Use this free online ovulation calculator to estimate your most fertile days based on the first day of your last menstrual period and your average cycle length. It calculates your likely ovulation date, fertile window, and next expected period using standard luteal-phase timing. This tool is provided for general, educational purposes only and is not a substitute for professional medical advice.
How to Use the Ovulation Calculator
Enter the first day of your last menstrual period in the "First Day of Last Menstrual Period" field.
Enter your average cycle length in days (between 15 and 45) in the "Average Cycle Length (days)" field.
Click "Calculate Ovulation" to generate your results.
Review your estimated ovulation date, fertile window, and next period estimate in the result box.
How the Dates Are Worked Out
Ovulation date: first day of last period + (cycle length − 14) days.
Fertile window: from 5 days before the estimated ovulation date to 1 day after it, a span of 7 days.
Next period: first day of last period + cycle length.
The method counts backwards from the next period rather than forwards from the last one. The phase after ovulation, called the luteal phase, is fairly steady at around 14 days for most people (usually somewhere between 12 and 16). The phase before ovulation is the part that stretches or shrinks. So in a 35-day cycle ovulation tends to happen around three weeks in, not in the middle.
Worked Example
Say your last period started on 1 September 2026 and your cycles are usually 28 days. The calculator adds 28 − 14 = 14 days, giving an estimated ovulation date of 15 September 2026. The fertile window runs from 10 September to 16 September, and the next period is expected on 29 September.
With a 32-day cycle and the same start date, everything moves four days later: ovulation around 19 September, a fertile window of 14 to 20 September, and the next period around 3 October.
Estimated Ovulation Day by Cycle Length
Counting the first day of your period as day 1, this is where the calculator places ovulation and the fertile window for common cycle lengths:
Cycle length
Estimated ovulation
Fertile window
21 days
Day 8
Days 3–9
24 days
Day 11
Days 6–12
26 days
Day 13
Days 8–14
28 days
Day 15
Days 10–16
30 days
Day 17
Days 12–18
32 days
Day 19
Days 14–20
35 days
Day 22
Days 17–23
Why the Fertile Window Lasts About a Week
An egg survives for only about 12 to 24 hours after it is released, but sperm can survive in the reproductive tract for up to about 5 days. Intercourse in the days before ovulation can therefore lead to pregnancy, while the chance drops quickly once the egg is gone. The two or three days leading up to and including ovulation are generally the most fertile.
Making the Estimate More Accurate
Use a real average. Note the start dates of your last three to six periods and average the gaps instead of assuming 28 days.
If your cycles vary, run the calculator twice, once with your shortest cycle and once with your longest, and treat everything between the two windows as potentially fertile.
Watch for body signs. Around ovulation, cervical mucus often becomes clear, slippery and stretchy, similar to raw egg white.
Ovulation test kits, sold at most chemists, detect the surge in luteinising hormone (LH) that usually comes a day or so before ovulation.
Basal body temperature rises slightly, typically by about 0.2 to 0.5 °C, after ovulation. It confirms ovulation after the fact, which helps you learn your pattern over several cycles.
When to Talk to a Doctor
Doctors usually suggest a fertility check-up if you have been trying to conceive for 12 months without success, or after 6 months if you are 35 or older. See a gynaecologist sooner if your periods are very irregular, absent for months, or very painful, since conditions such as PCOS and thyroid disorders can affect ovulation and are treatable. This calculator gives general estimates only and is not medical advice.
Frequently Asked Questions
It estimates your ovulation date by adding your cycle length minus 14 days to the first day of your last menstrual period, based on the typical 14-day luteal phase. Your fertile window is then calculated as the 5 days before ovulation through 1 day after it.
Day 14 only fits a 28-day cycle, and even then, counting the first day of your period as day 1, the calculator shows day 15 because it adds 14 days to your start date. Ovulation happens about 14 days before the next period, so shorter cycles ovulate earlier and longer cycles later.
You can, but the estimate is less reliable. Enter your shortest and longest recent cycles separately to see the full range of possible fertile days, and consider ovulation test kits or tracking cervical mucus for a more direct signal. Very irregular cycles are worth discussing with a doctor.
This is not recommended. Calendar-based estimates cannot account for a cycle that runs early or late, and ovulation can shift with stress, illness or travel. If you want to avoid pregnancy, speak to a doctor about a reliable method of contraception.
Home urine tests are most reliable from the day your period is due, which is roughly 14 days after ovulation. Testing earlier can give a negative result even if you are pregnant, so repeat the test a few days later if your period still has not started.