Ovulation Calculator
Last period + cycle length — ovulation day and fertile window, estimated.
How this is calculated
ovulation ≈ last period + cycle length − 14 · fertile window = ovulation − 5 … + 1
Worked example: last period starting 1 July with a 28-day cycle puts estimated ovulation on 15 July, the fertile window 10–16 July, and the next period around 29 July. The "−14" is the luteal phase — the half of the cycle that stays near-constant across people — which is why the estimate counts back from the NEXT period, not forward from the last.
What this page is and isn't
It's honest calendar arithmetic — the right starting point for conception timing and cycle awareness, and the same math midwives use for quick estimates. It is not a diagnosis, not contraception, and no match for your body's actual signals: LH test strips and temperature tracking confirm what arithmetic can only predict. Irregular cycles (varying more than a few days) make calendar estimates genuinely unreliable — that's a conversation with a clinician, not a calculator. Informational only — not medical advice.
Frequently asked questions
How is the ovulation date estimated?
Ovulation typically happens about 14 days BEFORE the next period (the luteal phase is the stable half of the cycle). So: last period start + cycle length − 14. For a 28-day cycle that's day 14; for a 33-day cycle, day 19 — which is why "day 14" advice fails anyone with longer or shorter cycles.
What is the fertile window?
Sperm survive up to about 5 days; the egg lives roughly a day. The window therefore spans the 5 days before ovulation through the day after — the range this page highlights. The two days right before ovulation carry the highest probability.
How accurate is this?
It's a calendar estimate. Real cycles vary month to month, and ovulation timing shifts with stress, illness, and travel. Ovulation predictor kits (LH strips), basal body temperature, and cervical-mucus tracking read your actual body; this page reads arithmetic. Use it for planning, confirm with signals.
Can I use this as contraception?
No. Calendar math has a substantial real-world failure rate — cycles are simply not regular enough. Fertility-awareness methods that work require training and multiple daily signals; this calculator is for conception planning and cycle awareness only. Not medical advice.