Zerdly

Ovulation Calculator

Predict your ovulation day and most fertile window based on your cycle length with this free ovulation calculator in your browser.

100% FreeRuns in your browserBasic
How to use this calculator

Assumed 14 unless you know yours.

These dates stay in this browser tab. Nothing is saved, put in the page address or sent anywhere.

Estimates

Enter the first day of your last period and your cycle length.

Estimate only — not medical advice. Cycles, ovulation and due dates vary between people and from cycle to cycle. Confirm with a healthcare professional.

How the method works

Cycle variation widens every estimate
Illustrative method
Cycle variation widens every estimateA cycle-day axis from day 1 to day 32. For cycles of 26 to 30 days and a 14 day luteal phase, ovulation is estimated between cycle days 13 and 17, the fertile-window estimate spans days 8 to 17, and the next period between days 27 and 31.Day 1 = period start (entered date)Fertile-window estimatedays 8–17Ovulation estimatedays 13–17Next period estimatedays 27–311714212832cycle day
Cycles of 26–30 days with a 14-day luteal assumption: next period on cycle day 27–31, ovulation on cycle day 13–17 (next start − 14), fertile-window estimate days 8–17 (ovulation − 5 to ovulation). These are ranges of estimates, not certainties or safe days.

Irregular cycles, recent hormonal contraception, postpartum and breastfeeding weaken every calendar estimate. Fixed example values for illustration — not your result.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the first day of your last period, your average cycle length and, if you know it, your luteal phase (14 days is assumed).
  2. Optionally enter your shortest and longest recent cycles to see how much the estimate moves.
  3. Read the estimated ovulation date and cycle day, the fertile interval, the extra caution day and the next period.

How it works

Next period = last period + cycle length; ovulation = next period − luteal phase. The cycle day is counted from day 1 = the first day of your period.

The estimated fertile interval is the five days before ovulation and the day itself. One extra day after ovulation is shown separately as caution — none of this is a clinically exact window.

With a shortest and longest cycle, ovulation is shown as a range — a what-if envelope, not a probability.

Irregular cycles, PCOS, hormonal medication, breastfeeding and the months after birth all weaken a calendar estimate, and it can never tell you which days are safe for contraception.

Worked example

Last period 1 Jan 2026, 28-day cycle, 14-day luteal phase. Next period 29 Jan; estimated ovulation 15 Jan (cycle day 15); fertile interval 10–15 Jan, with 16 Jan as the extra caution day. If cycles run 26–30 days, ovulation moves between 13 and 17 Jan.

Common mistakes

  • Treating the fertile interval as exact.
  • Using the calendar method for contraception.
  • Leaving the luteal phase at 14 days when you know yours is different.

Tips

  • Estimate only — not medical advice. Dates and fertile windows vary between people and cycles; confirm with a healthcare professional.

Frequently asked questions

How reliable is this?

Cycles vary, so it's an estimate. Ovulation tests or tracking give a more personal result.

Privacy?

Runs entirely in your browser. Your inputs are not saved or transmitted.

Related tools

Explore more in Family, Parenting & Custody.

Cite this tool

Zerdly. (2026). Ovulation Calculator. Retrieved from https://www.zerdly.com/tools/ovulation-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.