Ovulation calculator - fertile days
Calculate the estimated ovulation date, fertile window and estimated due date from your last period.
Estimate only. Actual ovulation may differ.
Approximate calculation. The exact amount is confirmed by the Social Insurance Board or the Tax and Customs Board - check the official source.
Last updated: 2026-06-03
How to calculate ovulation?
Ovulation happens about 14 days before the next period starts. The fertile window spans roughly 5 days before to 1 day after ovulation: sperm survive up to 5 days, while the egg can be fertilised for 12-24 hours. The calendar method assumes a regular cycle; if your cycle length varies, ovulation tests or basal temperature tracking are more accurate. This is an estimate - it does not replace medical advice and is not a method of contraception.
How it is calculated
Formula
ovulation ≈ LMP + (cycle length − 14). The fertile window is −5 to +1 days around ovulation. Estimated due date = LMP + 280 days.
Example
LMP 1 January, cycle 28 days → ovulation ≈ 15 January.
Frequently asked questions
When does ovulation occur?+
Ovulation occurs about 14 days before the next period starts, depending on the cycle length.
How accurate is the ovulation estimate?+
It is an estimate based on an average cycle. Actual ovulation can vary, so the result is indicative.
How are fertile days calculated?+
Fertile days start about 5 days before ovulation and end 1 day after it. Ovulation occurs roughly 14 days before the next period, so around day 14 of a 28-day cycle. The calculator shows the full fertile window.
Does the calculator work with irregular cycles?+
If your cycle length varies by more than a couple of days, the calendar method is imprecise. Enter the average length of your recent cycles and back it up with ovulation tests or basal temperature tracking.
When should I take a pregnancy test?+
A test is reliable from the first day of a missed period, about 14 days after ovulation. Testing earlier can give a false negative.
When will my next period start?+
The next period starts roughly one cycle length after the previous one began: with a 28-day cycle, 28 days later. The calculator computes the date from the cycle length you enter.
What is a normal cycle length?+
A cycle of 21-35 days is considered normal; the average is 28 days. If your cycle is consistently shorter, longer or very irregular, consult a gynaecologist.