Ovulation calculator and fertile window
Work out your ovulation date and six-day fertile window from your last period and cycle length. Nothing is uploaded and nothing is stored unless you ask.
- Runs in your browser
- Nothing you enter is sent
- Free, no account
Nothing you type here is sent anywhere.
The dates are used by JavaScript running inside this page and are discarded when you close the tab. There are no cookies, no analytics and no account. Storage on this device is off unless you switch it on below, and you can delete it in one click.
First day of one period to the first day of the next.
Ovulation to the next period. Far steadier than cycle length — 14 days if you don't know.
ovulation = first day of last period + (cycle length − luteal phase) ovulation = 25 Dec 2025 + (28 − 14) days ovulation = 25 Dec 2025 + 14 days = 8 Jan 2026 fertile window = ovulation − 5 days … ovulation day fertile window = 3 Jan … 8 Jan next period = first day of last period + cycle length next period = 25 Dec 2025 + 28 days = 22 Jan 2026
| Cycle | Period starts | Fertile window | Ovulation | Due date if conceived |
|---|---|---|---|---|
| 1 | Thu 25 Dec 2025 | 3 Jan – 8 Jan | Thu 8 Jan 2026 | Thu 1 Oct 2026 |
| 2 | Thu 22 Jan 2026 | 31 Jan – 5 Feb | Thu 5 Feb 2026 | Thu 29 Oct 2026 |
| 3 | Thu 19 Feb 2026 | 28 Feb – 5 Mar | Thu 5 Mar 2026 | Thu 26 Nov 2026 |
Count backwards, not forwards
The single most useful fact about ovulation timing is that it is anchored to the next period, not the last one. The luteal phase — from ovulation to the start of bleeding — is remarkably consistent within a person, usually 12 to 14 days, while the follicular phase before ovulation is where nearly all cycle-length variation lives.
So "day 14" is only correct for a 28-day cycle. On a 32-day cycle with a 14-day luteal phase, ovulation is around day 18. On a 24-day cycle it is around day 10. That is why this calculator asks for your luteal phase length as a separate input instead of assuming half your cycle.
The six-day fertile window
Conception is only possible on about six days per cycle: the five days before ovulation and the day of ovulation itself. The asymmetry has a biological cause.
| Cell | Survival | Consequence |
|---|---|---|
| Sperm | Up to 5 days in fertile cervical mucus | Intercourse before ovulation can still result in conception |
| Egg | About 12–24 hours after release | The window closes almost immediately after ovulation |
Probability is not flat across those six days. Studies of couples timing intercourse consistently find the highest conception rates in the two days immediately before ovulation, with the day of ovulation itself slightly lower and the fifth day before it much lower.
What a calendar prediction can and cannot do
This tool projects from an average. It cannot know that you ovulated late this month because of illness, travel, stress or a change in sleep — and cycle-to-cycle variation of several days is entirely normal even in people who describe their cycles as regular. Large data sets of tracked cycles show that only a minority of people ovulate on the day a calendar method predicts.
Three methods observe rather than predict, and they pair well with this:
- LH test strips. Detect the luteinising hormone surge that precedes ovulation by 24–36 hours. Start testing a few days before the predicted date.
- Basal body temperature. Rises by about 0.3 °C after ovulation and stays up. It confirms ovulation happened but only in retrospect.
- Cervical mucus. Becomes clear, stretchy and slippery in the fertile window. The cheapest and most immediate signal, and the one taught first in fertility awareness.
Not a contraceptive method
Calendar-only prediction is among the least reliable ways to avoid pregnancy, with typical-use failure rates commonly quoted between 12% and 24% per year. Properly taught symptothermal fertility awareness — combining temperature, mucus and calendar rules with instruction — performs far better, but it is a different practice from reading a date off a web page. Do not use this tool as contraception.
When to seek advice
General guidance in most health systems is to seek advice after twelve months of trying under the age of 35, or six months at 35 and over. Sooner if cycles are absent, very irregular, shorter than 21 days or longer than 35, or if there is a known condition such as PCOS or endometriosis. A short luteal phase — under 10 days — is also worth mentioning.
Why this page has no account and no cookies
Reproductive health data is among the most sensitive information a person can enter into a website, and the standard app model — upload it, store it, sync it, monetise it — creates a permanent record that can be breached, sold or subpoenaed. This page removes the possibility rather than promising restraint: the arithmetic runs in your browser, there is no server behind it, and no request leaves the page while you use it.
Saving your dates on this device is off by default. If you switch it on, the values are written to your own browser's local storage — never transmitted — and the delete button removes them immediately. You can verify all of this by opening your browser's network tab, or simply by turning off Wi-Fi: the tool works identically offline.
Questions
When do I ovulate?
About 14 days before your next period starts, not 14 days after the last one began. That is why this calculator asks for your luteal phase length: the second half of the cycle is far more consistent than the first, so counting backwards from the next period is more accurate than counting forwards.
How long is the fertile window?
Six days: the five days before ovulation plus the day itself. Sperm can survive up to five days in fertile cervical mucus, while an egg is viable for around 24 hours. Conception rates are highest in the two days immediately before ovulation.
How accurate is a calendar prediction?
It is an estimate from an average, and cycle length varies month to month for most people. Ovulation predictor kits detecting the LH surge, basal body temperature charting and cervical mucus observation all measure what actually happened rather than predicting it — use one of them alongside this.
Can I use this to avoid pregnancy?
No. Calendar-only methods are among the least reliable forms of contraception, with typical-use failure rates around 12–24% per year. Symptothermal fertility awareness methods taught by an instructor are a different thing entirely.
Is my reproductive health data safe here?
It never leaves your device. This page has no server, no cookies, no analytics and no account. Saving your dates on this device is off by default; if you turn it on, the data is written to your own browser storage and the delete button removes it immediately.