Ovulation Calculator
Find when you ovulate and your most fertile days from the first day of your last period and your average cycle length. This calculator also shows the best days to try, when a home pregnancy test would be reliable, the due date if you conceive this cycle, and a calendar of the next several cycles.
Formula
Worked example
Last period on 10 May, a 28-day cycle and a 14-day luteal phase put ovulation on day 14 (24 May). The fertile window runs roughly 19 to 25 May, the best days to try are 22 to 24 May, the next period is due 7 June, and conceiving would give a due date around 14 February next year.
How ovulation timing is estimated
Ovulation usually happens about 14 days before your next period, because the luteal phase (from ovulation to menstruation) is relatively consistent. So the calculator counts your cycle length and subtracts the luteal phase to estimate the ovulation day from the first day of your last period. The follicular phase before ovulation is the part that varies most between people and cycles, which is why the cycle length matters so much. If a luteinizing hormone kit or basal body temperature chart has told you your own luteal length, set it under advanced options for a sharper estimate.
Your fertile window and the best days to try
You are fertile for about six days each cycle: the five days leading up to ovulation and ovulation day itself. This is because sperm can survive up to five days in the reproductive tract, while the egg is viable for around 12 to 24 hours after release. The highest-probability days are the two days just before ovulation through ovulation day, so the calculator flags those as the best days to try. Having intercourse every day or every other day across the fertile window gives the best chance of conception.
Pregnancy test timing, due date and the cycle calendar
If conception happens, the calculator estimates the earliest reliable home pregnancy test date, which is around the day your next period is due, when the pregnancy hormone hCG is usually high enough to detect. It also projects a due date by adding 266 days to ovulation (the average length of pregnancy from conception). The fertility calendar repeats your cycle forward for several months so you can plan ahead, though each projected cycle assumes the same length and real cycles drift.
Why estimates are approximate
Cycle length, stress, illness and other factors can shift ovulation, so a calendar estimate is a starting point rather than a guarantee. People with irregular cycles, or cycles shorter than 21 or longer than 35 days, will find calendar prediction less reliable. Methods that detect the body’s actual signals, ovulation predictor kits that measure the luteinising hormone surge, or basal body temperature charting, give a more precise indication for a particular cycle.
Phases of a 28-day cycle
| Phase | Cycle days | What happens |
|---|---|---|
| Menstruation | Days 1-5 | The uterine lining sheds |
| Follicular | Days 1-13 | An egg matures; lining rebuilds |
| Fertile window | Days 10-15 | Highest chance of conception |
| Ovulation | ~Day 14 | The egg is released |
| Luteal | Days 15-28 | Lining thickens; period if no pregnancy |
A typical cycle, counting from day 1 (the first day of your period). Cycle length and ovulation timing vary between people and from month to month.
Frequently asked questions
How many days am I fertile, and which are best?
About six days per cycle: the five days before ovulation plus ovulation day. Sperm can survive several days, which extends the window before the egg is released. The two days just before ovulation through ovulation day are the highest-probability days, which is why the calculator highlights them as the best days to try.
When can I take a pregnancy test?
A home test is usually reliable from about the day your next period is due, which this calculator shows. Testing earlier can give a false negative because the pregnancy hormone hCG may not be high enough yet. If an early test is negative but your period does not arrive, test again a few days later.
How is the due date worked out?
If you conceive around ovulation, the due date is estimated by adding 266 days (about 38 weeks) to the ovulation date, since that is the average length of pregnancy measured from conception. This matches the standard rule of 280 days from the first day of the last period for a 28-day cycle.
What if my cycle is irregular?
Calendar estimates assume reasonably regular cycles. If yours vary a lot, tracking several months, using ovulation predictor kits, or charting basal body temperature will give a more reliable picture than dates alone. You can also set your own luteal phase length under advanced options if a kit or chart has measured it.
Can I use this to avoid pregnancy?
It is not a reliable contraceptive method on its own. Calendar-based estimates have a meaningful failure rate because ovulation can shift. Speak to a healthcare provider about effective contraception.