Your Ovulation & Fertile Window Calculator
Enter the first day of your last period and your average cycle length to estimate when you ovulate and which days you are most likely to conceive.
Quick samples
Add your last period to see your fertile window.
Enter the first day of your last period and your average cycle length (or try a quick sample), and your ovulation day appears here.
Ovulation is estimated as about fourteen days before your next period. The fertile window covers the five days before it plus the day of ovulation, when conception is most likely.
The formula
Next period = last period + cycle length · Ovulation ≈ next period − 14 · Fertile window = ovulation − 5 to ovulation + 1
- Cycle length — Days from the start of one period to the start of the next.
- − 14 days — The roughly fixed luteal phase, so ovulation sits about two weeks before the next period.
- − 5 days — How long sperm survive, opening the fertile window before ovulation.
- + 1 day — The short life of the egg after release, closing the window soon after ovulation.
Typical ovulation day by cycle length
| Cycle length | Approx. ovulation day | Peak-fertile days |
|---|---|---|
| 24 days | Day 10 | Days 5–10 |
| 28 days | Day 14 | Days 9–14 |
| 32 days | Day 18 | Days 13–18 |
| 35 days | Day 21 | Days 16–21 |
Cycle day one is the first day of your period. The luteal phase is assumed at 14 days; individual luteal phases run roughly 12–16 days.
This ovulation calculator estimates the day you are most likely to release an egg and the six-day fertile window around it. Enter the first day of your last period and your average cycle length, and you get a projected ovulation date, a fertile window, and a nudge for your next period.
Whether you are trying to conceive or trying to avoid it, knowing your likely fertile days is useful. The window is narrow — sperm survive a few days but the egg only lives about a day — so timing matters far more than any other single factor in natural conception.
The estimate rests on one reliable rule: ovulation tends to happen about fourteen days before your next period starts, no matter how long your cycle is. A longer cycle mostly delays ovulation rather than moving that back-count, which is why cycle length is the key input here.
How ovulation is estimated
A menstrual cycle has two halves with ovulation in the middle, but the halves are not equal. The first half, before ovulation, varies a lot between people and months; the second half, after ovulation, is remarkably steady at roughly twelve to fourteen days for most.
Because that luteal phase is stable, the simplest estimate counts backwards from your next period. Take your last period, add your cycle length to find the next one, then subtract fourteen days to place ovulation.
That is why a twenty-eight-day cycle ovulates around day fourteen while a thirty-four-day cycle ovulates nearer day twenty. The egg is released later, but it is still about two weeks before the period that follows.
What your fertile window really means
The fertile window is not a single day. Sperm can survive inside the reproductive tract for up to about five days, so intercourse in the days before ovulation can still result in conception when the egg appears.
The egg itself only lives around twelve to twenty-four hours after release, so the window closes quickly once ovulation has passed. That is why the calculator shows the five days before ovulation plus a day after, rather than just the ovulation date.
Fertility peaks in the two to three days leading up to ovulation. If you are trying to conceive, regular intercourse across the window covers the timing better than trying to hit one exact day.
A worked example
Suppose the first day of your last period was 1 June and your cycles run a steady 28 days. Adding 28 days puts your next period at 29 June, and counting back fourteen days places ovulation around 15 June.
The fertile window then spans roughly 10 June to 16 June, with the days just before 15 June being the most fertile. Any of that stretch could lead to conception, so it is a range to plan within, not a deadline.
Now imagine a 34-day cycle with the same 1 June start. The next period lands 5 July, so ovulation moves to about 21 June and the whole window shifts later. The rule holds; only the length changes the placement.
How to use this calculator well
Use the first day of your most recent period — the day full bleeding began, not spotting — and your average cycle length. If you track your cycles, an honest average over the last few months is better than a single number.
The quick-sample buttons load common cycle lengths so you can see how the window moves before entering your own dates. This makes it easy to understand why length matters more than you might expect.
If your cycles are irregular, treat the result as a rough centre rather than a precise forecast. Ovulation predictor kits, cervical mucus changes and a sustained basal-temperature shift all give real-time confirmation that a calendar cannot.
- Try ovulation predictor kits to catch the LH surge that precedes ovulation by a day or two.
- Watch for fertile cervical mucus, which is clear and stretchy as ovulation approaches.
- A resting basal-temperature rise confirms ovulation after it has already happened.
