Ovulation Calculator
Your fertile window, ovulation day and the next six cycles
Cycle length is counted from the first day of one period to the first day of the next. Leave the luteal phase at 14 days unless a doctor or an ovulation test has told you yours, because it is the setting that changes the answer most.
The next six cycles
| Cycle | Fertile window | Ovulation | Period due | Due date if you conceive | When |
|---|
How to Use
- 1Enter the first day of bleeding in your last period, which is cycle day 1.
- 2Set your average cycle length. If it varies, use a typical recent one.
- 3Read the fertile window, the six days ending on ovulation day.
- 4Open the table to plan the next six cycles, each with the due date if you conceive.
How It Is Worked Out
- Next periodLast period + cycle length
- OvulationNext period − luteal phase
- Fertile windowOvulation − 5 days to ovulation
- Peak daysOvulation − 2 days to ovulation
- Due dateOvulation + 266 days
Quick Examples
Tap a cycle length to load it above.
Everything about the Ovulation Calculator
Which six days actually matter, and how far to trust a calendar estimate.
This ovulation calculator estimates the day you are most likely to release an egg and, more usefully, the six-day fertile window around it. It works backwards from your next expected period rather than forwards from your last one, which is the part most calendar tools get wrong.
This is not birth control. Calendar-based methods have a real-world failure rate of roughly one in four over a year. Ovulation shifts by several days from cycle to cycle even in people with regular periods, and sperm can survive long enough to meet an egg released days later. If you are trying to avoid pregnancy, use an actual contraceptive method and talk to a healthcare professional.
Why we count backwards
A menstrual cycle has two halves. The follicular phase runs from the first day of bleeding until ovulation, and it is the elastic one. It is what stretches when a cycle runs to 33 days and shortens when it runs to 25. The luteal phase runs from ovulation to the next period and is remarkably stable, usually 12 to 16 days for any given person.
So the reliable anchor is the end of the cycle, not the start. Counting forwards from your last period ("ovulation is day 14") only works if your cycle happens to be 28 days. Counting backwards from the next expected period by your luteal phase length works whatever your cycle length is. That is what this calculator does: ovulation ≈ next period − luteal phase.
| Cycle length | Ovulation (14-day luteal) | Fertile window |
|---|---|---|
| 24 days | Around cycle day 11 | Days 6 to 11 |
| 28 days | Around cycle day 15 | Days 10 to 15 |
| 32 days | Around cycle day 19 | Days 14 to 19 |
| 35 days | Around cycle day 22 | Days 17 to 22 |
You will also see "day 14" quoted for a 28-day cycle. The one-day difference comes from whether ovulation day itself is counted inside the luteal phase. Both conventions are in common use and the real biological variation is several times larger than that single day, so it is not worth losing sleep over.
The fertile window is lopsided on purpose
Sperm can survive up to about five days in fertile cervical mucus. An egg survives only 12 to 24 hours after release. That asymmetry means almost all the chance of conceiving sits in the days before ovulation, not after it.
- 1Five days before ovulation
The window opens. Conception is possible because sperm present now can still be waiting when the egg arrives.
- 2Two days before, and ovulation day
Peak fertility. Every published dataset puts the highest per-cycle conception rates in this narrow band, which is why the calculator highlights it separately.
- 3The day after ovulation
The window has essentially closed. The egg is no longer viable, so timing anything later in the cycle does not help.
How accurate is this, honestly
For someone with genuinely regular cycles, a calendar estimate typically lands within a day or two of actual ovulation. For anyone else it can be out by a week. Studies tracking hormones have found that only around 30% of people have their fertile window entirely where a textbook calendar would put it.
If you want to know when you are ovulating rather than when you probably will:
- Ovulation predictor kits detect the LH surge in urine and give 24 to 36 hours' notice. This is the most practical method for timing.
- Basal body temperature rises about 0.3 °C after ovulation. It confirms ovulation happened but only after the window has closed, so it maps your pattern rather than predicting this cycle.
- Cervical mucus becomes clear, stretchy and slippery in the fertile days, so it is free, and a genuinely useful signal once you learn to read it.
Using a kit alongside this calendar is the sensible combination: the calendar tells you roughly when to start testing, the kit tells you when it is actually happening.
When to stop guessing and ask
Speak to a healthcare professional if your cycles are shorter than 21 days or longer than 35, if their length swings wildly, if your luteal phase is consistently under 10 days, or if you have been trying to conceive for a year, or six months if you are over 35. None of that means something is wrong, but it is beyond what a calendar can answer.
Already know you are pregnant? The pregnancy calculator works out your due date and how far along you are from a period date, a scan or an IVF transfer. Once the baby arrives, the age calculator gives their exact age in weeks and days for the growth charts.
Medical disclaimer: this calculator provides general estimates from calendar averages only. It is not medical advice, cannot diagnose anything, and must not be relied on as contraception. Always consult a qualified healthcare professional about fertility or contraception.
Frequently Asked Questions
When do I ovulate?
Roughly 14 days before your next period starts, not 14 days after your last one. For a 28-day cycle those happen to be nearly the same day, but for a 35-day cycle ovulation is around day 21 rather than day 14. Enter your own cycle length above and the calculator counts backwards from your next expected period for you.
What is the fertile window and when is it?
It is the six days ending on ovulation day: the five days before, plus ovulation itself. It is lopsided because sperm survive up to about five days while an egg lasts only 12 to 24 hours, so the days before ovulation carry almost all the chance of conceiving. The two days before ovulation and ovulation day are the peak.
Can I use this calculator as birth control?
No. Calendar methods fail for roughly one in four people over a year of typical use, because ovulation moves between cycles and sperm survive for days. This tool is designed for planning conception, not avoiding it. If you are trying to prevent pregnancy, use a proper contraceptive method and speak to a healthcare professional.
How accurate is an ovulation calculator?
For regular cycles it usually lands within a day or two of real ovulation. For irregular cycles it can be out by a week or more. Hormone-tracking studies have found only about 30% of people ovulate exactly where a textbook calendar predicts, so treat these dates as a starting point rather than a fact.
What if my cycles are irregular?
A calendar estimate is much weaker when cycle length varies, because the whole calculation depends on knowing when the next period will arrive. Enter your shortest recent cycle to get the earliest likely window, then confirm with ovulation predictor kits or cervical mucus changes. If your cycles regularly fall outside 21 to 35 days, it is worth seeing a healthcare professional.
What is the luteal phase and why does it matter here?
It is the stretch between ovulation and your next period, usually 12 to 16 days and fairly constant for any one person. Because it barely changes while the first half of the cycle does, counting backwards from the next period by the luteal phase is far more reliable than counting forwards. Leave it at 14 days unless a doctor or ovulation tracking has told you yours.
How can I confirm I actually ovulated?
Ovulation predictor kits detect the LH surge and give 24 to 36 hours' warning, which is the most practical way to time things. A basal body temperature rise of about 0.3 °C confirms ovulation happened, but only after the fertile window has closed. Clear, stretchy cervical mucus is a free everyday signal that the window is open.
Is the ovulation calculator free and is my data stored?
It is completely free with no sign-up, and every date is calculated inside your browser. Nothing about your cycle is sent to a server, logged or saved anywhere, which matters more than usual for health information.