When will I ovulate based on my last period?

This calculator places ovulation about 14 days before your next expected period: ovulation date = LMP + (cycle length − 14). Default (LMP May 1, 2026, 28-day cycle) → ovulation May 15, 2026 (cycle day 15). A 32-day cycle (LMP June 1, 2026) → Jun 19, 2026 (day 19). A 25-day cycle → Aug 21, 2026 (day 12).

What is the fertile window and how many days is it?

Modeled as 5 days before ovulation through 1 day after — sperm can live ~5 days and the egg ~12–24 hours after release. Default → May 10, 2026 to May 16, 2026 (7 calendar days). Peak timing often cited: May 13, 2026 – May 15, 2026 (two days before through ovulation day).

How does cycle length affect ovulation day?

Ovulation cycle day = cycle length − 14 (fixed luteal assumption). Examples: 24-day → day 11; 28-day → day 15; 30-day → day 17; 32-day → day 19; 35-day → day 22. Longer cycles shift fertile days later — do not assume day 14 if your cycle is usually 32+ days.

How accurate is an ovulation calculator?

Fair for regular cycles within a few days; one study context suggests calendar methods are weaker than LH urine tests for pinpointing ovulation. Real luteal phases vary (~10–16 days), and stress, illness, or PCOS can shift ovulation. Use this for planning, then confirm with ovulation predictor kits (LH surge ~24–48 h before ovulation) or basal body temperature rise (~0.2–0.5°F after ovulation).

When am I most likely to get pregnant?

Highest conception odds are usually intercourse during the fertile window, especially 1–2 days before ovulation. Default window May 10, 2026–May 16, 2026 with peak band May 13, 2026–May 15, 2026. Every-other-day sex across the window is a common TTC approach; daily is fine if sperm parameters are normal (per ASRM-style guidance).

Can I use this calculator to avoid pregnancy?

No. Calendar rhythm methods have high typical-use failure rates compared with condoms, pills, IUDs, or implants. Ovulation can move month to month, and sperm survive up to ~5 days — you can conceive from sex before visible “fertile” signs. Use FDA-approved contraception if pregnancy prevention is the goal.

What if my periods are irregular?

If cycle length varies by more than ~7 days month to month, calendar estimates lose precision. Track 3–6 cycles to find your range, then use LH kits or BBT. Example: if cycles swing 26–34 days, ovulation might land anywhere from day 12 to day 20 — a single 28-day assumption would mislead. See a clinician if cycles are very irregular, absent, or under 21 / over 35 days persistently.

How do ovulation predictor kits (OPKs) differ from this tool?

OPKs detect luteinizing hormone (LH) surge, usually 24–48 hours before ovulation — closer to real-time than LMP math. This calculator might show ovulation on May 15, 2026 while an LH kit peaks 1–2 days earlier or later in the same cycle. Best practice for TTC: use calendar estimates to start testing OPKs ~4 days before expected ovulation, then time intercourse after a positive LH test.

How long is the egg viable after ovulation?

Roughly 12–24 hours after release. That is why the model includes only 1 day after ovulation in the fertile window. Sperm longevity (up to ~5 days) is why intercourse before ovulation still works — default fertile start is May 10, 2026 (5 days before estimated ovulation).

When is my next period expected?

Next period start = LMP + cycle length. Default → May 29, 2026 (28 days after May 1, 2026). 32-day example → Jul 3, 2026. If your period is >7 days late and pregnancy is possible, take a pregnancy test.

What is basal body temperature (BBT) tracking?

Take temperature immediately on waking before getting up. After ovulation, progesterone often raises BBT ~0.2–0.5°F (0.1–0.3°C) sustained until the next period. BBT confirms ovulation retrospectively (not in advance) — pair with calendar estimates: e.g. if this tool predicts ovulation May 15, 2026, look for a sustained rise starting within 1–3 days after.

Is this medical advice?

No. Educational calendar estimates only. Fertility treatment, postpartum cycles, perimenopause, and medical conditions need personalized care from OB-GYN, reproductive endocrinology, or primary care clinicians.

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Fertility · calendar method

Ovulation Calculator — Fertile Window & Ovulation Date

Free ovulation calculator: enter your last period (LMP) and average cycle length to estimate ovulation (cycle − 14 days), your fertile window, peak conception days, and next period. Not for birth control.

Last updated: May 25, 2026

Default 28-day cycle → ovulation May 15, 2026 (day 15)

Fertile window May 10, 2026 – May 16, 2026 (7 days)

Next period ~May 29, 2026

Tracking full cycle phases? Period tracker & ovulation

Ovulation Calculator

Estimate ovulation, fertile window, and next period from LMP + cycle length

First day of your last period (LMP)

Average cycle length

Estimated ovulation (cycle day 15)

Friday, May 15, 2026

Fertile window

May 10, 2026toMay 16, 2026

7 calendar days (5 days before ovulation through day after)

Peak timing often cited: May 13, 2026 – May 15, 2026

Next period expected

May 29, 2026

Estimated ovulation 2026-05-15 (cycle day 15). Fertile window 2026-05-10–2026-05-16 (7 days). Next period ~2026-05-29 if cycles stay 28 days. Not for birth control.

