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.
Examples: same period, different cycle length
Suppose your last period started on 1 September 2026. For three common cycle lengths the calculator gives:
- 24-day cycle: ovulation around 11 September, fertile window 6-12 September, next period 25 September.
- 28-day cycle: ovulation around 15 September, fertile window 10-16 September, next period 29 September.
- 32-day cycle: ovulation around 19 September, fertile window 14-20 September, next period 3 October.
Why ovulation is not always mid-cycle
The examples show that the time from period to ovulation changes with the cycle, while the time from ovulation to the next period stays around 14 days. For most women the second half of the cycle is fairly stable; it is the first half that tends to vary.
That is why the rule of thumb "ovulation is on day 14" does not fit everyone. A woman with a 32-day cycle who focuses on day 14 will probably be several days too early for her real fertile window. With a short 24-day cycle, the fertile window can start during the last days of the period.
How to measure your cycle length correctly
Cycle length is counted from the first day of one period up to the day before the next period starts. If a period started on 3 August and the next on 31 August, the cycle was 28 days. Light spotting before a period does not count as the start; use the first day of normal bleeding.
A single cycle can differ because of stress, illness, travel or lack of sleep. For a better estimate, note the length of your last 3-6 cycles and enter the average. The calculator accepts cycle lengths of 20-45 days: smaller entries are treated as 20 and larger ones as 45 so the result does not become unrealistic.
Irregular cycles: calculate the whole range
If your cycles vary, run the calculation twice, with your shortest and your longest cycle. For example, if your cycles over the past six months were 26-33 days and your last period started on 1 September 2026:
Taken together, the fertile window may fall anywhere between 8 and 21 September, almost two weeks. This shows honestly why the calendar alone is not enough with an irregular cycle and why additional body signs help.
- Shortest cycle, 26 days: ovulation around 13 September, fertile window 8-14 September.
- Longest cycle, 33 days: ovulation around 20 September, fertile window 15-21 September.
How to refine the calendar estimate with body signs
The calculated date tells you when to start paying attention. These methods help pinpoint actual ovulation in your own body:
- Ovulation (LH) tests: start a couple of days before the calculated fertile window. A positive test means ovulation usually follows within the next 24-36 hours.
- Cervical mucus: before ovulation it becomes clear, slippery and stretchy, like raw egg white. This is one of the signs of the most fertile days.
- Basal body temperature: measure every morning at the same time before getting up. After ovulation the temperature rises by about 0.2-0.5 °C and stays higher. It confirms ovulation after the fact, so it is most useful for spotting your pattern over several cycles.
A cycle diary: adapting the estimate to your own body
For each cycle, note the start date of your period, the cycle length, the day of a positive ovulation test and, if you like, the days of the temperature rise and fertile mucus. After just 3 cycles you will see whether your actual ovulation lines up with the calculated date.
If, for example, with a 28-day cycle your LH test turns positive around day 9-10 after your period starts for three cycles in a row, you probably ovulate a few days earlier than the calculator predicts. It then makes sense to shift your own fertile window a few days earlier too and start testing sooner. A diary like this is also useful at a gynaecologist appointment, since it shows your cycle pattern better than memory does.
Common misconceptions and when to see a doctor
Many people wait for ovulation day or a positive test, but the chance of conception is highest from intercourse in the two days before ovulation and on the day itself. Sperm can wait for the egg, but the egg does not wait long. If you are trying to conceive, having sex every 1-2 days throughout the fertile window is a sensible approach.
The estimated due date shown is always last period + 280 days and does not take cycle length into account. With a long cycle, an ultrasound may later move the date.
See a gynaecologist if you have not conceived after a year of regular unprotected intercourse, or after about six months if you are over 35. Go sooner if periods have stopped for more than 3 months, cycles are very irregular or painful, or you have a known condition that affects fertility.
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.