Your next period usually starts one cycle length after the first day of your last one. Ovulation tends to occur about 14 days before the next period, so in a 28-day cycle on day 14. The fertile window is the five days before ovulation plus the day itself.
How is the next period predicted?
A menstrual cycle is counted from the first day of bleeding to the day before the next period starts. The average is 28 days, but anything from 21 to 35 days is considered normal for adults. If your last period started on 1 March and your cycle is 28 days, your next one is expected on 29 March. The calculator adds your cycle length to the start date and repeats this to give the next four cycles.
When do you ovulate?
Ovulation (the release of an egg) usually happens about 12 to 16 days before the next period begins, because the luteal phase after ovulation is relatively fixed at around 14 days, while the first half of the cycle varies. The calculator subtracts the luteal phase from your cycle length. With a 28-day cycle that gives day 14, with a 32-day cycle day 18. Women with longer or shorter cycles therefore don’t ovulate on day 14, which is a common misunderstanding.
The fertile window
Sperm can survive up to five days, and the egg lives for about 12 to 24 hours. The fertile window is therefore the five days before ovulation plus the day of ovulation. The two days before and the day of ovulation are the most fertile. If you want to get pregnant, intercourse every one to two days in that window is a good approach; ovulation tests detect the luteinising hormone surge that precedes ovulation by one to two days.
This calculator is not contraception
Calendar predictions assume a regular cycle and are only estimates. Stress, illness, travel, breastfeeding and hormonal conditions can shift ovulation. The calendar method alone has a high failure rate as contraception. If you want reliable protection, speak with your huisarts about options.
When to see a doctor
- Cycles shorter than 21 or longer than 35 days, or very irregular.
- Periods that stop for three months or more without pregnancy.
- Very heavy bleeding, severe pain, or bleeding between periods.
- Trying to conceive for 12 months (6 months if over 35) without success.
If you are tracking a pregnancy or use your cycle for planning, tools like the Calorie Calculator can help when your needs change. Parents planning ahead can also look at the Child Height Calculator.
How to use this calculator
- Pick the first day of your most recent period.
- Enter your usual cycle length, counted from day one of one period to day one of the next.
- Optionally adjust period length and luteal phase if you know them.
- Read the predicted dates for the next four cycles.
Key figures at a glance
| Average cycle | 28 days (21 to 35 normal) |
|---|---|
| Luteal phase | About 14 days |
| Fertile window | 5 days before ovulation + ovulation day |
| Egg lifespan | 12 to 24 hours |
Frequently asked questions
Subtract your luteal phase (usually 14 days) from your cycle length and count from the first day of your last period. In a 28-day cycle ovulation is around day 14.
About six days: the five days before ovulation and the day of ovulation.
Yes, in short cycles. Sperm can survive five days, so ovulation soon after the period ends can lead to pregnancy.
It is accurate for regular cycles but less so when cycles vary by more than a few days.
Cycles shift with age, stress, weight changes and illness. Persistent irregularity is worth discussing with a doctor.