Default snapshot (LMP May 1, 2026, 28-day cycle)

Ovulation

May 15, 2026

Cycle day 15

Fertile window

May 10, 2026 – May 16, 2026

7 calendar days

Peak timing (often cited)

May 13, 2026 – May 15, 2026

2 days before through ovulation day

How the calendar method works

Ovulation is estimated as 14 days before the next period start. If your average cycle is 28 days, ovulation lands around cycle day 15 (because 28 − 14 = 14 days after LMP, plus day 1 = LMP).

Formulas used

Ovulation date = LMP + (cycle length − 14)
Fertile start = ovulation − 5 days
Fertile end = ovulation + 1 day
Next period = LMP + cycle length

Cycle length vs ovulation day

Cycle length	Ovulation cycle day	Fertile window (cycle days)	Note
24 days	Day 11	Day 6–12	Earlier fertile phase
26 days	Day 13	Day 8–14	Earlier fertile phase
28 days	Day 15	Day 10–16	Average textbook cycle
30 days	Day 17	Day 12–18	Slightly later ovulation
32 days	Day 19	Day 14–20	Slightly later ovulation
35 days	Day 22	Day 17–23	Longer cycle pattern

Worked scenarios

Average 28-day cycle

LMP May 1, 2026 — textbook day-14 ovulation

LMP May 1, 2026 · 28 days

Ovulation: May 15, 2026 (day 15)

Fertile: May 10, 2026 – May 16, 2026

Next period: May 29, 2026

Longer 32-day cycle

LMP June 1, 2026 — ovulation shifts later

LMP Jun 1, 2026 · 32 days

Ovulation: Jun 19, 2026 (day 19)

Fertile: Jun 14, 2026 – Jun 20, 2026

Next period: Jul 3, 2026

Shorter 25-day cycle

LMP August 10, 2026 — earlier ovulation

LMP Aug 10, 2026 · 25 days

Ovulation: Aug 21, 2026 (day 12)

Fertile: Aug 16, 2026 – Aug 22, 2026

Next period: Sep 4, 2026

24-day cycle

Early ovulation around cycle day 10

LMP Mar 1, 2026 · 24 days

Ovulation: Mar 11, 2026 (day 11)

Fertile: Mar 6, 2026 – Mar 12, 2026

Next period: Mar 25, 2026

35-day cycle

Late ovulation around cycle day 21

LMP Jan 15, 2026 · 35 days

Ovulation: Feb 5, 2026 (day 22)

Fertile: Jan 31, 2026 – Feb 6, 2026

Next period: Feb 19, 2026

30-day cycle

Ovulation around cycle day 16

LMP Apr 10, 2026 · 30 days

Ovulation: Apr 26, 2026 (day 17)

Fertile: Apr 21, 2026 – Apr 27, 2026

Next period: May 10, 2026

Calendar method vs LH kits vs BBT

Calendar (this tool)

Fast planning from LMP + average cycle. Best when cycles are regular.

Default fertile window spans 7 days.

LH ovulation tests

Detect hormone surge ~24–48 h before ovulation. Start testing ~4 days before estimated ovulation.

Helpful when cycle length varies month to month.

Basal body temperature

Confirms ovulation after it happens via sustained temperature rise.

Pair with calendar estimates — not a standalone forward predictor.

Factors that shift ovulation

Stress, illness, jet lag, and poor sleep can delay or advance ovulation.
PCOS, thyroid disorders, hyperprolactinemia, and perimenopause widen cycle variability.
Recent hormonal contraception stop, postpartum, or breastfeeding may pause ovulation.
If trying to conceive for 12 months (<35) or 6 months (35+) without success, seek fertility evaluation.

Limitations

Calendar method assumes a stable ~14-day luteal phase — real luteal length varies (~10–16 days)
Stress, illness, travel, PCOS, thyroid issues, and perimenopause can shift ovulation unpredictably
Not reliable as birth control — sperm survival up to ~5 days makes “safe days” estimates risky
Irregular cycles need LH tests, basal body temperature, or clinician-guided tracking
Does not replace preconception care, fertility treatment, or emergency contraception advice

References

Not birth control. Do not use fertile-day estimates to prevent pregnancy. For medical questions about irregular cycles, pain, or infertility, consult a licensed clinician.

