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Ovulation Calculator

100% Free

Estimate your fertile window, ovulation day, next period, and an estimated due date from your last period and average cycle length — an educational calendar estimate, in-browser.

Fertile Window
Cycle Aware
Instant
100% Client-Side

1. Your cycle

Most cycles are about 14 days. Leave as is if unsure.

Educational estimate only — not medical advice and not a contraception method. Cycles vary month to month; this calendar model can be off by several days. For health or fertility decisions, consult a doctor.

2. Estimates

Enter the first day of your last period and your average cycle length to estimate your fertile window.

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How to Use

  1. 1 Pick the first day of your last period
  2. 2 Enter your average cycle length (28 days is the default)
  3. 3 Adjust the luteal phase only if you know yours differs from 14 days
  4. 4 Read your estimated fertile window and ovulation day
  5. 5 Check the next period, due-date estimate, and upcoming cycles

Features

  • Estimated ovulation day and 6-day fertile window
  • Next expected period start date
  • Estimated due date if conception occurs (Naegele's rule)
  • Current cycle day based on today's date
  • Predictions for the next three cycles
  • Log periods, symptoms, and get reminders with Pro

Why it Matters

Knowing roughly when ovulation happens helps people who are trying to conceive time intercourse, and helps anyone understand their cycle better. A calendar estimate is a useful starting point — though it is only an estimate and should never be used as a contraception method.

★★★★★

Use Cases

Trying to Conceive

Time the fertile window when planning a pregnancy

Cycle Awareness

Understand your own rhythm and when your period is due

Plan Ahead

Anticipate periods around travel or events

Talk to a Doctor

Bring a clear cycle summary to an appointment

What this tool does

The Ovulation Calculator estimates your fertile window, most likely ovulation day, next expected period, and an estimated due date if conception occurs — all from the first day of your last period and your average cycle length. It also shows your current cycle day and the next three cycles.

How it works

Ovulation is estimated as cycle length minus the luteal phase (about 14 days for most people), counted from the start of your last period. The fertile window covers the five days before ovulation plus the day after, since sperm survive several days and the egg lives roughly a day. The due-date estimate uses Naegele’s rule (last period + 280 days).

Important: this is an estimate, not medical advice

This calculator is educational only. It is not medical advice, not a contraception method, and not a substitute for professional care or for tracking real ovulation signs. Cycles vary, so estimates can be off by several days. For health or fertility decisions, talk to a doctor.

Free vs Pro

The free tool gives one private estimate and stores nothing. A proposed Pro upgrade would let you log periods and symptoms, get predictions that learn from your own data, and receive reminders — built privacy-first, since reproductive-health data is sensitive. The single estimate stays free.

Privacy

Everything runs locally in your browser. Nothing you enter is uploaded, logged, or stored.

Frequently Asked Questions

How is the fertile window calculated?
Ovulation is estimated as your cycle length minus your luteal phase, counted from the first day of your last period — for a 28-day cycle with a 14-day luteal phase, that's around day 14. The fertile window spans the five days before ovulation plus the day after, because sperm can survive several days and the egg lives about 24 hours.
How accurate is this calculator?
It's a calendar estimate based on average values, so it can be off by several days, especially if your cycles are irregular. Real ovulation shifts month to month. For more precision, track ovulation signs such as basal body temperature or ovulation tests, and treat this tool as a starting point.
Can I use this as birth control?
No. This calculator is not a contraception method and must never be relied on to prevent pregnancy. Calendar estimates are far too imprecise for that purpose. Speak to a healthcare provider about effective contraception.
How is the due date estimated?
It uses Naegele's rule — last-period date plus 280 days (40 weeks) — which is the standard first estimate. An actual due date is confirmed by a clinician, often with an ultrasound, and may differ.
Is my data private?
Yes. Everything runs entirely in your browser. The dates you enter are never uploaded, logged, or stored — refreshing the page clears them. Reproductive-health data is sensitive, so the free tool deliberately stores nothing.
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Want to understand the math?

Browse all guides →
Understanding Your Fertile Win
ovulation

Understanding Your Fertile Window and Ovulation

How the menstrual cycle works, what the fertile window is, how ovulation is estimated from cycle length, why calendar estimates are imprecise, and when to see a doctor.

25 Jun 2026

5 min read

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