BCBetter Calculators

Ovulation Calculator

Estimate your ovulation date, fertile window, and days until next ovulation.

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Enter your values and click Calculate

How It Works

Ovulation timing is estimated using the luteal phase constant: ovulation occurs approximately 14 days before the start of the next period. For any cycle length, ovulation day = cycle length − 14. For a 28-day cycle, ovulation is on day 14. For a 32-day cycle, it is on day 18. The fertile window runs from 5 days before ovulation through 1 day after — this accounts for sperm lifespan (up to 5 days in fertile cervical mucus) and egg viability (12–24 hours post-ovulation). Current cycle day = days since the first day of your last period. Days until ovulation = ovulation day − current cycle day. If that number is negative, you are past ovulation in the current cycle and the calculator shows days until next cycle's ovulation. These are statistical estimates based on average cycle patterns; actual ovulation varies. For reliable detection, use LH ovulation predictor kits or basal body temperature charting.

Examples

Standard 28-Day Cycle, Day 10
LMP started 10 days ago on a 28-day cycle.
Result: Ovulation expected in 4 days (day 14). Entering fertile window in 1 day.
Longer 32-Day Cycle, Day 14
LMP started 14 days ago with a 32-day cycle.
Result: Ovulation expected in 4 days (day 18). Currently 1 day into the fertile window.
Past Ovulation
LMP started 20 days ago on a 28-day cycle — ovulation has passed.
Result: Past ovulation this cycle. Next ovulation in approximately 22 days.

Frequently Asked Questions

How accurate is this ovulation calculator?
This calculator provides an estimate based on the assumption that you have a regular cycle and that ovulation occurs 14 days before your next period. In reality, cycle length can vary month to month, and the timing of ovulation shifts with it. Studies show that even women with regular cycles can ovulate anywhere from day 10 to day 21. For conception planning, confirm ovulation with LH surge test strips (OPKs), which detect the hormone surge that triggers ovulation 24–36 hours in advance.
What is the fertile window?
The fertile window is the period during which unprotected sex can result in pregnancy. It spans the five days before ovulation (because sperm can survive in the reproductive tract for up to five days in fertile cervical mucus) plus ovulation day itself and the day after (because the egg is viable for 12–24 hours after release). This gives a window of approximately seven days, with the highest probability of conception on the two to three days immediately before and on ovulation day.
What signs indicate I am near ovulation?
Physical signs of approaching ovulation include: changes in cervical mucus (becoming clear, slippery, and stretchy — like raw egg white), a slight increase in basal body temperature (0.2–0.5°F above your baseline, detected the day after ovulation), mild one-sided pelvic pain (mittelschmerz), increased libido, and breast tenderness. LH ovulation predictor kits detect the luteinizing hormone surge that triggers ovulation 24–36 hours before the egg is released — the most practical method for timing intercourse.
Can I get pregnant outside the fertile window?
The probability of conception is very low outside the fertile window, since sperm typically survive no more than five days and the egg is viable for only 12–24 hours. However, unexpected early or late ovulation can shift the fertile window, making unprotected sex at any point in the cycle carry some risk. This calculator is for informational and conception-planning purposes only and should not be used as a form of contraception.

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