How to use
- Pick the first day of your last period.
- Enter your cycle length and how many days your period usually lasts.
- Read your next period, ovulation day and fertile window, and check the calendar and upcoming cycles.
Options explained
- Cycle length
- Days from the first day of one period to the first day of the next — usually 21 to 35. If it varies, use the average of the last few months.
- Ovulation day
- Usually about 14 days before the next period, so around day 14 of a 28-day cycle.
- Fertile window
- The five days before ovulation and the day after. Sperm can survive about five days; the egg lives about a day.
- Safe period
- The days between the end of your period and the start of the fertile window, and between the end of the fertile window and your next period. Short cycles may have no pre-ovulation safe days. The chance of pregnancy is lower, not zero.
Example: a 28-day cycle
Last period started 1 September and lasts 5 days:
| Event | Date |
|---|---|
| This period | 1–5 September |
| Fertile window | 10–16 September |
| Ovulation | 15 September |
| Next period | 29 September |
Frequently asked questions
Can I use the safe days for contraception?
No. Ovulation can shift with stress, illness or travel, and pregnancy is possible outside the predicted window. Use a reliable method of contraception.
What if my cycle is irregular?
Use your average cycle, but expect the dates to be less accurate. Ovulation tests or basal body temperature can pin down ovulation. Cycles regularly shorter than 21 or longer than 35 days are worth discussing with a doctor.
When is the best time to try to conceive?
The two days before ovulation and the day of ovulation have the highest chance. Having sex every other day through the fertile window works well.
Is my data saved?
Nothing is uploaded. Everything is calculated in your browser.
Privacy
This tool runs entirely in your browser. Your files and text are processed on your device and are never uploaded to our servers.