Ovulation Calculator
Ovulation date ยท Fertile window ยท Peak days ยท Test timing
๐ธ Enter the first day of your last period and your cycle length to see your ovulation date, fertile window, and peak days.
What This Ovulation Calculator Shows
An ovulation calculator estimates the day an ovary is most likely to release an egg, based on the first day of your last period and your usual cycle length. From that date it marks the fertile window, the days when sex can lead to pregnancy, and the peak days when the chance is highest.
This tool also accounts for cycles that vary from month to month, colors each day of the cycle on a calendar, and tells you when to start ovulation (LH) tests, when a pregnancy test is worth taking, and the due date if you conceive this cycle. A schedule projects the next 3, 6, or 12 cycles.
It is built for people trying to conceive. It does not confirm ovulation, and it is not a method of birth control.
Ovulation Formula
Ovulation date
Fertile window and peak days
Irregular cycles
Due date if you conceive
Ovulation Day by Cycle Length
Luteal phase of 14 days. Day 1 is the first day of your period.
| Cycle | Ovulation | Fertile days | Next period |
|---|---|---|---|
| 21 days | Day 8 | Days 3 to 9 | Day 22 |
| 24 days | Day 11 | Days 6 to 12 | Day 25 |
| 26 days | Day 13 | Days 8 to 14 | Day 27 |
| 28 days | Day 15 | Days 10 to 16 | Day 29 |
| 30 days | Day 17 | Days 12 to 18 | Day 31 |
| 32 days | Day 19 | Days 14 to 20 | Day 33 |
| 35 days | Day 22 | Days 17 to 23 | Day 36 |
| 40 days | Day 27 | Days 22 to 28 | Day 41 |
People often say ovulation happens on "day 14" of a 28 day cycle. Counting the first period day as day 1, 14 days later is cycle day 15, which is the convention used here and in the standard due date rule.
How Ovulation and the Fertile Window Work
The menstrual cycle runs from the first day of one period to the first day of the next. ACOG explains that the time from ovulation to the next period is the most consistent part of the cycle, at about 14 days. That is why a longer cycle moves ovulation later: the first half of the cycle stretches while the second half stays about the same.
An egg survives about 24 hours after release, while sperm can live about 3 and sometimes up to 5 days. ACOG therefore places the fertile time from 5 days before ovulation to 1 day after. In the study by Wilcox and colleagues, which followed 625 cycles in 221 women, conception occurred only when sex took place during the 6 days ending on ovulation day.
| Day of intercourse | Chance of conception |
|---|---|
| 5 days before ovulation | About 10% |
| 4 days before | About 16% |
| 3 days before | About 14% |
| 2 days before | About 27% |
| 1 day before | About 31% |
| Ovulation day | About 33% |
How to Use the Calculator: Worked Example
- Enter the first day of your last period. Use the first day of full flow, not spotting.
- Choose your average cycle length. Average your last 3 to 6 cycles if you track them.
- Set regularity. If your cycle moves by several days, pick the matching range.
- Read the results. The calendar colors period, fertile, peak, and ovulation days.
Example: last period March 1, 28 day cycle, 14 day luteal phase. Ovulation is estimated for March 15, the fertile window runs March 10 to 16, and the peak days are March 13 to 15. LH tests start March 12. The next period is due March 29, the first day a home pregnancy test is worth taking. The due date if you conceive is December 6.
With a 35 day cycle, ovulation moves to March 22 and the next period to April 5. If the 30 day cycle varies by 4 days, ovulation could fall anywhere from March 13 to March 21.
Confirming Ovulation: Signs and Tests
LH tests. Urine ovulation tests detect the rise in luteinizing hormone that triggers egg release. ACOG notes that a positive result suggests ovulation within the next 24 to 48 hours. The calculator suggests starting tests 3 days before the earliest likely ovulation date.
Cervical mucus. In the days before ovulation, mucus increases and becomes thin and slippery. After ovulation it becomes thicker and less noticeable.
Basal body temperature. Resting temperature rises by about 0.5 to 1ยฐF around ovulation and stays higher until the next period. It confirms ovulation after the fact, so it is most useful for learning your pattern over several cycles.
Accuracy and Common Mistakes
- Assuming every cycle is 28 days. Enter your own average; a 24 or 35 day cycle moves ovulation by a week.
- Counting from the end of your period. Day 1 is the first day of bleeding.
- Waiting for ovulation day. The days before ovulation carry most of the chance, so start trying early in the window.
- Testing too early. A negative test before a missed period is not reliable.
- Using the calendar as contraception. ACOG reports 12 to 24 pregnancies per 100 women in the first year of typical use of fertility awareness methods.
Stress, illness, travel, breastfeeding, recently stopping hormonal birth control, PCOS, thyroid conditions, and approaching menopause can shift or stop ovulation. In these cases rely on LH tests and clinical advice rather than dates.
Trying to Conceive: When to Seek Help
ACOG states that for healthy couples in their 20s or early 30s, the chance of pregnancy is about 25 to 30 percent in any single cycle, falling below 10 percent per cycle by age 40.
Experts recommend an infertility evaluation after 1 year of regular sex without birth control, after 6 months if you are older than 35, and right away if you are older than 40. Problems with ovulation are the most common cause of female infertility.
After a positive test, the due date calculator and pregnancy calculator track weeks and milestones. The period calculator projects period dates.
Frequently Asked Questions
Sources & References
- American College of Obstetricians and Gynecologists, Fertility Awareness Based Methods of Family Planning (FAQ024, reviewed February 2025). acog.org
- American College of Obstetricians and Gynecologists, Evaluating Infertility (FAQ136, reviewed May 2026). acog.org
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517 to 1521. doi.org/10.1056/NEJM199512073332301
- Office on Women's Health, U.S. HHS, Knowing if you are pregnant (updated September 2025). womenshealth.gov
- ACOG Committee Opinion 700, Methods for Estimating the Due Date. acog.org
All dates and values are estimates. This calculator is not birth control and does not replace advice from a qualified healthcare professional. Reference data reviewed: 2026.