Ovulation and Period Calculator
Enter the first day of your last period and the usual length of your cycle. You will see your likely ovulation day, fertile window and the dates of your next three periods.
The next three cycles
These are estimates based on an average cycle. Real cycles vary, so do not rely on them to avoid pregnancy.
How ovulation is calculated
Your menstrual cycle runs from the first day of one period to the first day of the next. The second half of the cycle, from ovulation to the next period, is fairly steady at about 14 days for most women. That is why ovulation is estimated as 14 days before the next period, not 14 days after the last one.
- Next period = first day of last period + cycle length
- Ovulation day ≈ next period − 14 days
- Fertile window = the 5 days before ovulation and ovulation day
Worked example
Your last period started on 1 June and your cycle is 30 days long. Your next period is due on 1 June + 30 days = 1 July. Ovulation is about 14 days earlier, on 17 June, and your fertile window runs from 12 June to 17 June.
How to work out your cycle length
Mark the first day of bleeding on a calendar for three or more months and count the days between each start date. If the numbers differ, use the average. A cycle between 21 and 35 days is common. If your cycles are very irregular, the dates above will be less accurate.
Signs that ovulation is near
- Clear, stretchy vaginal discharge, similar to raw egg white
- A small rise in body temperature after ovulation
- Mild pain on one side of the lower tummy for some women
- A positive result on an ovulation test kit from a pharmacy
When to see a health worker
See a doctor or nurse if your periods stop for more than three months without pregnancy, if cycles are regularly shorter than 21 days or longer than 35, if bleeding is very heavy, or if you have been trying to get pregnant for a year (six months if you are over 35).
Frequently Asked Questions
Already pregnant? Use the Pregnancy Due Date Calculator to see how many weeks along you are.