Ovulation Calculator

Estimate your ovulation date, fertile window, and next period from the start of your last menstrual period and cycle length. Educational estimate only — not a method of contraception.

Examples

Average 28-day cycle

Last period started 1 April 2026, average 28-day cycle.

First day of last period: Apr 1, 2026
Average cycle length: 28 days

Estimated ovulation date

Apr 14, 2026

Fertile window start

Apr 9, 2026

Fertile window end

Apr 15, 2026

Estimated next period

Apr 29, 2026

Day of cycle

Cycle length

28 days

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How It Works

Formula

d_{\text{ov}} = L - 14

D_{\text{ov}} = D_0 + (d_{\text{ov}} - 1)

D_{\text{fw,start}} = D_0 + (\max(1, d_{\text{ov}} - 5) - 1)

D_{\text{fw,end}} = D_0 + d_{\text{ov}}

D_{\text{next}} = D_0 + L

Variables, symbols and units

$D_0$: First day of the last menstrual period
$L$: Average cycle length(days)
$d_{\text{ov}}$: Day of cycle on which ovulation is predicted (1-indexed)
$D_{\text{ov}}$: Calendar date of predicted ovulation
$D_{\text{fw,start}}$: Start of the fertile window (5 days before ovulation, floor at day 1)
$D_{\text{fw,end}}$: End of the fertile window (1 day after ovulation)
$D_{\text{next}}$: Estimated start of the next period

Calculation method explained

Enter the first day of your last menstrual period and your average cycle length. The calculator uses your average cycle length to place an approximate ovulation date, then marks a broad fertile window around that estimate. The next period date is your last period plus your cycle length.

References and source material

Examples

Average 28-day cycleApr 1, 2026 · 28 days → Apr 14, 2026

Last period started 1 April 2026, average 28-day cycle.

First day of last period: Apr 1, 2026
Average cycle length: 28 days

Estimated ovulation date: Apr 14, 2026

Shorter 24-day cycleApr 1, 2026 · 24 days → Apr 10, 2026

Last period 1 April 2026, shorter 24-day cycle — ovulation arrives sooner.

First day of last period: Apr 1, 2026
Average cycle length: 24 days

Estimated ovulation date: Apr 10, 2026

Longer 35-day cycleApr 1, 2026 · 35 days → Apr 21, 2026

Last period 1 April 2026, longer 35-day cycle — ovulation later in the cycle.

First day of last period: Apr 1, 2026
Average cycle length: 35 days

Estimated ovulation date: Apr 21, 2026

Frequently Asked Questions

How does the calculator estimate ovulation?

It uses a simplified cycle-length estimate to place an approximate ovulation date within the cycle. So for a 28-day cycle, ovulation lands around day 14; for a 30-day cycle, around day 16.

What is the fertile window?

It is the span of days each cycle when intercourse is most likely to result in pregnancy. Sperm can survive in the reproductive tract for up to about 5 days, and the egg lives roughly 12–24 hours after release, so the calculator highlights a broad fertile window around the estimated ovulation date.

Can I use this as birth control?

No. Ovulation timing varies from cycle to cycle and can shift unpredictably due to stress, illness, travel, hormonal changes, and other factors. This calculator is an educational estimate only; do not rely on it to prevent pregnancy. Talk to a healthcare provider about contraceptive options.

How accurate is this?

It is an estimate, not a measurement. The calculation assumes a regular cycle of the length you entered and a textbook 14-day luteal phase. Real cycles vary, and ovulation can occur a few days earlier or later than predicted. Tracking basal body temperature or using ovulation tests gives more individual signal.

What if my cycle is irregular?

For irregular cycles, predictions become less reliable because the average cycle length used here may not reflect the next cycle. Average several recent cycles, but treat the result as a rough window rather than a date. A clinician can help interpret patterns.

Ovulation Calculator

Average 28-day cycle

How It Works

Examples

Frequently Asked Questions

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