Ovulation Calculator

Direct answer

Your estimated ovulation day is the first day of your last menstrual period (LMP) plus your cycle length minus 14 days:

Ovulation ≈ LMP + (Cycle length − 14 days)

Your fertile window is the 5 days before ovulation plus ovulation day itself — 6 days in total. For example, with LMP January 1, 2026 and a 28-day cycle, ovulation is estimated on January 15, 2026, the fertile window is January 10–15, 2026, and the next period is expected on January 29, 2026.

Estimates only, not a substitute for medical advice. These dates are estimates based on average cycle timing. Ovulation varies from cycle to cycle, and this calculator cannot confirm whether or when you ovulated. Talk to a doctor or other qualified health professional about fertility, pregnancy, or any health concern.

The tool

Ovulation Calculator



Ovulation formula: January 1, 2026 plus 14 days equals ovulation on January 15, 2026
Figure 1. Ovulation formula — LMP Jan 1, 2026 + (28 − 14) days = ovulation Jan 15, 2026 — averagecalculatoronline.com

Inputs

FieldDefaultUnit
First day of last menstrual period14 days before todaydate
Average cycle length28days

Outputs

OutputFormulaDefault result
Estimated ovulation dateLMP + (cycle − 14) daysToday
Fertile window startOvulation − 5 days5 days before today
Fertile window endOvulation dayToday
Next period expectedLMP + cycle days14 days from now

Defaults on load: LMP 14 days ago, cycle 28 days → ovulation today, fertile window the five days before today through today, next period in 14 days. The tool must show these results before the visitor types anything, and must recalculate on every input change.

How it works

  1. Enter the first day of your last period. This is day 1 of the cycle — the first day of menstrual bleeding, not the last day.
  2. Enter your average cycle length. A cycle is counted from the first day of one period to the first day of the next. The average of your last few cycles is more useful than a single cycle.
  3. Subtract 14 days. Ovulation usually happens about 14 days before the next period, so the estimate is LMP + (cycle length − 14). For a 28-day cycle that is day 14; for a 30-day cycle it is day 16; for a 24-day cycle it is day 10.
  4. Read the fertile window. The 5 days before ovulation plus ovulation day itself are the days when conception is most likely, because sperm can survive in the body for several days before the egg is released.
  5. Read the next period date. Adding the full cycle length to the LMP gives the expected first day of the next period.
Fertile window from January 10 to January 15, 2026, six days ending on ovulation day
Figure 2. Fertile window — Jan 10 to Jan 15, 2026, 6 days ending on ovulation day — averagecalculatoronline.com

Ovulation day by cycle length (recomputed)

All rows use the same LMP: January 1, 2026. Ovulation = LMP + (cycle − 14); fertile window = ovulation − 5 days to ovulation day; next period = LMP + cycle length.

Cycle lengthOvulation dayFertile windowNext period
21 daysJanuary 8, 2026January 3–8, 2026January 22, 2026
22 daysJanuary 9, 2026January 4–9, 2026January 23, 2026
23 daysJanuary 10, 2026January 5–10, 2026January 24, 2026
24 daysJanuary 11, 2026January 6–11, 2026January 25, 2026
25 daysJanuary 12, 2026January 7–12, 2026January 26, 2026
26 daysJanuary 13, 2026January 8–13, 2026January 27, 2026
27 daysJanuary 14, 2026January 9–14, 2026January 28, 2026
28 daysJanuary 15, 2026January 10–15, 2026January 29, 2026
29 daysJanuary 16, 2026January 11–16, 2026January 30, 2026
30 daysJanuary 17, 2026January 12–17, 2026January 31, 2026
31 daysJanuary 18, 2026January 13–18, 2026February 1, 2026
32 daysJanuary 19, 2026January 14–19, 2026February 2, 2026
33 daysJanuary 20, 2026January 15–20, 2026February 3, 2026
34 daysJanuary 21, 2026January 16–21, 2026February 4, 2026
35 daysJanuary 22, 2026January 17–22, 2026February 5, 2026

A longer cycle moves ovulation later; a shorter cycle moves it earlier. The fertile window is always 6 days long in this estimate. If ovulation leads to a pregnancy, the due date calculator can estimate the due date from your last period, and the pregnancy calculator shows the trimester dates and current week.

