When exactly do I ovulate?

Ovulation usually happens about 14 days before your next period, not 14 days after your last one. For a 28-day cycle that is around day 14, but for longer or shorter cycles it shifts based on your cycle length.

Does the fertile window work with irregular periods?

It is less precise with irregular cycles because the calendar method relies on predictability. Use it as a starting point and lean on physical signs like cervical mucus and ovulation tests, and consider tracking a few cycles.

Does this calculator replace medical advice?

No. It offers general educational guidance for planning conception. If you have been trying for over a year, or six months if you are over 35, or have very irregular cycles, please see your provider.

Maternal Health · Dexocalc

Fertile Window Calculator

Find your most fertile days for conceiving — your calculator maps your ovulation day, your full 6-day fertile window, and your two peak days, then forecasts the next three cycles so you can plan with confidence.

Medically reviewed by Charlotte Rose, RN Reviewed for accuracy: July 2026 · Method per Johns Hopkins & OWH

This calculator gives you a personalised estimate based on the calendar method. It works best if your cycles are fairly regular — if they vary a lot, pair it with ovulation tests or cervical-mucus tracking, and remember it isn’t a reliable form of contraception.

First day of your last period

The day your most recent period started — the first day of full flow.

Average cycle length: 28 days

2028 (average)45

Count from the first day of one period to the first day of the next. Not sure? 28 days is the average.

Enter your last period date and cycle length to see your fertile window, peak days, and a three-cycle forecast.

How the fertile window calculator works

If you’re trying to conceive, timing is everything — and this fertile window calculator takes the guesswork out of it. It uses two simple pieces of information, the first day of your last period and your average cycle length, to pinpoint the days each month when you’re most likely to get pregnant. Behind the scenes, it works out your ovulation day, then maps the six-day fertile window that leads up to and includes it, and highlights your two peak fertility days when conception chances are highest.

The science is straightforward. Ovulation happens about 14 days before your next period starts, so the calculator counts forward from your last period using your cycle length and subtracts the luteal phase. Because sperm can survive in the body for up to five days and the egg lives for around a day after release, your fertile window spans roughly six days — the five days before ovulation plus ovulation day itself. If you’d like to double-check your dates first, our Reverse Due Date Calculator and Basal Body Temperature Charting Tool are helpful companions.

Quick answer

Your fertile window is the six days ending on ovulation day — the five days before ovulation plus ovulation day itself. For a 28-day cycle, ovulation is usually around day 14, making days 9 to 14 the fertile window. The two most fertile days are the day before and the day of ovulation, when the chance of conceiving peaks at roughly 30% per cycle for healthy couples.

What is the fertile window?

The fertile window is the short stretch of your cycle when having sex can lead to pregnancy. It exists because of two biological timers: sperm can wait in the reproductive tract for up to five days, and a released egg survives for only about 12 to 24 hours. Put those together and you get a window of about six days — the five days before ovulation, when sperm can be waiting, plus ovulation day, when the egg appears. Outside this window, pregnancy is very unlikely, which is exactly why identifying it matters so much when you’re trying to conceive.

How to calculate your ovulation and fertile days

The calculation rests on one reliable fact: the luteal phase — the time between ovulation and your next period — is fairly constant at around 14 days, even when cycle lengths vary. So the formula is simply:

Ovulation day = (first day of last period + cycle length) − 14

For a 28-day cycle that lands on day 14; for a 32-day cycle, day 18; for a 24-day cycle, day 10. Your fertile window is then the five days before that day plus ovulation day itself. This is why two people with the same period date but different cycle lengths can have completely different fertile windows — and why using your own cycle length, rather than assuming 28 days, makes the estimate far more accurate.

Fertile window by cycle length

Here’s roughly when ovulation and the fertile window fall for common cycle lengths, counting from the first day of your period.

Cycle length	Ovulation (approx.)	Fertile window (cycle days)	Peak days
24 days	Day 10	Days 5–10	Days 9–10
26 days	Day 12	Days 7–12	Days 11–12
28 days	Day 14	Days 9–14	Days 13–14
30 days	Day 16	Days 11–16	Days 15–16
32 days	Day 18	Days 13–18	Days 17–18
35 days	Day 21	Days 16–21	Days 20–21

When are you most fertile?

Not every day in the fertile window is equal. Conception chances climb as ovulation approaches, peaking on the two days before ovulation and ovulation day itself. Research shows intercourse during those peak days gives the highest pregnancy rates — around 30% per cycle for healthy couples in their twenties and early thirties. The days at the start of the window carry a lower but still real chance, thanks to sperm survival. If you’re trying to conceive, the simplest advice from fertility specialists is to have sex every one to two days across your whole fertile window, so healthy sperm are always ready when the egg arrives.

