Ovulation Calculator — Fertile Window & Cycle Dates
Your fertile window opens in 0 days and ovulation follows 0 days from today.
You are in the fertile window right now — ovulation is estimated in 0 days.
This cycle's window has closed. The next one is dated above, 0 days away.
Cycle day : You are bleeding — the start of a new cycle. Day 1 is the first day of full flow, not spotting the evening before.
Cycle day : Follicular phase. A follicle is maturing and oestrogen is climbing; the fertile window has not opened yet.
Cycle day : You are inside the fertile window. Sperm can survive here long enough to meet the egg when it is released.
Cycle day : Estimated ovulation day. The egg is viable for roughly 12 to 24 hours from here.
Cycle day : Luteal phase. Ovulation has passed, progesterone is high, and this stretch stays close to the same length every cycle.
If conception happens in the upcoming cycle, implantation would fall around — and the estimated due date would be —.
Your next cycles
| Period | Fertile window | Peak days | Ovulation | Test from | Due date |
|---|
Peak days are the three with the highest conception odds. The due date column assumes birth 266 days after ovulation, the same 40-week figure a clinic would use.
Conception odds, day by day
Probabilities of pregnancy from a single act of intercourse, measured against a confirmed ovulation day. The peak sits before ovulation, not on it — which is why waiting for a positive ovulation test already costs you the two best days.
| Timing | Chance | Notes |
|---|---|---|
| 5 days before ovulation | ~4% | The outer edge of sperm survival. |
| 4 days before | ~10% | Cervical mucus starts to turn clear and stretchy. |
| 3 days before | ~15% | Fertile mucus established. |
| 2 days before | ~28% | Highest single-day odds in most datasets. |
| 1 day before | ~27% | An LH test usually turns positive here. |
| Ovulation day | ~20% | Egg released; a basal temperature rise follows. |
| 1 day after | ~8% | The window is effectively shut. |
Signs that confirm the prediction
A calendar predicts; your body reports. Use both — the prediction tells you when to start paying attention, and the signs tell you whether this cycle is behaving as expected.
| Sign | When | What it tells you |
|---|---|---|
| LH surge on an ovulation test | 24–36 hours before | The most practical confirmation at home. Test in the early afternoon, not first thing — LH rises later in the day for most people. |
| Egg-white cervical mucus | 2–4 days before | Clear, slippery and stretchy. This is the texture that keeps sperm alive for days rather than hours. |
| Basal body temperature rise | 1–2 days after | Confirms ovulation happened but cannot predict it. Useful for learning your luteal-phase length, which this calculator asks for. |
| One-sided twinge (mittelschmerz) | Around the day itself | Only about one person in five notices it, and it is not reliable enough to plan around on its own. |
This calculator is a planning aid, not medical advice, and it is not a contraceptive method. If you have been trying for a year without success — six months over the age of 35 — that is the point at which guidelines suggest getting checked.
Enter the first day of your last period and your average cycle length to see when you ovulate, the six-day fertile window around it, and the dates of your next few periods. The calculation counts backwards from your next period rather than assuming day 14, so it stays correct on a 24-day or a 38-day cycle. Everything runs in your browser — no account, no tracking, and no cycle dates leaving your device.
How it works
- 1 Enter the first day of your last period Use the day proper bleeding started, not the day spotting began. If you are not sure, pick the closest date you remember — the fertile window shifts by exactly as many days as the start date is off.
- 2 Set your cycle length and luteal phase Cycle length is the gap from one period's first day to the next, averaged over a few months. Leave the luteal phase at 14 days unless temperature or progesterone tracking has shown you something different; that figure is what places ovulation day.
- 3 Read the fertile window and plan ahead The summary gives your next ovulation date, the fertile window around it, and a countdown. The calendar below projects the same dates for up to twelve cycles, with the earliest reliable test date and the due date that would follow conception in each one.
Your data stays private
All processing happens entirely in your browser. No files, text, or data are ever sent to our servers. You can disconnect from the internet and this tool will still work.
Frequently asked questions
- How does an ovulation calculator work?
- It counts backwards from your next expected period, not forwards from your last one. The luteal phase — ovulation to period — stays close to 14 days in most people, while the follicular phase is what varies. So ovulation lands on cycle length minus luteal phase: day 14 of a 28-day cycle, but day 21 of a 35-day cycle. The fertile window is then the five days before ovulation plus the day itself.
- When am I most likely to get pregnant?
- The two days before ovulation. Studies tracking conception against confirmed ovulation dates put the per-day probability at roughly 25–30% there, falling to about 20% on ovulation day itself and under 10% the day after. Intercourse every day or every other day across the whole six-day window beats trying to hit one perfect day.
- How long is the fertile window?
- Six days: five before ovulation and ovulation day. That length comes from sperm, not the egg — sperm survive up to five days in fertile cervical mucus, while the egg itself is viable for only 12 to 24 hours. Once that day has passed, the window is closed until the next cycle.
- Is ovulation always on day 14?
- No, and this is the single most common mistake. Day 14 is the midpoint of a textbook 28-day cycle. A large cohort study of over 600 cycles found ovulation spread across cycle days 8 to 20 even among women with regular periods. If your cycle is not 28 days, enter the real figure — the calculator will move the window accordingly.
- Can I use this calculator to avoid pregnancy?
- No. Calendar prediction is far too coarse for contraception — stress, illness, travel and ordinary variation can move ovulation by days, and sperm already in the reproductive tract do not care about your calendar. Typical-use failure rates for calendar methods run around 15% a year. Use it to find the likely window, not to find safe days.
- What if my cycles are irregular?
- Averaging will give you a rough window, but treat it as a starting point and confirm with evidence. Ovulation tests, cervical mucus changes and basal body temperature all track the current cycle rather than a prediction from last month. If cycles vary by more than about eight days, or run shorter than 21 or longer than 35, that is worth raising with a doctor.
- How soon can I take a pregnancy test?
- Around 14 days past ovulation, which the calendar below dates for every cycle. Implantation happens 6 to 12 days after ovulation and hCG only becomes detectable in urine a few days later, so testing at 9 or 10 days past ovulation produces false negatives more often than answers.
- Is my data sent anywhere?
- No. The whole thing is date arithmetic in JavaScript on your device. Nothing is uploaded, stored or logged, and the page keeps working with the network off — which matters more than usual for cycle data, given how often period-tracking apps have ended up selling it.