How to use
- Pick the first day of your last period. A date seven days ago is filled in to start.
- Enter your average cycle length (from the first day of one period to the day before the next) and how many days your period usually lasts.
- You get your estimated ovulation day, fertile window, next period and a schedule for the next six cycles, plus which cycle day today is.
This calculator provides general information only and is not medical advice. Do not use it as birth control.
How it is calculated
| Item | Calculation |
|---|---|
| Next period | First day of last period + cycle length |
| Ovulation day | Next period − 14 days |
| Fertile window | 5 days before ovulation through ovulation day (6 days) |
The luteal phase — from ovulation to the next period — is fairly consistent and averages about 14 days. Differences in cycle length mostly come from the phase before ovulation, so people with longer cycles ovulate later.
The six-day fertile window comes from a study by Wilcox and colleagues at the US National Institute of Environmental Health Sciences, published in the New England Journal of Medicine in 1995. Conception occurred only from intercourse within the six days ending on ovulation day, and the chance rose the closer intercourse was to ovulation — sperm can survive in the body for several days, while the egg lives for about a day.
Examples
| Last period began | Cycle | Fertile window | Ovulation | Next period |
|---|---|---|---|---|
| Oct 1, 2026 | 28 days | Oct 10 – 15 | Oct 15 (cycle day 15) | Oct 29 |
| Oct 1, 2026 | 32 days | Oct 14 – 19 | Oct 19 (cycle day 19) | Nov 2 |
Things to keep in mind
- Ovulation can move by several days from one cycle to the next because of stress, illness, weight change or breastfeeding. Calendar predictions are not a reliable form of contraception.
- If your cycles are often shorter than 21 or longer than 35 days, or irregular, predictions can be well off — that is worth discussing with a doctor.
- To pinpoint ovulation, combine this with ovulation predictor kits (which detect the LH surge), basal body temperature charting or ultrasound monitoring.
- For help trying to conceive or with fertility concerns, talk to a health professional.
FAQ
How do I work out my cycle length?
Count from the first day of one period (day 1) up to the day before the next period starts. Averaging your last few cycles gives a better estimate. The date difference calculator can count the days for you.
Can I get pregnant outside the fertile window?
Yes. Ovulation may come earlier or later than predicted, so pregnancy is possible on days the calendar marks as outside the window.
If I’m pregnant, when is my due date?
Enter the first day of your last period in the due date calculator to see your due date and how many weeks along you are.