Period & Ovulation Fertility Calendar Online Free
Enter the first day of your last period, typical cycle length and period length to project the next several period start dates, estimated ovulation (cycle − 14), the fertile window (five days before ovulation through ovulation day) and a color-coded 35-day calendar of menstruation, follicular, fertile, ovulation and luteal phases. Everything is computed on-device from the calendar rules you provide — useful for planning, literacy and cycle awareness, not contraception guarantees.
Period & Ovulation Fertility Calendar In-Browser Workspace
Input data or select files to begin. 100% private, client-side execution with zero server transmission.
How to Use Period & Ovulation Fertility Calendar
- Step 1: Enter the first day of the last menstrual period (LMP start).
- Step 2: Set average cycle length (18–60 days) and period length (1–15 days).
- Step 3: Click Predict Period & Ovulation.
- Step 4: Read current cycle day/phase, estimated ovulation and fertile range.
- Step 5: Scan the 35-day grid and next period list for upcoming dates.
About Period & Ovulation Fertility Calendar
A typical anovulatory-free menstrual cycle is often modeled near 28 days with ovulation ~14 days before the next period (luteal phase fairly constant ~12–16 days). Ovulation day estimate = cycle_length − 14 days after LMP (or equivalently 14 days before the projected next start). The fertile window spans approximately the five days before ovulation plus ovulation itself, because sperm can survive up to about five days while the oocyte is viable roughly 12–24 hours. Menstruation is modeled as the first periodLen days of each cycle; the follicular phase runs until ovulation; the luteal phase runs after. The tool advances cycle index from elapsed days since LMP, then projects period starts by adding multiples of cycle_length. Phase labels in the 35-day grid: period (rose), follicular (sky), fertile (green tint), ovulation (solid green), luteal (amber). Irregular cycles, stress, illness, PCOS, breastfeeding and hormonal contraception break these assumptions — windows shift by days to weeks. Calendar methods are high typical-use failure rates for pregnancy avoidance if used alone. This calculator does not confirm ovulation (no LH, temperature or ultrasound data) and does not diagnose disorders.
Frequently Asked Questions
Is this a reliable contraception method?
No. Calendar-only estimates have high typical-use failure rates. Use clinician-backed contraceptive methods for pregnancy prevention.
Why ovulation at cycle − 14?
The luteal phase is relatively stable (~12–16 days); ovulation is counted backward from the next expected period.
What if my cycles are irregular?
Predictions become wide error bands. Track several cycles or use medical evaluation for persistent irregularity.
Does hormonal birth control fit this model?
Withdrawal bleeds on combined pills are scheduled by the pack, not spontaneous ovulation — treat those schedules differently.
Do you store my period data?
No — dates are computed in the browser and never uploaded.