Ovulation calculator
The dates stay in this tab. Nothing is uploaded.
Next cycles
Cycle 1
Cycle 2
Cycle 3
Ovulation ≈ next period − 14 days; fertile window = 5 days before through ovulation day.
An estimate, not contraception or medical advice. Cycles vary from month to month.
How to use this tool
See the fertile window, likely ovulation and next period for three cycles. Nothing is uploaded.
Open Ovulation calculator
A sample last period 10 days ago and a 28-day cycle are filled. Three cycles sit on the right.
Enter your last period
Type the first day of your last period. The dates are in this tab only.
Set your cycle length
Type the average length of your cycle, 21 to 45 days. The dates move with it. Copy puts them on the clipboard.
FAQ
How is the ovulation day estimated?
Ovulation usually happens about 14 days before the next period. With a 28-day cycle that is day 14; with a 32-day cycle, day 18.
Is anything uploaded?
No. The dates stay in this tab and are not stored.
What is the fertile window?
The five days before ovulation plus ovulation day. Sperm can survive up to five days and the egg about a day, so these six days carry the highest chance of pregnancy.
Can I use this as contraception?
No. Cycles vary, and calendar estimates miss many ovulations. Use a reliable method of contraception, and ask a doctor if you are planning a pregnancy.
My cycle is irregular. Will it work?
Less well. Use the average length of your last six cycles. Ovulation tests or basal temperature tracking give a better signal for irregular cycles.