CalcLibrary

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.

days
Examples

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

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
14
Cycle length
28 days

Today is on or after the estimated start of your next period.

Estimate based on a regular cycle. Ovulation timing varies from cycle to cycle and is not exact; this is **not a contraceptive method** and should not be used to prevent pregnancy. For fertility planning, conception, or contraception, talk to a qualified healthcare provider.

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Examples

How It Works

Formula

dov=L−14d_{\text{ov}} = L - 14

Dov=D0+(dov−1)D_{\text{ov}} = D_0 + (d_{\text{ov}} - 1)

Dfw,start=D0+(max⁡(1,dov−5)−1)D_{\text{fw,start}} = D_0 + (\max(1, d_{\text{ov}} - 5) - 1)

Dfw,end=D0+dovD_{\text{fw,end}} = D_0 + d_{\text{ov}}

Dnext=D0+LD_{\text{next}} = D_0 + L

Variables

D0D_0

First day of the last menstrual period

LL

Average cycle length(days)

dovd_{\text{ov}}

Day of cycle on which ovulation is predicted (1-indexed)

DovD_{\text{ov}}

Calendar date of predicted ovulation

Dfw,startD_{\text{fw,start}}

Start of the fertile window (5 days before ovulation, floor at day 1)

Dfw,endD_{\text{fw,end}}

End of the fertile window (1 day after ovulation)

DnextD_{\text{next}}

Estimated start of the next period

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.

Frequently Asked Questions

01How 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.
02What 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.
03Can 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.
04How 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.
05What 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.

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