Home › Calculators › Ovulation Calculator

Ovulation Calculator

Wondering when you're most fertile, or when your next period is due? This free calculator estimates your ovulation day, fertile window, next expected period and an estimated due date from the first day of your last menstrual period and your cycle length — privately, with nothing uploaded.

First day of last menstrual period (LMP)

Average cycle length (days, 21-35)

Luteal phase length (days)

Who is this ovulation calculator for?

This ovulation calculator is a free, private way to estimate when you ovulate, your fertile window and your next period. It helps four groups in particular.

Trying to conceive: couples hoping for a pregnancy can use the ovulation calculator by last period to pinpoint the fertile days and time intercourse around the predicted ovulation day.
Tracking their cycles: anyone who wants to understand their body better can use the ovulation calendar to see how the ovulation day moves with their average cycle length, and to find their most fertile days.
Couples planning a family: partners timing a pregnancy can enter different cycle lengths to compare when the fertile window and ovulation day shift, making planning easier.
Health awareness: people who simply want to know when do I ovulate and understand the luteal phase calculator can use the result to learn the pattern of their cycle and recognise their fertile window.

Ovulation chart: cycle length to ovulation & fertile window

This table maps an average cycle length to the ovulation day and fertile window the calculator returns, using a fixed first day of the last menstrual period of March 1, 2026 and a 14-day luteal phase. The formula is ovulation day = LMP + (cycle length - luteal phase).

Cycle length	Ovulation day	Fertile window	Next expected period
21 days	March 8, 2026	March 3 - 9, 2026	March 22, 2026
24 days	March 11, 2026	March 6 - 12, 2026	March 25, 2026
28 days	March 15, 2026	March 10 - 16, 2026	March 29, 2026
30 days	March 17, 2026	March 12 - 18, 2026	March 31, 2026
35 days	March 22, 2026	March 17 - 23, 2026	April 5, 2026

All rows were computed with the page's formula: ovulation day = first day of the last menstrual period + (cycle length - 14-day luteal phase), and the fertile window is the five days before ovulation plus the day after.

What is ovulation?

Ovulation is the moment during the menstrual cycle when a mature egg is released from one of the ovaries. It usually happens about once each cycle, typically around the middle of a 28-day cycle, which is roughly day 14. Once the egg is released, it survives for a day or so, while sperm can live inside the body for several days. That combination is why the fertile window spans the days just before and after ovulation — intercourse in that window is when conception is most likely.

The ovulation day depends on your cycle length and how long your luteal phase lasts, so it is not the same for everyone. Knowing your own pattern helps you predict the most fertile days, plan for a pregnancy, or simply understand your cycle. Because ovulation can shift slightly from month to month, this calculator gives an estimate rather than an exact certainty.

How to use this ovulation calculator

Enter the first day of your last menstrual period (LMP).
Set your average cycle length — 28 days is the typical default, or anywhere from 21 to 35.
Set your luteal phase length, usually around 14 days.
Click Calculate Ovulation to see your ovulation day, fertile window, next expected period and an estimated due date.

Example

If your last menstrual period started on March 1, 2026 with an average cycle length of 28 days and a 14-day luteal phase, the estimated ovulation day is March 15, 2026 (March 1 + 14 days). Your fertile window is March 10 to March 16, 2026. Your next expected period is March 29, 2026, and the estimated due date would be December 6, 2026. These are the exact figures this calculator returns.

Common use cases

Planning a pregnancy by timing intimacy around the most fertile days.
Predicting when the next period will arrive using the ovulation calculator cycle length.
Monitoring the luteal phase to understand cycle health and irregularity.
Learning a personal ovulation calendar to answer "when do I ovulate".

Pro tips

Use the ovulation calculator 28 day cycle default first, then adjust to your own length for a closer estimate.
Combine the result with cervical mucus or an ovulation test to confirm when your ovulation day really occurs.
If your cycles vary, enter your average — an irregular cycle can shift the fertile window by a day or two.
Treat the due date as a guide and confirm a real pregnancy with a doctor or midwife.

Ovulation Calculator FAQs

How is the ovulation day calculated?

The calculator takes the first day of your last menstrual period (LMP), then adds the number of days between your cycle length and luteal phase. Ovulation day = LMP + (cycle length - luteal phase).

What is the fertile window?

The fertile window is the five days before ovulation plus the day after. The calculator shows this as the range from (ovulation - 5 days) to (ovulation + 1 day).

When should I start trying to conceive?

Your highest chance of conceiving is in the fertile window around ovulation, especially the two or three days right before ovulation on the calculated day.

What if my cycle is irregular?

Enter your cycle length and the calculator returns an estimate, but an irregular cycle makes the predicted day less precise. Track a few cycles or confirm with a doctor for a personalized plan.

What is the luteal phase?

The luteal phase is the time after ovulation and before the next period. It is usually around 14 days, which is why the calculator uses 14 as a default.

What does the estimated due date mean?

The due date is a guide only. It adds 280 days (40 weeks) to the first day of your last period, so treat it as an estimate and confirm with a doctor or midwife.

Can I use this with a longer cycle?

Yes. A longer cycle pushes ovulation later, so the calculator adjusts the ovulation day, fertile window and next period to match whatever cycle length you enter.

How accurate is an ovulation calculator?

It is a good estimate for a regular cycle, but real ovulation can shift by a day or two. Signs like cervical mucus and basal body temperature can add accuracy.

Is this calculator free and private?

Yes. It is free, runs entirely in your browser, and no date you enter is uploaded anywhere.

What does LMP mean?

LMP stands for the first day of your last menstrual period. It is the starting point the calculator uses to count forward to ovulation and your next period.