How to use
- Enter the first day of your last period.
- Enter your average cycle length: the days from the first day of one period to the first day of the next. Leave it at 28 if you’re not sure.
- See your fertile window, estimated ovulation day, next period and your next 3 cycles.
How the fertile window is calculated
- Next period = first day of your last period + your cycle length.
- Ovulation ≈ next period − 14 days. NHS guidance says ovulation is usually 12 to 16 days before a period starts.
- Fertile window = the 5 days before ovulation plus ovulation day. In a study of 221 women, conception happened only after sex in this 6-day stretch (Wilcox, NEJM 1995).
In that study, the chance of conceiving rose from about 10% five days before ovulation to about 33% on ovulation day.
Examples
Last period on 21 October 2026:
- 28-day cycle: fertile 30 October – 4 November, ovulation Wednesday, 4 November, next period 18 November.
- 35-day cycle: fertile 6–11 November, ovulation on cycle day 22, next period 25 November.
Fertile days by cycle length
| Cycle length | Fertile days | Ovulation |
|---|---|---|
| 21 days | Days 3 to 8 | Day 8 |
| 24 days | Days 6 to 11 | Day 11 |
| 26 days | Days 8 to 13 | Day 13 |
| 28 days | Days 10 to 15 | Day 15 |
| 30 days | Days 12 to 17 | Day 17 |
| 32 days | Days 14 to 19 | Day 19 |
| 35 days | Days 17 to 22 | Day 22 |
Day 1 is the first day of your period. The table assumes a 14-day luteal phase.
How accurate is it?
It’s a calendar estimate, and real cycles vary:
- In over 600,000 tracked cycles, only 13% lasted exactly 28 days, and the average luteal phase was 12.4 days, not 14 (Bull, 2019).
- Only about 30% of women had their fertile window entirely within days 10 to 17 (Wilcox, BMJ 2000).
Ovulation tests can help pinpoint your own fertile days.
Not a method of contraception
There is no reliably “safe” time: ovulation can shift even in regular cycles. Fertility-awareness methods have typical-use failure rates of 2% to 23% (CDC), so use a reliable method if you don’t want to get pregnant.
When to see a doctor
- Your periods are irregular, missing, or usually come less than 21 or more than 35 days apart.
- You’re under 35 and haven’t conceived after a year of trying, or 35 or older after 6 months (CDC).
Day 14 or day 15, and why calculators differ
Counting 14 days back from the next period (day 29 of a 28-day cycle) lands on day 15. Many sources round this to “around day 14”.
Some calculators add 1 or 2 days after ovulation, or start the window 7 days before it. We follow the study data: 6 days ending on ovulation day.
A shorter luteal phase moves ovulation later. With a 12-day luteal phase, a 28-day cycle ovulates on day 17.
Already pregnant? The due date calculator works out your due date and how far along you are.