Ovulation Calculator

Written byOleh Soloninupdated 25 September 2026

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 day15

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.

How to use

Enter the first day of your last period.
Enter your average cycle length — from the first day of one period to the first day of the next.
Keep the luteal phase at 14 days, or enter your own if ovulation tests have shown it.
The result gives the ovulation day, the fertile window and the date of the next period.

Ovulation by cycle length

The calculator counts the luteal phase back from the next period: ovulation = first day of the cycle + (cycle length − luteal phase) days. The first day of the period is cycle day 1, so on a 28-day cycle ovulation on 15 July is day 15; on a 24-day cycle it is day 11, on a 35-day cycle day 22.

The fertile window here is seven days: the five before ovulation, the day itself and the day after.

Period started on 1 July 2026, luteal phase 14 days. Every row comes from the calculator on this page.
Cycle, days	Cycle day of ovulation	Ovulation	Fertile window	Next period
24	11	11 July	6–12 July	25 July
26	13	13 July	8–14 July	27 July
28	15	15 July	10–16 July	29 July
30	17	17 July	12–18 July	31 July
32	19	19 July	14–20 July	2 August
35	22	22 July	17–23 July	5 August

If your luteal phase is shorter than 14 days

Fourteen days is a textbook convention. An analysis of over 600,000 cycles (Bull and colleagues, 2019) found an average luteal phase of 12.4 days, and it was the first, follicular phase that varied most from cycle to cycle.

If ovulation tests have told you your luteal phase, enter it: on a 28-day cycle with a 12-day luteal phase, ovulation falls on cycle day 17, not day 15.

A 28-day cycle, period started on 1 July 2026. Every row comes from the calculator on this page.
Luteal phase, days	Cycle day of ovulation	Ovulation	Fertile window
11	18	18 July	13–19 July
12	17	17 July	12–18 July
13	16	16 July	11–17 July
14	15	15 July	10–16 July
16	13	13 July	8–14 July

What this calculator shows

The estimated ovulation date
A seven-date window: the five days before ovulation, the day itself and the day after
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

What day do I ovulate on a 30-day cycle?

Cycle day 17 with a 14-day luteal phase, counting the first day of the period as day 1: a period starting on 1 July puts ovulation around 17 July and the fertile window at 12–18 July, 14 days before the next period on 31 July.

Why count back from the next period?

Because the first, follicular phase is the part that changes most from cycle to cycle, while the luteal phase is steadier. Counting back from the expected period is therefore more reliable than counting forward from the last one.

How long is the fertile window?

The calculator shows seven days: the five before ovulation, the day itself and the day after. Wilcox and colleagues' classic 1995 study found a six-day window ending on ovulation day; sperm survival, not egg survival, sets the width.

When will my next period start?

One cycle length after the first day of the last one: for a period starting on 1 July and a 28-day cycle, on 29 July.

Can I use this as contraception?

No. Cycles shift with stress, illness and travel, and a calendar calculation cannot see that. For contraception, use a method designed for it.

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 cycle day 19 rather than day 15 — the extra days land before ovulation, not after.

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