How accurate a calendar estimate is
Calendar math is a good average but a poor day-by-day predictor. Stress, illness, travel and normal hormonal variation can shift ovulation by several days in any given cycle, even when your periods feel regular.
This means the estimated ovulation date is best read as the middle of a plausible range, with the fertile window already widened to absorb some of that uncertainty. It is a planning aid, not a guarantee.
For conception, that is usually fine, because a wide window plus regular intercourse covers the timing. For avoiding pregnancy by dates alone, a calendar estimate is far too unreliable, and reliable contraception is the safer route.
Ovulation after contraception, postpartum and in perimenopause
The calendar method assumes a natural, regularly cycling body, and that assumption falls away in a few common situations. After stopping hormonal contraception it can take a cycle or two for ovulation to settle into a predictable rhythm, so a calculated date may not reflect what is actually happening while your cycle reboots.
Postpartum cycles are variable, especially if you are breastfeeding, and ovulation can occur before the very first period returns. That means a memory of 'no periods yet' is not the same as 'not fertile', which matters both if you are hoping to conceive and if you are not.
In the late thirties and forties cycles often lengthen and become irregular as fertility declines, shifting ovulation later and less predictably. In every one of these phases treat the estimate as a centre rather than a certainty, and lean on real-time signs or a clinician rather than dates alone.
Key terms
- Ovulation
- The release of an egg from the ovary, the event that makes conception possible.
- Fertile window
- The run of days when intercourse can lead to pregnancy — the five days before ovulation plus the day of.
- Luteal phase
- The fixed second half of the cycle after ovulation, usually twelve to sixteen days.
- Cycle day one
- The first day of full menstrual bleeding, used as the start of the count.
Pro tips
- Enter your average cycle length, not your shortest — the mean of recent cycles is the best anchor.
- Aim for the days before ovulation, not just the ovulation day, since sperm survive several days.
- Pair the calendar with an ovulation predictor kit to confirm the surge in real time.
Common mistakes
- Assuming ovulation always lands on day fourteen, whatever the cycle length.
- Counting cycle day one from spotting rather than full bleeding.
- Relying on a calendar estimate alone to avoid pregnancy.
Ovulation calculator — your questions
How do I know which day I ovulate?
The reliable estimate is to subtract about fourteen days from the start of your next expected period, because the luteal phase after ovulation is fairly fixed. So if your cycles run twenty-eight days, you likely ovulate around day fourteen; if they run thirty-four days, nearer day twenty. The calculator does exactly this from your last period and your average cycle length.
How long is the fertile window?
About six days. Sperm can survive up to roughly five days in the reproductive tract, and the egg lives only around twelve to twenty-four hours after release. Putting those together, the window opens about five days before ovulation and closes about a day after it. Your highest chance is in the two to three days immediately before ovulation, when healthy sperm are already waiting for the egg.
Does this work if my cycles are irregular?
Only roughly. The method assumes a predictable next period, which irregular cycles do not provide, so the estimated ovulation date can be several days off. A calendar still gives a sensible centre, but you will do better with real-time signs: ovulation predictor kits that catch the LH surge, changes in cervical mucus, or a sustained basal-temperature rise. If irregular cycles are extreme or you are also bleeding between periods, it is worth speaking to a clinician.
How accurate is an ovulation calculator?
As an average, reasonably; as a day-by-day prediction, not very. Ovulation shifts with stress, illness, travel and normal hormonal variation, even in people with regular cycles. The tool places ovulation about fourteen days before your next period and widens the fertile window to absorb some uncertainty, so treat the date as the middle of a plausible range. For timing conception, regular intercourse across the window beats chasing one exact day.
Can I use this to avoid pregnancy?
Not reliably on its own. Calendar estimates are too imprecise about ovulation to depend on for contraception, and fertile-window avoidance needs additional confirmation such as temperature and mucus tracking done with training. If preventing pregnancy matters, use dependable contraception. This calculator is designed mainly to help people understand their likely fertile days when trying to conceive.
What if I do not remember my last period?
Start from the best date you have and note the uncertainty. If you track your cycle in an app, pull the most recent period from there. If you genuinely cannot recall and your cycles are irregular, an ovulation predictor kit is more helpful than a calendar estimate, since it detects the hormonal surge directly. Over a cycle or two of tracking you will have the last-period and cycle-length numbers this tool needs.