Ovulation Calculator

First day of last period

Cycle length, days

Advanced options

Luteal phase, days

Result

Estimated ovulation2026-07-15
Fertile window2026-07-10 — 2026-07-16
Next period2026-07-29
Cycle day14

This result is informational only and is not professional advice.

Counting back from the next period, not forward from the last

Estimate the likely day of ovulation and the fertile window around it, based on cycle length and the luteal phase.

Ovulation is timed better by counting backwards. The luteal phase — from ovulation to the next period — is fairly consistent at about 14 days, while the first half of the cycle varies a lot between people.

The fertile window opens before ovulation rather than on the day, because sperm can survive around five days while the egg lasts roughly 24 hours.

What this calculator shows

The estimated ovulation date
The fertile window across roughly six days
The expected start of the next cycle

What to keep in mind

Assumes a regular cycle with a consistent luteal phase; irregular cycles make calendar methods unreliable.
This is an estimate, not contraception, and not a substitute for medical advice or ovulation testing.

FAQs

Can I use this as contraception?

No. Calendar methods have a high failure rate because cycles shift with stress, illness and travel. For contraception, use a method designed for it.

Why count back from the next period?

Because the luteal phase is the stable part of the cycle at around 14 days. The follicular phase before ovulation is what varies, so counting forward from the last period is less reliable.

How long is the fertile window?

About six days: five before ovulation plus the day itself. Sperm survival, not egg survival, is what makes it that wide.

What if my cycles are irregular?

Then a calendar estimate is weak. Ovulation tests, basal temperature tracking or clinical monitoring give far better information.

Worked example

A 28-day cycle

Input: Period started 1 July, 28-day cycle

Output: Ovulation around 15 July, fertile window 10–16 July

Note: On a 32-day cycle ovulation shifts to about day 18, not day 16 — the extra days land before ovulation, not after.

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