Ovulation calculator
Estimate your most fertile days, your ovulation date and when your next period is due — from the first day of your last period and your usual cycle length.
A simple, calendar-based estimate computed entirely in your browser. Nothing you enter is sent anywhere. It is for general awareness and family planning, not contraception or medical diagnosis.
Your cycle
The first day of bleeding in your most recent menstrual period.
From the first day of one period to the first day of the next. Most cycles run 21–35 days; 28 is the textbook average.
This is an estimate based on a regular cycle and a fixed 14-day luteal phase. It is not a contraceptive method and cannot confirm ovulation — see the notes below.
Estimated ovulation day
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Enter the first day of your last period to begin
Most fertile
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Fertile window
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Next period
—
Cycle length
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| Last period began (LMP) | — |
| Next period due | — |
| Ovulation | — |
| Fertile window opens | — |
| Fertile window closes | — |
An estimate only. The fertile window assumes sperm survive up to about five days and the egg about one day after release. Real cycles vary month to month, the luteal phase is not always exactly 14 days, and stress, illness, breastfeeding and conditions such as PCOS can shift or prevent ovulation. For contraception, family planning, or if your cycle is irregular, consult a doctor or use a clinically validated method.
From your last period to your fertile days
The calculator pins ovulation to a stable luteal phase, then opens a fertile window around it to reflect how long sperm and egg survive.
Next period
Add your average cycle length to the first day of your last period (LMP) to project when the next period should begin.
Ovulation
Count back 14 days from that projected period — the luteal phase is fairly constant, so this is more reliable than counting forward.
Fertile window
Open the window five days before ovulation and close it one day after, covering how long sperm and the egg can survive.
Ovulation, answered
How does this ovulation calculator work?+
It uses the standard calendar method. From the first day of your last period (LMP) and your average cycle length, it estimates the next period as LMP + cycle length, then places ovulation about 14 days before that next period — because the luteal phase (ovulation to period) is fairly constant at roughly 14 days for most people.
What is the fertile window?+
The fertile window is the span of days in your cycle when intercourse can lead to pregnancy. Because sperm can survive up to about five days and a released egg lives about a day, this tool marks the window as the five days before ovulation through the day after — six days in total, with the highest chance on the two days before and the day of ovulation.
Why is ovulation counted back from the next period, not forward from the last one?+
The phase before ovulation (the follicular phase) varies a lot between people and cycles, while the phase after ovulation (the luteal phase) is more stable at around 14 days. Counting back 14 days from the expected next period is therefore more reliable than counting a fixed number of days forward from your last period — especially if your cycle is longer or shorter than 28 days.
How accurate is a calendar-based estimate?+
It is an estimate, not a measurement. It assumes regular cycles and a 14-day luteal phase, but real cycles shift month to month and the luteal phase can be anywhere from about 11 to 17 days. Stress, illness, travel, breastfeeding, thyroid issues and conditions such as PCOS can move or stop ovulation. Ovulation predictor kits, basal body temperature charting and ultrasound are more precise.
Can I use this for birth control?+
No. This calculator is for general awareness and is not a contraceptive method. Calendar prediction alone is unreliable for preventing pregnancy. If you want to avoid pregnancy, use a clinically proven method and speak to a health professional.
My cycle is irregular — is this useful?+
If your cycle length varies widely, a single calendar estimate will be less dependable because the LMP-plus-cycle assumption breaks down. Track several cycles to find your shortest and longest, consider ovulation predictor kits, and consult a doctor — irregular cycles can sometimes signal an underlying condition worth checking.
Sources & data note
Based on the standard calendar method: next period = LMP + cycle length, ovulation ≈ next period − 14 days (a ~14-day luteal phase), and a fertile window from 5 days before ovulation to 1 day after. These are widely used clinical rules of thumb; individual cycles vary and the luteal phase is not always exactly 14 days. This tool gives an indicative estimate only — it is not contraception or a medical diagnosis. Verify timing with ovulation predictor kits or a health professional, especially for irregular cycles.