Frequently Asked Questions

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Fertility · calendar method

Ovulation Calculator — Fertile Window & Ovulation Date

Last updated: May 25, 2026

Default 28-day cycle → ovulation May 15, 2026 (day 15)

Fertile window May 10, 2026 – May 16, 2026 (7 days)

Next period ~May 29, 2026

Tracking full cycle phases? Period tracker & ovulation

Ovulation Calculator

Estimate ovulation, fertile window, and next period from LMP + cycle length

First day of your last period (LMP)

Average cycle length

Estimated ovulation (cycle day 15)

Friday, May 15, 2026

Fertile window

May 10, 2026toMay 16, 2026

7 calendar days (5 days before ovulation through day after)

Peak timing often cited: May 13, 2026 – May 15, 2026

Next period expected

May 29, 2026

Estimated ovulation 2026-05-15 (cycle day 15). Fertile window 2026-05-10–2026-05-16 (7 days). Next period ~2026-05-29 if cycles stay 28 days. Not for birth control.

Default snapshot (LMP May 1, 2026, 28-day cycle)

Ovulation

May 15, 2026

Cycle day 15

Fertile window

May 10, 2026 – May 16, 2026

7 calendar days

Peak timing (often cited)

May 13, 2026 – May 15, 2026

2 days before through ovulation day

How the calendar method works

Ovulation is estimated as 14 days before the next period start. If your average cycle is 28 days, ovulation lands around cycle day 15 (because 28 − 14 = 14 days after LMP, plus day 1 = LMP).

Formulas used

Ovulation date = LMP + (cycle length − 14)
Fertile start = ovulation − 5 days
Fertile end = ovulation + 1 day
Next period = LMP + cycle length

Cycle length vs ovulation day

Cycle length	Ovulation cycle day	Fertile window (cycle days)	Note
24 days	Day 11	Day 6–12	Earlier fertile phase
26 days	Day 13	Day 8–14	Earlier fertile phase
28 days	Day 15	Day 10–16	Average textbook cycle
30 days	Day 17	Day 12–18	Slightly later ovulation
32 days	Day 19	Day 14–20	Slightly later ovulation
35 days	Day 22	Day 17–23	Longer cycle pattern

Worked scenarios

Average 28-day cycle

LMP May 1, 2026 — textbook day-14 ovulation

LMP May 1, 2026 · 28 days

Ovulation: May 15, 2026 (day 15)

Fertile: May 10, 2026 – May 16, 2026

Next period: May 29, 2026

Longer 32-day cycle

LMP June 1, 2026 — ovulation shifts later

LMP Jun 1, 2026 · 32 days

Ovulation: Jun 19, 2026 (day 19)

Fertile: Jun 14, 2026 – Jun 20, 2026

Next period: Jul 3, 2026

Shorter 25-day cycle

LMP August 10, 2026 — earlier ovulation

LMP Aug 10, 2026 · 25 days

Ovulation: Aug 21, 2026 (day 12)

Fertile: Aug 16, 2026 – Aug 22, 2026

Next period: Sep 4, 2026

24-day cycle

Early ovulation around cycle day 10

LMP Mar 1, 2026 · 24 days

Ovulation: Mar 11, 2026 (day 11)

Fertile: Mar 6, 2026 – Mar 12, 2026

Next period: Mar 25, 2026

35-day cycle

Late ovulation around cycle day 21

LMP Jan 15, 2026 · 35 days

Ovulation: Feb 5, 2026 (day 22)

Fertile: Jan 31, 2026 – Feb 6, 2026

Next period: Feb 19, 2026

30-day cycle

Ovulation around cycle day 16

LMP Apr 10, 2026 · 30 days

Ovulation: Apr 26, 2026 (day 17)

Fertile: Apr 21, 2026 – Apr 27, 2026

Next period: May 10, 2026

Calendar method vs LH kits vs BBT

Calendar (this tool)

Fast planning from LMP + average cycle. Best when cycles are regular.

Default fertile window spans 7 days.

LH ovulation tests

Detect hormone surge ~24–48 h before ovulation. Start testing ~4 days before estimated ovulation.

Helpful when cycle length varies month to month.

Basal body temperature

Confirms ovulation after it happens via sustained temperature rise.

Pair with calendar estimates — not a standalone forward predictor.

Factors that shift ovulation

Stress, illness, jet lag, and poor sleep can delay or advance ovulation.
PCOS, thyroid disorders, hyperprolactinemia, and perimenopause widen cycle variability.
Recent hormonal contraception stop, postpartum, or breastfeeding may pause ovulation.
If trying to conceive for 12 months (<35) or 6 months (35+) without success, seek fertility evaluation.

Limitations

Calendar method assumes a stable ~14-day luteal phase — real luteal length varies (~10–16 days)
Stress, illness, travel, PCOS, thyroid issues, and perimenopause can shift ovulation unpredictably
Not reliable as birth control — sperm survival up to ~5 days makes “safe days” estimates risky
Irregular cycles need LH tests, basal body temperature, or clinician-guided tracking
Does not replace preconception care, fertility treatment, or emergency contraception advice

References

Not birth control. Do not use fertile-day estimates to prevent pregnancy. For medical questions about irregular cycles, pain, or infertility, consult a licensed clinician.