Ovulation & Fertile Window
Pinpoint your most fertile days. Enter your last period and cycle length — we'll map your ovulation day, full fertile window, and the next three cycles.
Find your fertile window
Just two numbers. We'll plot your whole cycle.
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Your cycle at a glance
Menstruation
Days 1–5. Uterine lining sheds. Hormones are at their lowest.
Follicular phase
An egg matures in the ovary. Oestrogen rises; uterine lining thickens.
Fertile window
Five days before ovulation plus ovulation day. Sperm can survive up to five days.
Ovulation
The peak fertile day. A mature egg is released. Viable for ~24 hours.
Luteal phase
After ovulation. Progesterone rises. If no pregnancy, period begins.
Your fertile window
Conception is most likely across this seven-day span. The highlighted day is ovulation.
Your next three cycles
Use these projected dates to plan ahead — but remember, real cycles vary.
How to boost your odds this cycle
- Time intercourse across the windowSperm live up to five days. Aim for every 1–2 days during your fertile window rather than timing a single day.
- Watch for ovulation signsCervical mucus becoming clear and stretchy ("egg-white" consistency) is the most reliable natural indicator.
- Take folic acid daily400 µg daily for at least a month before conception reduces neural tube defect risk. Higher doses if clinically advised.
- Use an ovulation predictor kit (OPK)Detects the LH surge 24–36 hours before ovulation. More reliable than calendar methods alone.
- Review medications and lifestyleSmoking, heavy alcohol and certain medications can affect fertility. Speak to your GP if TTC for 6+ months.
How the fertile window works
Ovulation is the single event that makes conception possible — the moment a mature egg is released from the ovary into the fallopian tube. The egg remains viable for only about 24 hours. Sperm, by contrast, can survive inside the female reproductive tract for up to five days. Put those two facts together and the "fertile window" becomes six days long: the five days before ovulation plus ovulation day itself.
In a textbook 28-day cycle with a 14-day luteal phase, ovulation happens on day 14 — counted from day 1 of your last period. But most women's cycles aren't textbook. The key insight is that the luteal phase (ovulation to period) is remarkably stable at around 14 days, while the follicular phase (period to ovulation) is what varies. That's why the calculator works backward: cycle length minus luteal phase gives you the day of ovulation.
On a 32-day cycle with a 14-day luteal phase, ovulation happens on day 18 — not day 16. On a 24-day cycle with a 14-day luteal phase, ovulation happens on day 10 — well inside what most people think of as still being "their period." These are the cases where standard calendar methods fail and why specifying your luteal phase matters.
A window, not a day
Even with perfect calendar information, ovulation still varies by a day or two each cycle. The most accurate way to confirm it is an ovulation predictor kit (OPK) detecting your LH surge, or a follicle tracking scan measuring the egg follicle directly.
If you've been trying for 6+ months with regular cycles, or 3+ months with irregular ones, speak to your GP or book a follicle tracking scan.
Signs you're ovulating
The calculator predicts ovulation from your cycle — but your body gives signals too. Watching for these alongside the calendar dramatically improves timing.
Cervical mucus changes
Mucus becomes clear, stretchy and slippery — like raw egg white — 2–3 days before ovulation. This is the most reliable natural indicator.
Mid-cycle twinge (Mittelschmerz)
A mild, one-sided lower abdominal pain around ovulation. Many women feel nothing; roughly 20% get this recognisable cramp.
Basal body temperature rise
Your resting temperature rises ~0.3°C after ovulation and stays elevated until your period. Confirms ovulation has happened — but only in hindsight.
LH surge on an OPK
Ovulation predictor kits detect the surge of luteinising hormone that triggers ovulation, typically 24–36 hours beforehand. The most practical advance warning.
Heightened libido
A natural increase in sex drive around ovulation — biology quietly encouraging timing. Not everyone notices, but it's real and well-documented.
Breast tenderness
Some women feel tender breasts in the few days after ovulation, as progesterone rises. Often a late-phase confirmation rather than a predictor.
When to seek a fertility scan
The calculator helps with timing, but it can't confirm that ovulation is actually happening — or that there's no underlying issue.
Frequently asked questions
How accurate is a calendar-based ovulation calculator?
Accurate to within 1–3 days for most women with regular cycles. Much less accurate for irregular cycles, PCOS, or recent changes (postpartum, stopping contraception, stress). A calendar method is a starting point — not a substitute for ovulation predictor kits, basal body temperature tracking, or a follicle tracking scan, all of which give real-time confirmation.
Why does my luteal phase matter?
Because the luteal phase (ovulation to period) is the most stable part of your cycle — typically 12–16 days. The follicular phase (period to ovulation) is what varies. To calculate ovulation accurately from cycle length, you subtract luteal phase length. A woman with a 32-day cycle and a 14-day luteal phase ovulates on day 18, not day 16.
My cycles are irregular. Can I still use this?
You can, but the result will be less reliable. If your cycles vary by more than 7 days month-to-month, calendar-based prediction becomes guesswork. Options include: using an ovulation predictor kit (OPK) for real-time LH detection, tracking basal body temperature across cycles to spot patterns, or booking a follicle tracking scan that watches the follicle develop directly.
When is the best time to have intercourse to conceive?
Every 1–2 days throughout your fertile window gives you the best odds. The two days before ovulation and ovulation day itself are the highest-probability days. Sperm take time to reach the fallopian tube — so intercourse in the days leading up to ovulation often works better than waiting for the exact day.
How long should we try before seeking help?
Standard NHS guidance is 12 months of regular, unprotected intercourse before referral if you're under 35; 6 months if you're 35 or over. That said, if you have irregular cycles, known gynaecological issues (PCOS, endometriosis), previous miscarriages, or have stopped hormonal contraception within the last few months and cycles haven't returned, you don't need to wait. Speak to your GP or book a fertility assessment sooner.
Does this calculator work for people with PCOS?
Not reliably. PCOS often means anovulatory cycles — cycles where ovulation doesn't happen at all, or happens unpredictably. The calendar method assumes regular ovulation, which PCOS can disrupt. If you have PCOS and are trying to conceive, a follicle tracking scan is much more useful: it shows whether the follicle is developing and whether ovulation actually occurs.
I've just come off the pill — when will my fertile window return?
Usually within 1–3 months, though it can take longer. Hormonal contraception suppresses ovulation, and it takes time for your natural cycle to resume. Cycles in the first few months post-pill are often irregular, which makes the calculator less reliable. A few cycles of tracking — with an OPK or a scan — will give you a clearer picture.
Confirm ovulation with a follicle tracking scan
We watch your follicle develop across the cycle and confirm ovulation has happened. The definitive answer to "am I ovulating?". Discreet specialist care in Kensington.