Ovulation and Fertile Window Calculator
Estimate your likely ovulation date and fertile window using your cycle information. This calendar estimate cannot confirm ovulation and should not be used as contraception.
Enter your cycle details to estimate a likely fertile window and ovulation date.
How the fertile-window estimate is calculated
The next period is estimated from the first day of the last period plus the average cycle length. The likely ovulation date is the estimated next period minus the selected luteal-phase length.
The displayed fertile window runs from five days before through one day after estimated ovulation. ACOG explains that sperm can sometimes survive up to five days and an egg about 24 hours after ovulation.
Estimated ovulation day by cycle length
Calendar methods count backwards from the next expected period, not forwards from the last one. With a 14-day luteal phase, ovulation is estimated at cycle length minus 14 - which is why a longer cycle shifts ovulation later rather than stretching the whole cycle evenly.
Day 1 is the first day of bleeding. These are estimates for a regular cycle; if yours varies from month to month, the actual day can fall well outside the window shown.
| Cycle length | Estimated ovulation | Fertile window |
|---|---|---|
| 21 days | Day 7 | Days 3-7 |
| 24 days | Day 10 | Days 6-10 |
| 26 days | Day 12 | Days 8-12 |
| 28 days | Day 14 | Days 10-14 |
| 30 days | Day 16 | Days 12-16 |
| 32 days | Day 18 | Days 14-18 |
| 35 days | Day 21 | Days 17-21 |
When a calendar estimate is less reliable
There is no foolproof calendar date for ovulation. Clinical advice, ovulation testing, cervical-mucus observations, or other validated methods may provide more useful information for an individual situation.
- Cycles are irregular or outside the range entered
- Periods have recently started or are approaching menopause
- You recently gave birth, are breastfeeding, or stopped hormonal contraception
- Illness, stress, medication, or a health condition changes cycle timing
Contraception and medical safety
Do not treat dates outside the estimated window as safe days. If avoiding pregnancy, use a reliable contraceptive method. Seek advice from a qualified health professional if cycles change unexpectedly, fertility is a concern, or pregnancy would pose a medical risk.
When to seek advice rather than track
Calendar tracking assumes cycles are regular enough to predict from. Where they are not, the estimate carries little weight, and persistent irregularity is itself worth discussing with a doctor rather than working around with a calculator.
It is generally advised to seek medical advice if you have been trying to conceive for a year without success, or after six months if you are over 35, and sooner where there is a known condition affecting fertility. Cycles that are consistently very short or very long, that stop, or that change markedly are also reasons to speak to a clinician.
Ovulation predictor kits, basal body temperature charting, and clinical monitoring all give a more reliable picture than a calendar estimate, and a doctor can advise which is appropriate. This calculator is a rough guide for a regular cycle, and it is not a diagnostic tool.