Ovulation Calculator
Estimate when you ovulate and your most fertile days from the first day of your last period and your usual cycle length.
Last updated: June 20, 2026 · Medically reviewed by TG we-Calculate Editorial TeamFirst day of last period
days
Thu, Jan 15, 2026
- 1
Luteal phase (constant)
14The time from ovulation to the next period is the most stable part of the cycle at ~14 days. - 2
Ovulation — cycle day
28 − 14 = 14 - 3
Fertile window opens
cycle day 14 − 5 = 9 - 4
Fertile window closes
cycle day 14 + 1 = 15
How is ovulation calculated?
Ovulation is estimated as your next expected period minus 14 days, which equals the first day of your last period plus your cycle length minus 14. Your fertile window runs from about five days before ovulation through the day after, when conception is most likely.
Formula
How this is calculated
You enter two things: the first day of your last menstrual period (the LMP, a calendar date) and your usual cycle length in whole days — the number of days from the start of one period to the start of the next, typically 21–35. The calculator first projects your next period by adding the cycle length to the LMP. It then estimates ovulation using the luteal phase, the stretch between ovulation and the next period, which is the most stable part of the cycle at roughly 14 days. So ovulation = next period − 14 days = LMP + (cycle length − 14). The hero number is simply that count of days from your LMP (the "cycle day").
The fertile window is then built around ovulation: it opens 5 days before (sperm can survive in the reproductive tract for up to about 5 days) and closes 1 day after (the egg lives roughly 24 hours). Dates are computed in whole days, so month and leap-year rollovers are handled correctly.
This assumes a regular cycle and a fixed 14-day luteal phase, which is why it is an estimate, not a diagnosis. Inputs outside 21–45 days are rejected, and real ovulation can shift by several days, especially with irregular cycles — track basal body temperature, cervical mucus, or an ovulation test for confirmation.
Examples
| Input | Result |
|---|---|
| Last period 2026-01-01, 28-day cycle | Ovulation ~Jan 15; fertile Jan 10–16; next period Jan 29 |
| Last period 2026-01-01, 32-day cycle | Ovulation ~Jan 19; fertile Jan 14–20; next period Feb 2 |
About this calculator
This ovulation calculator estimates the day you are most likely to ovulate based on the first day of your last menstrual period (LMP) and the average length of your cycle. Because the luteal phase — the time between ovulation and your next period — is the most consistent part of the menstrual cycle and lasts about 14 days, ovulation is estimated as your next expected period minus 14 days, which equals LMP plus (cycle length − 14).
The fertile window spans the five days before ovulation through the day after, because sperm can survive in the reproductive tract for up to about five days while the egg lives for roughly 24 hours. Having intercourse during this window gives the best chance of conception.
These are statistical estimates, not a diagnosis. Ovulation timing varies between people and from cycle to cycle, especially with irregular cycles. For a more precise indication, track basal body temperature, cervical mucus, or use an ovulation predictor kit, and speak to a healthcare professional for personalised advice.
Frequently asked questions
You are most fertile in the few days leading up to and including ovulation. This calculator marks a fertile window from about five days before ovulation through the day after, with the day before and the day of ovulation being the most fertile.
It gives an estimate based on average cycle behaviour. Real ovulation can shift by several days, particularly with irregular cycles, so use it as a guide alongside body signs or an ovulation test.
For irregular cycles, calendar estimates are less reliable. Tracking basal body temperature, cervical mucus, or using ovulation predictor kits gives a better picture of your individual timing.
Also known as
Sources & references
TG we-Calculate Editorial Team. (2026). Ovulation Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/ovulation-calculator
TG we-Calculate Editorial Team. "Ovulation Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/ovulation-calculator.
TG we-Calculate Editorial Team, "Ovulation Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/ovulation-calculator
@misc{wecalculate_ovulation_calculator, title = {Ovulation Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/ovulation-calculator}}, year = {2026}, note = {TG we-Calculate} }
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