How to use it
- Enter the First day of your last period: the day the bleeding started.
- Set the Cycle length in days, or keep 28 if you do not know it.
- Set the Period length, then read the Next period starts, the Next ovulation and the Fertile window.
How the dates are estimated
A typical cycle is 21 to 35 days long and most often about 28; a period lasts between 2 and 7 days, usually about 5, according to the NHS. The next period starts one cycle length after the last one, and the table counts six cycles from the one that holds today.
Ovulation is counted back from the next period. In a study of normal cycles by Lenton and colleagues (1984), the luteal phase, from ovulation to the next period, averaged 14.13 days with a standard deviation of 1.41 days, so the calculator takes 14 days.
These dates are estimates and not medical advice. Cycles vary, and illness, stress, travel and medicines can shift them. If your periods are very irregular or you have missed periods, see a doctor.
Next period = first day of the period + cycle length
Ovulation = next period - 14 days
Fertile window = the 5 days before ovulation and the day of ovulation
Period ends = period starts + period length - 1 dayThe fertile window
In a study of couples by Wilcox and colleagues (1995), conception happened only after intercourse in a six-day period ending on the day of ovulation. The chance of conception was 0.10 when intercourse was five days before ovulation and 0.33 on the day of ovulation.
The ovulation day is itself an estimate, so the window is an estimate too. For a calendar with more detail, see the ovulation and fertile days calendar.
What this calculator does not do
- It is not contraception and does not protect against pregnancy or infection.
- It assumes every cycle has the length you entered. The rows further down the table are projections, not promises.
- It does not diagnose anything. Ask a doctor about irregular, very heavy or missed periods.
Worked example: last period on 1 November 2026, 28-day cycle
The next period starts 28 days after 1 November, on November 29, 2026. Ovulation is 14 days before that, on November 15, 2026.
The fertile window runs from five days before ovulation to the day of it, so from November 10 to November 15, 2026. The period ends on November 5 after five days.
| Date | Result |
|---|---|
| Period, 1 to 5 November 2026 | Current cycle |
| Fertile window | November 10 to November 15, 2026 |
| Ovulation | November 15, 2026 |
| Next period | November 29, 2026 |
Frequently asked questions
How do I find my cycle length?
Count the days from the first day of one period to the first day of the next, and average several months. A cycle of 28 days is common, but 21 to 35 days is typical.
Why is ovulation 14 days before the next period?
The luteal phase, from ovulation to the next period, averaged 14.13 days in the Lenton study, so the calculator counts back 14 days from the next period.
Can I use this as contraception?
No. Estimated fertile days are not reliable enough to prevent pregnancy. Talk to a pharmacist or doctor about contraception that suits you.
What if my periods are irregular?
The dates will be less accurate. Irregular periods, very heavy bleeding or missed periods are worth discussing with a doctor.
Is my date stored or sent to a server?
No. The dates are calculated in your browser and nothing you enter is sent to us.
Sources
- NHS: Periods, cycle length and period length
- Wilcox AJ, Weinberg CR, Baird DD 1995, N Engl J Med: timing of intercourse and ovulation (PubMed)
- Lenton EA et al. 1984, Br J Obstet Gynaecol: luteal phase length (PubMed)
Figures and text last checked on Tuesday, November 10, 2026.