Find your fertile window, ovulation day, and the four highest-probability days to try for conception — based on your last period and cycle length.
Results update live as you change inputs.
Best days to try for conception
Ovulation day
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cycle day —
Fertile window
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6-day span
Days until
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ovulation
Next period
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if not pregnant
| Cycle | Fertile window | Ovulation | Best days |
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The Method
Ovulation reliably happens 14 days before the next expected period, because the luteal phase is the stable part of the menstrual cycle. From the LMP and your cycle length we can pinpoint ovulation, then build the 6-day fertile window around it (sperm live up to 5 days, the egg ~24 hours). The four highest-probability days run from 2 days before ovulation through 1 day after.
Calendar method
About This Tool
A conception calculator — also called a fertility window calculator or best days to conceive calculator — identifies the days in your cycle when pregnancy is most likely. Unlike a conception date calculator (which works backwards from a known pregnancy), this tool projects forwards from your last period to help you plan intercourse for conception.
Every menstrual cycle has only a 6-day window when conception is possible — the 5 days before ovulation plus ovulation day itself. The 4 peak fertile days are the 2 days before ovulation, ovulation day, and the day after. Couples who time intercourse to this 4-day window dramatically increase conception odds compared to random timing.
Our calculator uses the standard calendar method: ovulation = LMP + (cycle length − 14). It works best when your cycles are regular (varying by less than ±3 days). For irregular cycles, combine the prediction with ovulation predictor kits (OPKs), basal body temperature (BBT) tracking, or cervical-mucus monitoring — these detect ovulation as it actually happens.
This free online conception calculator runs entirely in your browser — no sign-up, no data stored. Display up to 6 cycles ahead to plan around travel, holidays, or work commitments. If you've tried for 12+ months (or 6+ months over age 35) without success, see a fertility specialist.
Ovulation Day
Pinpoints ovulation 14 days before each expected next period.
Fertility Probability
Visual day-by-day probability chart based on published research.
6-Cycle Projection
Project fertile windows up to 6 cycles ahead for long-range planning.
Cycle Adjustable
Works for any cycle from 21 – 45 days — adjusts ovulation automatically.
Live Updates
Change LMP or cycle length and every result recalculates instantly.
100% Private
All dates calculated in your browser — no account, no data stored.
Three quick inputs give you months of fertility predictions.
Pick the first day of your last period — the day bleeding started, not when it ended.
A standard cycle is 28 days. If yours runs shorter or longer, change it — ovulation shifts accordingly.
From 1 to 6 cycles. Three is a sensible default — covers about 3 months of trying.
The headline dates are the 4 peak fertile days. Aim for intercourse every other day during this window.
The visual chart shows daily conception probability across the 8-day window — purple bars mark peak days.
The table lists fertile windows for the next 1–6 cycles so you can plan ahead.
Common questions about conception timing, fertile windows, and maximising your chances.
The fertile window is the 6-day period in each cycle when conception is possible: the 5 days before ovulation plus ovulation day itself. Sperm can survive in the female reproductive tract for up to 5 days, while the egg is viable for only 12–24 hours after release. Pregnancy from intercourse outside this window is biologically unlikely.
Published research (Wilcox 1995, NEJM) shows the highest conception probability on the day before ovulation (~32%), followed by ovulation day itself (~33%) and 2 days before (~30%). The 4-day window of 2 days before through 1 day after ovulation captures most of the total cycle conception probability. Aim for intercourse every 1–2 days throughout this window.
For regular cycles (varying by ±2 days), calendar prediction is typically within 1–2 days of true ovulation. For irregular cycles, accuracy drops significantly. Adding biological signs — ovulation predictor kits, basal body temperature tracking, or cervical mucus changes — gives much better real-time accuracy than the calendar alone.
Most fertility specialists recommend every 1–2 days throughout the fertile window. Daily intercourse is fine — it does not reduce sperm quality in healthy men. Less frequent (every 3–4 days) misses ovulation if timing is slightly off. The most common evidence-based advice is "intercourse every other day starting 5 days before expected ovulation."
The cycle has two phases. The follicular phase (period → ovulation) varies between women and from cycle to cycle. The luteal phase (ovulation → next period) is biologically fixed at ~14 days. So a 28-day cycle puts ovulation at day 14, but a 32-day cycle puts it at day 18 and a 24-day cycle puts it at day 10 — always 14 days before the next bleed.
Yes, but use the prediction as a starting point only. Track your actual cycle lengths for at least 6 months and enter the average. Crucially, supplement the calendar with biological markers — ovulation predictor kits (urine LH surge) typically detect ovulation 24–36 hours in advance and work for most women regardless of cycle regularity.
Conception from intercourse outside the 6-day window is very unlikely but not zero. Late ovulation in an unusually long cycle, or early ovulation with exceptional sperm survival, can occasionally produce surprise pregnancies. This is why "rhythm" methods of contraception have a relatively high failure rate (13–24% typical use).
For both partners: maintain a healthy BMI (20–25), stop smoking, limit alcohol to ≤7 drinks/week (women) or ≤14 (men), get 7–9 hours sleep, manage stress, and exercise moderately. For her: take a daily prenatal with folic acid (400 mcg). For him: keep testicles cool (avoid hot tubs, tight underwear), eat zinc-rich foods. These changes take ~3 months to influence egg or sperm quality.
The standard guidelines: see a fertility specialist after 12 months of well-timed intercourse without pregnancy if you're under 35, or 6 months if you're 35 or older. Earlier evaluation is warranted if you have very irregular cycles, history of pelvic surgery or infection, suspected endometriosis, prior chemotherapy, or known male-factor concerns.
This calendar method predicts ovulation in advance based on cycle math. An OPK detects the LH (luteinising hormone) surge in your urine that happens 24–36 hours before ovulation — it tells you "it's happening soon." Both tools complement each other: the calendar tells you when to start testing with OPKs, OPKs tell you exactly when to time intercourse.
The length of the fertile window stays the same, but conception probability per cycle drops with age. At 25, a healthy couple has roughly a 25% chance of conception per cycle. By 35 it's 15%, by 40 around 5%, and by 43 under 2%. The calendar window remains useful — it just takes more attempts on average for older couples.
No. This is an educational tool for healthy adults with regular cycles. For diagnosed infertility, recurrent pregnancy loss, PCOS, endometriosis, or any concerning gynaecological history, work with a reproductive endocrinologist who can order proper hormone testing, imaging, and tailored treatment (Clomid, IUI, IVF) where appropriate.