Ovulation Calculator

Your estimated ovulation date and fertile window — from your last period and cycle length

Your Cycle

First Day of Last Period

Average Cycle Length (days)

First day of one period to the first day of the next — 28 is average

Estimate only: The calendar method assumes a ~14-day luteal phase, but ovulation timing varies cycle to cycle — even in regular cycles. Ovulation predictor kits and basal temperature tracking are far more precise. This tool is not reliable as contraception, and irregular cycles make calendar estimates especially rough.

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Quick summary

Ovulation usually happens about 14 days before your next period, so this calculator estimates it as your cycle length minus 14 days after your last period started: day 14 of a 28-day cycle, day 21 of a 35-day cycle. Your fertile window is the five days before ovulation through the day after. It is a calendar estimate only; ovulation tests and temperature tracking are more precise, and it is not reliable contraception.

Reviewed by Simple Calculators TeamLast reviewed July 13, 2026

For information only — not medical advice. Results are estimates from published formulas and can't account for your health history. Talk to a doctor or other qualified clinician before making health decisions.

How to use this calculator

Enter the First Day of Last Period — the first day of full bleeding, not spotting.
Enter your Average Cycle Length (days), counted from the first day of one period to the first day of the next; use an average of your last few cycles, or 28 if you don't know.
Read your Estimated Fertile Window, the six days when conception is possible, and the estimated ovulation date inside it.
Check Next Period Expected and Due Date if Conceived, and update the date each cycle. To pinpoint ovulation, start ovulation predictor tests a few days before the window begins.

Ovulation Formulas

Estimated Ovulation Date (Calendar Method)

\text{Ovulation} = \text{LMP} + (C - 14) \text{ days}

LMP: First day of your last period
C: Average cycle length in days
14: Assumed luteal phase length (ovulation to next period)

Worked example

Last period started July 1, 2026; 28-day cycle

Given: LMP = Jul 1, 2026
C = 28 days
Calculation: $\text{Ovulation} = \text{Jul 1} + (28 - 14) = \text{Jul 1} + 14 \text{ days};\quad \text{Next period} = \text{Jul 1} + 28 \text{ days}$
Result: Ovulation ≈ July 15, 2026; next period expected July 29, 2026

Fertile Window

\text{Fertile window} = [\,\text{Ovulation} - 5,\ \text{Ovulation} + 1\,] \text{ days}

Ovulation: Estimated ovulation date from the formula above
−5: Sperm can survive up to about five days
+1: The egg survives about a day after release

Worked example

Same last period (July 1, 2026) with a 32-day cycle

Given: LMP = Jul 1, 2026
C = 32 days
Calculation: $\text{Ovulation} = \text{Jul 1} + 18 = \text{Jul 19};\quad \text{Jul 19} - 5 = \text{Jul 14},\ \text{Jul 19} + 1 = \text{Jul 20}$
Result: Fertile window ≈ July 14 – July 20, 2026

Frequently asked questions

When do I ovulate?

Most people ovulate roughly cycle length minus 14 days after their period starts: day 14 of a 28-day cycle, day 16 of a 30-day cycle, day 21 of a 35-day cycle. The '14' is the luteal phase, which stays fairly constant across cycle lengths, while the first half of the cycle stretches or shrinks. Individual timing still varies from cycle to cycle.

How is the ovulation date calculated?

The calculator adds your cycle length minus 14 days to the first day of your last period. For example, with a last period starting July 1 and a 28-day cycle, estimated ovulation is July 15 and the next period is expected July 29. The fertile window is then set from five days before ovulation to one day after, so July 10 to July 16.

What are the best days to conceive?

The two or three days just before ovulation, plus ovulation day itself, are the most fertile. Because sperm can live up to five days in fertile cervical mucus while the egg survives about a day, having sex in the days before ovulation is more likely to lead to pregnancy than waiting until after — which is why the fertile window runs from five days before to one day after.

How accurate is a calendar ovulation calculator?

A calendar ovulation calculator gives a rough starting estimate, not a precise date. Even regular cycles can shift ovulation by a few days month to month, and stress, illness, and travel can move it further. Ovulation predictor kits (which detect the LH surge), basal body temperature, and cervical mucus tracking pinpoint it better — use the calendar to know when to start testing.

What if my periods are irregular?

If your cycle length varies a lot, a calendar estimate will be less reliable, because the calculator assumes one average cycle length. Ovulation predictor kits or tracking basal body temperature can help identify ovulation as it happens. Very irregular, very long, or absent periods can have medical causes, so talk to a doctor or other healthcare provider, especially if you are trying to conceive.

How is the 'Due Date if Conceived' worked out?

It is your last period's start date plus 280 days (Naegele's rule), adjusted for your cycle length: add one day for each day your cycle is longer than 28 days, or subtract one for each day it is shorter. This is the same method the pregnancy due date calculator uses. For a last period starting July 1, 2026, a 28-day cycle gives April 7, 2027 and a 32-day cycle gives April 11, 2027. Your doctor or midwife may adjust the date after an early ultrasound.

Can I use this to avoid pregnancy?

No. Calendar math is not reliable contraception — sperm longevity plus normal variation in ovulation mean 'safe days' are much less safe than they look. Fertility-awareness methods that do work as birth control require training and daily observations of body signs. Talk to a doctor or other healthcare provider about contraception options that suit you.

Key terms

Luteal Phase: Ovulation to the next period — relatively fixed at ~14 days, the anchor of the calculation.
Fertile Window: The ~6 days ending the day after ovulation when conception is possible.
LH Surge: The hormone spike 24–36 hours before ovulation that predictor kits detect.
Naegele's Rule: Due date = last period + 280 days, adjusted here by (cycle length − 28) days — how the due-date estimate is computed.

Methodology, sources & limitations

This page shows the formula, assumptions, and limitations used for this estimate. Rates, thresholds, and other time-sensitive inputs can change, so verify them against the relevant official source before relying on a result.

For our review approach and the primary sources we consult, see the methodology and source policy. If you spot an outdated figure or formula, please contact us with a correction.

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