Ovulation Calculator
Estimate your ovulation day and fertile window for the next 3 cycles. Everything runs in your browser — private and free.
| Cycle | Ovulation day | Fertile window | Period due |
|---|
Enter the first day of your last period to see your estimate.
Educational estimate only — not medical advice. Consult your doctor for family planning.
How it works
- Enter 2 details — the first day of your last period and your average cycle length.
- See your fertile window instantly — ovulation day plus the 5 days before it.
- Plan across 3 cycles — a table of upcoming ovulation days, fertile windows and period due dates.
How the fertile window is calculated
We use the standard clinical convention: ovulation happens about 14 days before your next period starts. So for a 28-day cycle starting 1 October, the next period is due around 29 October and ovulation lands around 15 October. The fertile window runs from 5 days before ovulation through ovulation day itself — because sperm can survive up to 5 days in the reproductive tract while the egg survives roughly 12–24 hours. Every date in the table is computed transparently from your two inputs; nothing is a black box.
Frequently asked questions
How accurate is this?
It’s an estimate based on averages, not a medical prediction. Ovulation usually happens about 14 days before your next period, but the exact day varies between cycles and between people. Ovulation predictor kits or basal body temperature tracking give more precise results.
What if my cycles are irregular?
The calculator is less reliable with irregular cycles. Track your last few cycles and use the average length, or talk to your doctor if your cycles are consistently irregular — it can be worth investigating.
Can I get pregnant outside the fertile window?
It’s unlikely but possible. Sperm can survive up to 5 days in the reproductive tract, and ovulation timing can shift from cycle to cycle, so the window is an estimate, not a guarantee.
Why 14 days before the next period?
This is the standard clinical convention. The luteal phase (ovulation to next period) is fairly consistent at around 14 days, while the follicular phase (period to ovulation) varies more — so counting back from the next period is more reliable than counting forward.
Can I use this to avoid pregnancy?
No. The calendar method alone has a high failure rate and this tool is an educational estimate, not contraception. Talk to your doctor about reliable birth control options.
Is my data private?
Completely. All calculations happen in your browser; nothing is sent to any server.
Sources and methodology
This calculator is an educational illustration, not a medical tool. Dates are estimated using the standard clinical convention that ovulation occurs approximately 14 days before the onset of the next menses, with the fertile window defined by sperm survival of up to 5 days and ovum viability of roughly 12–24 hours — consistent with patient education materials from bodies such as the American College of Obstetricians and Gynecologists (ACOG). Individual cycles vary, and no calendar estimate replaces clinical guidance. For family planning decisions, talk to your doctor.