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Ovulation Calculator

Estimate your ovulation date, fertile window, next period, and due date from the first day of your last period, cycle length, and luteal phase.

Calculated in your browser. Nothing is uploaded.

Days from one period to the next. Defaults to 28.

Days from ovulation to your period. Defaults to 14.

Enter the first day of your last period to estimate your ovulation date and fertile window.

How to calculate your ovulation date

  1. Enter your last period

    Pick the first day of your most recent period; the estimate appears as soon as the date is valid, with no button to press.

  2. Set cycle and luteal length

    Cycle length defaults to 28 days and the luteal phase to 14 days, so adjust them to your own averages if you know them.

  3. Read your fertile window

    See your estimated ovulation date, the fertile window around it, your next period, and an estimated due date.

  4. Copy the summary

    Use Copy fertility summary to put every date on your clipboard for a note or a calendar.

Why use this tool

Ovulation date and fertile window

From your last period, cycle length, and luteal phase it estimates the day you ovulate and the fertile window from five days before through the day after.

Next period and due date

It also shows when your next period is expected and the estimated due date for a pregnancy conceived during this cycle.

Upcoming cycles at a glance

A short projection lists the ovulation and period dates for your next few cycles so you can plan ahead.

Sensible input guards

Cycle length is held to 20 to 45 days and the luteal phase to 9 to 18 days, so a mistyped number cannot produce an impossible date.

Runs entirely in your browser

Every date you enter is processed on your device and is never uploaded, stored, or logged.

About this tool

This ovulation calculator turns three simple inputs, the first day of your last period, your average cycle length, and your luteal phase, into the dates that matter when you are trying to conceive or simply tracking your cycle. It works out when you are most likely to ovulate, the fertile window around that day, and when your next period is due, updating the moment your details are valid so there is no button to press.

Ovulation is estimated by counting your luteal phase back from your next expected period, which falls one cycle length after the first day of your last one. The fertile window spans the five days before ovulation through the day after, because sperm can survive for several days while the egg lasts about a day. Cycle length is kept within a plausible 20 to 45 days and the luteal phase within 9 to 18 days, so an obvious typo does not produce a nonsense date. You also get an estimated due date for a pregnancy conceived this cycle, counted 266 days from ovulation.

Use it to plan around your fertile days, to sanity-check a period-tracking app, or to line up an estimate before a clinic visit. If you already know you are pregnant, the due date calculator works from your last period instead, and the date difference tool measures any plain gap between two dates. These figures are estimates for a regular cycle, not medical advice, and they are not a reliable method of contraception.

Frequently asked questions

How is my ovulation date calculated?
It assumes you ovulate one luteal phase before your next period. Your next period falls one cycle length after the first day of your last one, so ovulation lands that many days earlier. With the default 28-day cycle and 14-day luteal phase, that is about 14 days after your period starts.
What is the fertile window?
It is the stretch of days when conception is most likely: the five days before ovulation plus the day of ovulation and the day after. Sperm can survive for several days while the egg is viable for roughly a day, so the two days ending on ovulation are the most fertile.
What if my cycle is irregular?
The estimate assumes a fairly regular cycle. If yours varies a lot, ovulation can happen earlier or later than predicted, so treat the dates as a guide. Cycle length is limited to 20 to 45 days and the luteal phase to 9 to 18 days.
Can I use this to avoid pregnancy?
No. This is an estimate for a regular cycle and is not a reliable method of contraception. If you want to prevent pregnancy, speak to a healthcare provider about proven options.
Is my data uploaded anywhere?
No. Everything runs in your browser; the dates you enter are never sent to a server, stored, or logged.

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