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Women’s Health · Cycle awareness

Menstrual Cycle & Ovulation Estimator Calculator

Estimate your next period, ovulation day, and fertile window.

Your details

First day of your last period

This is a cycle-based estimate — not a diagnostic test and not a contraceptive method. Individual cycles vary; use it as a rough guide.

Your cycle length
28days
< 21 days≥ 36 days
Typical adult cycle range

A regular 21-35 day cycle sits within the ACOG/FIGO adult reference range. Cycle length is one of several markers of a healthy ovulatory rhythm.

Fertile window (6 days)
Wed, 26 Aug 2026 — Mon, 31 Aug 2026
ACOG: 5 days before ovulation + ovulation day.
Estimated ovulation
Mon, 31 Aug 2026
in 13 days
Estimated next period
Mon, 14 Sep 2026
in 27 days
The following cycle
Ovulation Mon, 28 Sep 2026 · Period Mon, 12 Oct 2026

Cycle-based estimates. Not a diagnostic test. Not a contraceptive method. Individual variation of ±2-3 days is common even in regular cycles.

How the estimator works

Given your last menstrual period (LMP) and your average cycle length, we estimate ovulation ≈ LMP + (cycle_length − 14) days. The luteal phase (ovulation → next period) is remarkably stable at 12-16 days across most people, so the variable part of the cycle is the follicular phase before ovulation.

The 6-day fertile window

Sperm can survive in the female reproductive tract for up to 5 days. Combined with the day of ovulation, this creates a 6-day window (5 days before + ovulation day) during which conception is possible. Peak fertility is on the day before and the day of ovulation — endorsed by ACOG 2017.

Why this is not a contraceptive method

Cycle-based fertility awareness has a first-year unintended-pregnancy rate around 12-24% with perfect use and much higher with typical use. Reliable contraception (condoms, IUD, pill, implant, etc.) is a different conversation with a clinician.

When cycles are irregular

If cycles vary by more than ~7 days month-to-month, an average is a rough guide only. Ovulation prediction kits (LH strips), basal body temperature charting, or fertility monitoring apps that use multiple signals are more accurate for irregular cycles.

Limitations & caveats

  • Assumes a luteal-phase length of 14 days — this varies 12-16 days in most people.
  • Requires a reasonably regular cycle. Not accurate for cycles varying by more than 7 days month-to-month.
  • Not a contraceptive method — typical use failure rate is high.
  • Not appropriate during breastfeeding, perimenopause, or within 3 months of stopping hormonal contraception.
FAQ

Frequently asked questions

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Clinical Interpretation

Typical adult cycle rangeNormal

A regular 21-35 day cycle sits within the ACOG/FIGO adult reference range. Cycle length is one of several markers of a healthy ovulatory rhythm.

Common causes
  • •Consistent with a regular ovulatory cycle in most adults
Lifestyle considerations
  • •Track your cycle — patterns are one of the most sensitive early signals of health changes
  • •Protect sleep and manage stress — both influence cycle regularity
  • •Maintain iron-rich intake, especially if periods are heavy — see our Iron Deficiency Likelihood Screener
When to consult a healthcare professional

No specific action needed based on cycle length alone. Discuss any changes in flow, pain, or regularity with a clinician.

Medical Evidence

How we ground the Menstrual Cycle & Ovulation Estimator

Grade A · Clinical guidelines & meta-analysesLast reviewed Feb 12, 2026
Named clinical guidelines
  • ACOG Committee Opinion 651 — Menstruation as a Vital Sign (2015, reaffirmed 2020)
  • ACOG Committee Opinion 762 — Optimizing Natural Fertility (2017, reaffirmed 2022)
  • FIGO 2018 — Normal and abnormal uterine bleeding (FIGO AUB System 2)
  • WHO — Reproductive health indicators
References

The Evidence Foundation

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Elements84 brings reviewed health evidence, practical decision tools and plain-English explanations together without presenting educational guidance as diagnosis or treatment.

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