Ovulation day by cycle length: 21 days January 8, 24 days January 11, 28 days January 15, 32 days January 19, 35 days January 22, 2026
Figure 3. Ovulation day by cycle length (LMP Jan 1, 2026) — longer cycle, later ovulation — averagecalculatoronline.com

Worked examples (all recomputed)

Example 1 — A 28-day cycle, LMP January 1, 2026

  • Ovulation: January 1 + (28 − 14) = January 1 + 14 days = January 15, 2026
  • Fertile window: January 15 − 5 days = January 10, 2026 to January 15, 2026
  • Next period: January 1 + 28 days = January 29, 2026

Example 2 — A 30-day cycle: LMP February 10, 2026

  • Ovulation: February 10 + (30 − 14) = February 10 + 16 days = February 26, 2026
  • Fertile window: February 26 − 5 days = February 21, 2026 to February 26, 2026
  • Next period: February 10 + 30 days = March 12, 2026
Example with a 30-day cycle: last period February 10, 2026, ovulation February 26, 2026, next period March 12, 2026
Figure 4. Worked example — 30-day cycle, LMP Feb 10, 2026 → ovulation Feb 26, 2026, next period Mar 12, 2026 — averagecalculatoronline.com

Example 3 — A 24-day cycle: LMP March 5, 2026

  • Ovulation: March 5 + (24 − 14) = March 5 + 10 days = March 15, 2026
  • Fertile window: March 15 − 5 days = March 10, 2026 to March 15, 2026
  • Next period: March 5 + 24 days = March 29, 2026

Example 4 — A 35-day cycle: LMP April 18, 2026

  • Ovulation: April 18 + (35 − 14) = April 18 + 21 days = May 9, 2026
  • Fertile window: May 9 − 5 days = May 4, 2026 to May 9, 2026
  • Next period: April 18 + 35 days = May 23, 2026

Tips and mistakes

  • Use the first day of bleeding as day 1. Counting from the last day of a period, or from spotting, shifts every date on the page by several days.
  • Average several cycles. One unusual cycle (stress, illness, travel) is not your average. Add up your last 3–6 cycle lengths and divide by the number of cycles.
  • Do not rely on the 14-day rule alone if your cycles are irregular. The estimate assumes the second half of the cycle (after ovulation) is about 14 days. With irregular cycles the actual ovulation day can be several days earlier or later than the estimate.
  • The fertile window is wider than ovulation day. Conception is possible on any of the 6 days, not only on ovulation day itself, because sperm can survive for several days before the egg is released.
  • Do not use this calculator to avoid pregnancy. Calendar estimates are not reliable contraception. If you need to avoid pregnancy, talk to a doctor or other qualified health professional about methods that suit you.
  • Track more than dates. Cervical mucus, basal body temperature and ovulation test strips give additional signs, but none of them confirm ovulation on their own either.

Sources

Ovulation timing (ovulation about 14 days before the next period) follows the standard cycle model described by the American College of Obstetricians and Gynecologists (ACOG). For general pregnancy information, see MedlinePlus — Pregnancy. Reviewed by the Average Calculator Team. Last reviewed: 7 November 2026.

FAQs

How do you calculate an ovulation date?

Take the first day of your last menstrual period and add your cycle length minus 14 days: Ovulation ≈ LMP + (cycle length − 14). For example, with an LMP of January 1, 2026 and a 28-day cycle, ovulation is estimated on January 1 + 14 days = January 15, 2026.

What is the fertile window?

The fertile window is the 5 days before ovulation plus ovulation day itself — 6 days in total. For the default example (ovulation January 15, 2026) the fertile window is January 10–15, 2026. These are the days when conception is most likely.

How accurate is an ovulation calculator?

It is an estimate, not a measurement. It assumes the second half of the cycle is about 14 days and that your cycle length is stable. Real ovulation can fall several days earlier or later, especially with irregular cycles, stress, illness or recent hormonal changes. Only a clinician’s tests can confirm ovulation.

Can I ovulate on a different day than the estimate?

Yes. The estimate uses an average cycle length, so any cycle that is longer or shorter than your average moves ovulation. Even with regular cycles, the exact day varies from cycle to cycle.

What if my cycle length varies from month to month?

Use the average of your last 3–6 cycles as the input, and treat the result as a range rather than a single day. If your cycles are very irregular (for example, varying by more than 7 days), a calendar estimate is less useful and a doctor can advise on better ways to track ovulation.

Can I use this calculator to avoid pregnancy?

No. Calendar-based estimates are not reliable contraception, because ovulation can shift and sperm can survive for several days. If you want to avoid pregnancy, talk to a doctor or other qualified health professional about contraception methods.

Disclaimer

Estimates only, not a substitute for medical advice. The dates on this page are estimates calculated from the numbers you enter, using the standard calendar method (ovulation about 14 days before the next period). They do not diagnose, treat, or confirm ovulation, fertility, or pregnancy, and they are not reliable contraception. Always talk to a doctor or other qualified health professional about fertility, pregnancy, or any health concern.

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Last updated: November 7, 2026.