How does the Ovulation Calculator estimate my ovulation day?

It takes the first day of your last period and your average cycle length, then subtracts a standard 14-day luteal phase to estimate the day you likely ovulated in that cycle.

What is the fertile window shown by the ovulation calculator?

It is the estimated span when conceiving is most likely: five days before your estimated ovulation day through one day after, covering sperm survival time and the egg's short lifespan.

Is the Ovulation Calculator accurate for every cycle length?

It works for average cycles between 20 and 45 days. Outside that range it falls back to the standard 28-day assumption, since very irregular cycles make a fixed-formula estimate unreliable.

Does the Ovulation Calculator upload my cycle dates?

No. All the math runs locally in your browser. The dates you enter are never sent to a server or uploaded to ArrayKit.

What happens if I leave the cycle length blank?

The calculator defaults to a standard 28-day cycle, which is the average used by most ovulation and fertility calculators when a personal cycle length is not entered.

Ovulation Calculator Online

Estimate your ovulation day and fertile window from your last period, right in your browser. A general estimate, not medical advice.

The Ovulation Calculator runs entirely in your browser. Your last-period date and cycle length never leave your device. This is a general estimate, not medical advice.

Open the Conception Calculator

About Ovulation Calculator

The Ovulation Calculator estimates your ovulation day and fertile window from the first day of your last period and your average cycle length. It uses the standard luteal-phase formula — ovulation is roughly your cycle length minus 14 days after your last period started — then marks the fertile window as the five days before ovulation through the day after, when conceiving is most likely. It also projects your next expected period so you can plan ahead. This is a general estimate from a standard formula, not medical advice, since actual ovulation timing varies by person and cycle. Everything is calculated locally on your device: your dates never leave your browser.

Features

Estimates ovulation day from your last period and average cycle length
Highlights the fertile window — the days you are most likely to conceive
Projects your next expected period based on the same cycle length
Defaults to a standard 28-day cycle when you leave the field blank
Accepts any cycle length in the typical 20-45 day clinical range
Shows results as full readable dates plus a copyable summary
Flags an invalid date or an out-of-range cycle length before you copy results
Runs entirely on your device — your cycle dates are never uploaded

How to use the Ovulation Calculator

Enter the first day of your last period, or tap Today for the current date
Enter your average cycle length in days, or leave it at the 28-day default
Read your estimated ovulation day and fertile window
Copy the summary to save or share your estimated dates

Example

Input

last period 2026-06-01, 28d

Output

Ovulation: ~Jun 15

A 28-day cycle places ovulation 14 days after the last period started.

Common errors & troubleshooting

The estimated ovulation day looks too early or too late. — Double-check your average cycle length — a longer cycle shifts ovulation later, a shorter cycle shifts it earlier, since the calculator always assumes a 14-day luteal phase after ovulation.
Cycle length field shows an error. — Enter a whole number of days between 20 and 45. Leave it blank to use the standard 28-day default.
Results do not match a period-tracking app. — This calculator uses a fixed formula from your entered dates; apps that track basal body temperature, ovulation tests, or cycle history can be more accurate for your body.
Not sure which date counts as the last period. — Use the first day of bleeding, not the last day, as the last-period date — that is the standard reference point the formula expects.

Frequently asked questions

How does the Ovulation Calculator estimate my ovulation day?: It takes the first day of your last period and your average cycle length, then subtracts a standard 14-day luteal phase to estimate the day you likely ovulated in that cycle.
What is the fertile window shown by the ovulation calculator?: It is the estimated span when conceiving is most likely: five days before your estimated ovulation day through one day after, covering sperm survival time and the egg's short lifespan.
Is the Ovulation Calculator accurate for every cycle length?: It works for average cycles between 20 and 45 days. Outside that range it falls back to the standard 28-day assumption, since very irregular cycles make a fixed-formula estimate unreliable.
Is the ovulation calculator's result medical advice?: No. It is a general estimate from a standard formula, not medical advice. Actual ovulation timing varies by person and cycle — talk to a healthcare provider for fertility planning or concerns.
Does the Ovulation Calculator upload my cycle dates?: No. All the math runs locally in your browser. The dates you enter are never sent to a server or uploaded to ArrayKit.
What happens if I leave the cycle length blank?: The calculator defaults to a standard 28-day cycle, which is the average used by most ovulation and fertility calculators when a personal cycle length is not entered.

Related tools

Conception Calculator — Estimate the likely conception date from a due date or last period.
Period Calculator — Predict your next several periods and fertile windows from your cycle.
Due Date Calculator — Estimate a pregnancy due date from the last menstrual period (Naegele’s rule).
Pregnancy Conception Calculator — Estimate the conception date and fertile window from a known pregnancy.
Pregnancy Calculator — Track pregnancy progress, trimester and milestones from the due date or LMP.
Pregnancy Weight Gain Calculator — Estimate a healthy pregnancy weight-gain range from pre-pregnancy BMI.