Period & Ovulation Predictor

Enter your last period, cycle length and period duration. See predictions on a calendar you can browse month by month. Perfect for visual cycle tracking.

First day of your most recent period
Typical range: 21–35 days
How long your bleeding usually lasts
? Regular 28d cycle (LMP: Mar 15)
⚡ Short cycle 24d (LMP: Mar 10)
? Long cycle 35d (LMP: Mar 1)
? Irregular reference 30d (LMP: Apr 1)
100% private: All calculations are performed locally. No data leaves your device.

? Understanding your menstrual cycle & ovulation

The menstrual cycle is a natural, recurring process that prepares the female body for pregnancy. The period calculator uses the standard Knaus–Ogino method (calendar-based) to estimate ovulation: ovulation typically occurs 14 days before the next menstrual period. This tool applies clinically accepted formulas to predict fertile windows, next period, and cycle phases. The interactive calendar with month navigation allows you to visually explore predicted cycles across multiple months.

Clinical reference: The American College of Obstetricians and Gynecologists (ACOG) states that the ovulatory phase lasts about 6 days (“fertile window”) — the 5 days before ovulation and the day of ovulation. The luteal phase is relatively fixed (12–16 days). Our calculator assumes a 14‑day luteal phase for prediction.

⚙️ How this calculator works

  1. LMP input — first day of your last menstrual period.
  2. Cycle length — number of days from one period to the next (average 28 days).
  3. Period duration — typical bleeding length used for phase visualisation.
  4. Prediction logic: Next period date = LMP + Cycle length. Ovulation = Next period − 14 days. Fertile window = Ovulation − 5 days to Ovulation + 1 day.
  5. The calendar highlights menstruation, fertile days, ovulation, and next period for any month you browse.

? Cycle phases & hormonal insights

  • Menstrual phase (days 1–5+): Shedding of uterine lining; estrogen and progesterone low.
  • Follicular phase (variable): Follicle-stimulating hormone (FSH) rises, preparing an egg. Ends at ovulation.
  • Ovulation (≈ day 14 in 28d cycle): LH surge triggers egg release; highest conception probability.
  • Luteal phase (fixed ≈14 days): Progesterone stabilises endometrium; if no implantation, period begins.
Case example — Cycle tracking for family planning

A person with 30‑day cycles and LMP on April 1 receives predictions: next period April 30, ovulation around April 16, fertile window April 11–17. Using the month navigation, she can view April, May, or later months to see future fertile windows and periods. The calendar makes planning intuitive.

? Scientific basis & accuracy

Estimates rely on population averages: ovulation occurs 14 days (±2) before menses. The fertile window spans 6 days due to sperm survival (up to 5 days) and ovum viability (24h). According to a 2019 study in Human Reproduction, calendar methods have a typical-use effectiveness of ~76–88% when avoiding pregnancy. Our tool is intended for educational & awareness purposes, not as a contraceptive method. For medical conditions such as PCOS or irregular cycles, clinical consultation is essential.

Calculations:
Next Period = LMP + Cycle_Length
Ovulation Day = Next_Period − 14 days
Fertile Window Start = Ovulation − 5 days
Fertile Window End = Ovulation + 1 day

❌ Common myths vs. facts

  • Myth: Ovulation always occurs on day 14 of every cycle. Fact: Only true for perfect 28‑day cycles; many women ovulate earlier or later.
  • Myth: You cannot get pregnant during your period. Fact: Low probability but possible, especially with short cycles.
  • Myth: Cycle length never changes. Fact: Stress, illness, lifestyle, and age can alter cycle length.

? When should you consult a healthcare provider?

Irregular cycles (cycles varying >7 days), missed periods (not pregnant), severe pain, or abnormal bleeding deserve medical attention. Use this period calculator as a reference, but listen to your body and track physical symptoms for more accurate ovulation prediction.

Reviewed by evidence‑based gynecology references – This tool follows guidelines from the World Health Organization (WHO) family planning module and standard reproductive physiology textbooks (Williams Gynecology, 4e). Last update: April 2026.

? Frequently Asked Questions

The calculator gives an estimate based on the average cycle length you provide. For irregular cycles (variation >7 days), predictions are less reliable. We recommend tracking multiple cycles and consulting an app with longer history or a healthcare provider.

For regular cycles, the fertile window estimation is clinically reasonable. However, ovulation day can shift by 1–2 days. To increase accuracy, combine with basal body temperature (BBT) tracking or LH test strips.

No. This is an educational predictor; it should not replace medical contraception. Fertility awareness methods require specific training and daily charting.

Use the left/right arrows next to the month name to browse any month. All predicted dates (period, fertile window, ovulation, next period) are automatically highlighted based on your last period and cycle settings.
References: ACOG (2023) “Fertility Awareness-Based Methods of Family Planning”; Wilcox AJ et al. (N Engl J Med 1995); WHO Medical Eligibility Criteria for Contraceptive Use.