Signs you’re in your fertile window

Your body offers clues that back up the calendar. The clearest is cervical mucus: around ovulation it becomes clear, slippery, and stretchy, like raw egg white. A small rise in basal body temperature (about 0.5–1°F) confirms ovulation has just happened, and an ovulation predictor kit detects the surge in luteinising hormone (LH) that precedes ovulation by a day or so. Some people also notice mild one-sided twinges (mittelschmerz), a heightened libido, or breast tenderness. Combining these signs with this calculator gives you the most reliable picture of your fertile days.

What if your cycles are irregular?

The calendar method assumes a reasonably predictable cycle, so if yours swings by more than a week from month to month, a calculator alone will be less accurate. That doesn’t mean it’s useless — it still gives you a starting point — but you’ll want to lean more on the physical signs above, especially ovulation kits and cervical-mucus tracking, to catch your actual ovulation each cycle. Tracking your cycles for three to six months also helps you learn your personal average. If your periods are very irregular or absent, it’s worth speaking to your provider, as conditions like PCOS or thyroid issues can affect ovulation.

Related maternal health calculators

This tool is part of the Maternal Health Calculators hub. These companion tools cover cycle tracking, conception, and early pregnancy — each answers a different question, so choose whichever fits where you are.

Frequently asked questions

About six. Your fertile window is the five days before ovulation plus ovulation day itself, because sperm survive up to five days and the egg lives about a day. Of these, the two days before and including ovulation are your peak, highest-chance days.

Ovulation usually happens about 14 days before your next period, not 14 days after your last one. For a 28-day cycle that’s around day 14, but for longer or shorter cycles it shifts — this calculator works it out from your specific cycle length.

The two most fertile days are the day before ovulation and ovulation day itself, when conception chances peak at roughly 30% per cycle. Having sex every one to two days across your whole fertile window gives you the best overall odds.

It’s much less likely but not impossible, because ovulation timing can shift and sperm can survive several days. That’s also why the calendar method isn’t reliable for preventing pregnancy. If you’re avoiding pregnancy, use a proper contraceptive method.

For regular cycles it’s about 85–90% accurate for pinpointing the fertile window, though the exact ovulation day can still vary by a day or two. Accuracy drops with irregular cycles, stress, or illness. Combining it with ovulation kits or cervical-mucus tracking improves reliability.

The luteal phase is the time between ovulation and your next period. It’s more consistent than the first half of the cycle — usually around 14 days — which is what makes counting back from your next period reliable. If tracking shows yours differs, you can adjust it in the calculator.

The most reliable time is around the day your period is due, roughly 14 days after ovulation. Testing earlier risks a false negative because the pregnancy hormone hCG needs time to build up. This calculator shows your earliest sensible test date.

It’s less precise with irregular cycles because the calendar method relies on predictability. Use it as a starting point, but lean on physical signs like cervical mucus and ovulation tests, and consider tracking a few cycles to learn your pattern. Persistent irregularity is worth discussing with a provider.

No. It offers general, educational guidance for planning conception. If you’ve been trying for over a year — or six months if you’re over 35 — or have very irregular cycles, please see your provider, who can investigate and support you personally.

Methodology

This calculator uses the calendar (rhythm) method to estimate your fertile window. It first estimates ovulation as the first day of your last period plus your cycle length, minus the luteal phase (14 days by default, adjustable between 10 and 16). The fertile window is then defined as the five days before ovulation plus ovulation day itself, reflecting sperm survival of up to five days and an egg lifespan of about 12 to 24 hours. Peak fertility days are the two days before and including ovulation, where per-cycle conception probability is highest, around 30% for healthy couples; the day-by-day chart reflects the published relative probabilities across the window. The tool also projects the next three cycles, and, for conception this cycle, estimates the implantation window (about 6 to 10 days after ovulation), the earliest sensible pregnancy-test date (about 14 days after ovulation), and an estimated due date (280 days from the last period). All outputs are educational estimates. Ovulation timing varies between cycles even when they are regular, and the calendar method is not reliable for contraception. This tool is not a diagnosis or a substitute for care from a qualified provider.

Reviewer

Charlotte Rose, RN A professional nurse turned SEO strategist and web designer with 10+ years of experience in the online health field. Charlotte reviewed this calculator’s ovulation logic, fertility-window model, and educational content for clinical accuracy and a clear, supportive tone.

References & trusted sources

Last reviewed & updated: July 2026.