Ovulation Calculator

Predict your ovulation day and fertile window from your last period. Free ovulation calculator for conception and cycle tracking.

An ovulation calculator estimates when you are most likely to ovulate and the days around it that make up the fertile window. Ovulation typically happens about 14 days before the next period, so the calculator works backward from the first day of your last menstrual period (LMP) and your average cycle length to estimate ovulation day. The fertile window spans roughly the five days before ovulation plus ovulation day, because sperm can survive several days in the reproductive tract while the egg lives about a day after release. Use it whether you are trying to conceive or tracking your cycle for natural family planning. It gives a calendar estimate based on average cycles; real ovulation can shift with stress, illness, or irregular cycles, so it is most reliable for people whose cycles are fairly regular. Enter the days since your last period started and your average cycle length. The numbers run privately in your browser.

cycle

Result

14

Breakdown

Days since ovulation
0
Fertile window start (cycle day)
9
Fertile window end (cycle day)
14
Days until next period
14

About Ovulation Calculator

What is ovulation calculator?

An ovulation calculator estimates when you are most likely to ovulate and the days around it that make up the fertile window. Ovulation typically happens about 14 days before the next period, so the calculator works backward from the first day of your last menstrual period (LMP) and your average cycle length to estimate ovulation day. The fertile window spans roughly the five days before ovulation plus ovulation day, because sperm can survive several days in the reproductive tract while the egg lives about a day after release. Use it whether you are trying to conceive or tracking your cycle for natural family planning. It gives a calendar estimate based on average cycles; real ovulation can shift with stress, illness, or irregular cycles, so it is most reliable for people whose cycles are fairly regular. Enter the days since your last period started and your average cycle length. The numbers run privately in your browser.

How to use this calculator

  1. Enter your days since last period started.
  2. Enter your average cycle length.
  3. Click the "Calculate" button.
  4. View your result instantly — the main result is displayed prominently at the top with a detailed breakdown below it.

When to use this calculator

  • Timing intercourse for conception around the fertile window
  • Tracking ovulation for natural family planning
  • Estimating ovulation when cycles are fairly regular
  • Planning when to use ovulation predictor kits
  • Understanding how cycle length shifts the ovulation date

How the calculation works

Ovulation is estimated as cycle_length minus 14 days from the start of the period, because the luteal phase after ovulation is fairly fixed at about 14 days. The fertile window spans roughly ovulation day minus 5 days to ovulation day, since sperm can survive up to about 5 days and the egg about 1 day. For example, with a 28-day cycle and LMP 14 days ago: ovulation is estimated at day 14 of the cycle (28 - 14), which is today, and the fertile window ran from about day 9 to day 14. With a 32-day cycle, ovulation shifts to about day 18 (32 - 14), so a longer cycle pushes the fertile window later without shortening the luteal phase. Days since ovulation = lmp_days_ago minus ovulation_day, and the next period is due in cycle_length minus lmp_days_ago days.

Frequently Asked Questions

How is ovulation calculated?▼
Ovulation is estimated as your cycle length minus 14 days, counted from the first day of your last period. The 14 days come from the luteal phase, which is fairly fixed even when cycle length varies.
What is the fertile window?▼
The fertile window is the roughly six days each cycle when sex can lead to pregnancy: the five days before ovulation plus ovulation day itself. Sperm can survive up to about five days, while the egg lives about one day after release.
Can I use an ovulation calculator for birth control?▼
An ovulation calculator gives a calendar estimate only and is not reliable for avoiding pregnancy. Sperm survival varies, ovulation can shift with stress or illness, and cycles are often irregular. For birth control, use a method backed by your doctor.
When do I ovulate if my cycle is 28 days?▼
On a 28-day cycle, ovulation is estimated at day 14 (28 minus 14), counted from the first day of your period. The fertile window runs from about day 9 to day 14.
Does a longer cycle change ovulation day?▼
Yes. On a 32-day cycle, ovulation shifts to about day 18 (32 minus 14). A longer cycle pushes the fertile window later in the month without shortening the fixed 14-day luteal phase.
How accurate is an ovulation calculator?▼
It is most reliable for people whose cycles are fairly regular. Real ovulation can shift with stress, illness, travel, or irregular cycles, so for precision use ovulation predictor kits or basal body temperature tracking alongside it.
What if my cycle is irregular?▼
With irregular cycles the calendar estimate is a rough guide only. Track several cycles to find your average length, or use ovulation predictor kits to confirm the actual fertile window each month.
Is this ovulation calculator free?▼
Yes, this ovulation calculator is free, runs entirely in your browser, and stores no data. Enter the days since your last period started and your average cycle length to get your ovulation day and fertile window.

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