: what is normal, and when to see a doctor and PCOS basics.
When to see a doctor
If you have been trying to conceive for 12 months without success (6 months if you are 35 or older), or you rarely seem to ovulate because periods are very irregular or absent, speak to a doctor. Also see a doctor if you have severe pain around ovulation, very heavy bleeding, or periods that stop for three months or more without pregnancy. Guidance from the NHS and ACOG is linked from our editorial policy.
This is not contraception
Knowing roughly when you ovulate does not make the other days safe. Ovulation can come early or late in any cycle, sperm can live for up to five days, and a calculator only sees the dates you type in. Fertility awareness methods can work for some people but need careful daily tracking and are less effective than most other methods of contraception, as ACOG explains. For preventing pregnancy, talk to a doctor or health worker. More in the fertile window and why it is not contraception.
Your privacy
Dates you enter on this page are used only in your browser and are never sent, saved or shared. This site does not use analytics or ads. If you want the same ideas on your phone, the Cycella app keeps your logs on your device and needs no account. See how Cycella keeps a period tracker private.
Frequently asked questions
How do I calculate my ovulation date?
Take the date your next period is expected and count back 14 days. If you do not know that date, add your cycle length to the first day of your last period, then subtract 14. In a 28-day cycle that is day 14; in a 32-day cycle it is closer to day 18.
Why is ovulation not always on day 14?
Day 14 is only right for a 28-day cycle. Ovulation happens roughly 14 days before the next period, so a longer cycle ovulates later and a shorter cycle ovulates earlier. The time after ovulation is more constant than the time before it.
How long does ovulation last?
The egg survives only about 12 to 24 hours after it is released. The fertile window is longer, about six to seven days in this calculator, because sperm can survive up to five days in the body.
Can I use ovulation dates to avoid pregnancy?
No. A calculator estimates ovulation from an average, and it can be wrong by several days, especially when cycles are irregular. It is not a method of contraception.
Is the ovulation calculator accurate with irregular cycles?
Much less so. If your cycle length changes a lot, ovulation moves with it. Use the shortest and longest cycle boxes for a range, and consider ovulation tests or talking to a doctor if you are trying to conceive.
Does this ovulation calculator save my data?
No. It runs in your browser, nothing is sent or stored, and the site has no analytics.
Want this in your pocket, without typing dates in each time? Cycella is a free Android period tracker with no account, no ads and no analytics. It learns your own cycle length from what you log, works offline, and is free on Google Play. See what it does for Nepal, or read about how it keeps your data private.