Ovulation Calculator
Enter the first day of your last period and your usual cycle length to estimate when you ovulate, your six-day fertile window, and the same dates projected across your next six cycles — with the due date each cycle would produce. Not a method of birth control.
How your cycle compares
How ovulation timing works
A menstrual cycle has two halves. The first (follicular) phase varies from person to person and cycle to cycle; the second (luteal) phase — from ovulation to the next period — is comparatively fixed at about 14 days. That asymmetry is the whole trick: instead of counting 14 days forward from your period, you count 14 days back from the next expected one. Conception odds then hinge on overlap: sperm can wait up to five days for the egg, but the egg waits less than a day for sperm. Intercourse every day or two through the fertile window is what maximizes the overlap.
How it’s calculated
Ovulation ≈ last period date + (cycle length − 14 days), reflecting a ~14-day luteal phase. Fertile window = the 5 days before ovulation plus ovulation day (sperm survive up to ~5 days; the egg 12–24 hours); the last 3 of those days are the most fertile. Next period = last period + cycle length; each later cycle repeats the pattern. Due date if conceived = ovulation + 266 days (Naegele-consistent).
Informational only, not medical advice — calendar estimates cannot confirm ovulation, so verify with LH tests or your provider, and do not use this page for contraception.
Your next six cycles
Projections assume every cycle matches your average length; real cycles drift a few days.
Worked example
Last period starting June 10, 2026 with a 28-day cycle puts ovulation around June 24, with a fertile window of June 19–24 (most fertile June 22–24). The next period is expected July 8, and conceiving that cycle would give a due date near March 17, 2027. The following cycle’s fertile window runs July 17–22.
Common mistakes
- Assuming everyone ovulates on day 14 — with a 33-day cycle, ovulation is closer to day 19.
- Counting the cycle from the end of a period instead of the first day of bleeding.
- Missing that the window effectively ends on ovulation day — the egg survives less than 24 hours.
- Leaning on calendar math with irregular cycles instead of LH strips, temperature, or mucus signs.
Where it is used
- Timing intercourse or insemination when trying to conceive.
- Deciding which day to start LH (ovulation) test strips each cycle.
- Planning several months ahead with the six-cycle projection.
- Preparing for a fertility consult — typically after 12 months of trying (6 if 35 or older).
Frequently asked questions
How does the calculator estimate ovulation?
The luteal phase — ovulation to the next period — is fairly stable at about 14 days, while the first half of the cycle varies. So the estimate is: last period date + (cycle length − 14). For a 28-day cycle that is day 14; for a 35-day cycle it is day 21. It is a calendar estimate, and real ovulation commonly lands within a few days either side.
Why is the fertile window six days?
Sperm can survive in the reproductive tract for up to about five days, while the egg is fertilizable for only 12–24 hours after release. Intercourse on the five days before ovulation or on ovulation day itself can therefore lead to conception — the classic six-day window, with the last three days the most fertile.
Can I use this as birth control?
No. Calendar estimates miss early or late ovulation, which is common, and sperm survival stretches the risky range. Fertility-awareness methods that can be used for contraception require daily observations (temperature, cervical mucus, LH tests) and proper training — a calculator alone is not a contraceptive method.
What if my cycles are irregular?
The wider your cycles swing, the less a calendar can tell you. Track a few months to find your shortest and longest cycles, then confirm ovulation in real time with LH test strips (which surge 24–36 hours before ovulation), basal body temperature, or cervical-mucus changes. If cycles are consistently shorter than 21 days or longer than 35, mention it to your OB-GYN.
When should I take a pregnancy test?
Home urine tests are most reliable from the first day of your missed period — about 14 days after ovulation. Testing earlier often produces false negatives because hCG hasn’t accumulated yet; if the result is negative and your period still doesn’t come, retest after 2–3 days.