Ovulation Date Calculator

Work out ovulation date instantly with clear inputs, formula shown and shareable results.

About Ovulation Date Calculator

The luteal phase is the most consistent part of the cycle at around fourteen days, so ovulation is estimated by counting back from the expected next period rather than forward from the last one. A 28 day cycle from 20 August 2026 puts ovulation near 3 September, cycle day 15, with a fertile window from five days before to one day after.

Formulas

Ovulation day

day = cycle length - luteal phase length

Fertile window

from five days before ovulation to one day after

Worked example

With the default values below, Ovulation Date Calculator returns Estimated Ovulation Cycle Day of 15. Change any input above and every figure here is recalculated in your browser.

Inputs used

First day of the last period: 2026-08-20
Average cycle lengthaccepts 20–45 days: 28 days
Luteal phase lengthaccepts 9–17 days: 14 days

Results

Estimated Ovulation Cycle Day: 15
Estimated Ovulation Date: 2026-09-03
Fertile Window Opens: 2026-08-29
Fertile Window Closes: 2026-09-04

Step by step

1Values used
2Ovulation day
3Fertile window
4Estimated Ovulation Cycle Day= 15
5Estimated Ovulation Date= 2026-09-03
6Fertile Window Opens= 2026-08-29
7Fertile Window Closes= 2026-09-04

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or fees or charges into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Ovulation estimates from cycle averages are approximate and vary considerably between cycles. They are not reliable for contraception. For fertility or contraceptive advice, consult a healthcare professional, who may recommend ovulation testing rather than calendar estimation.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Ovulation estimates from cycle averages are approximate and vary considerably between cycles. They are not reliable for contraception. For fertility or contraceptive advice, consult a healthcare professional, who may recommend ovulation testing rather than calendar estimation.

Frequently Asked Questions

Why count back from the next period?

Because the luteal phase is relatively fixed while the follicular phase varies. Counting forward from the last period is less reliable in irregular cycles.

How long does sperm survive?

Up to five days in favourable cervical mucus, which is why the fertile window opens well before ovulation itself.