Period & Ovulation Calculator — Private Calendar

The Formula

The menstrual cycle is divided into the follicular phase (from period to ovulation), ovulation (~24 hours), and the luteal phase (after ovulation until next period). The luteal phase is relatively constant at 12–16 days, while the follicular phase varies. Ovulation occurs approximately cycle length minus luteal phase days from the start of your period.

Variable Definitions

How to Use This Calculator

1. 1

   Enter the first day of your last period (YYYY-MM-DD format).

2. 2

   Enter your average cycle length (most common: 28 days).

3. 3

   Optionally enter your luteal phase length if you know it (defaults to 14 days).

4. 4

   Review your current cycle day, phase, and predicted dates for the next 3 months.

5. 5

   Use the privacy notice: all data is processed locally — nothing is stored on servers.

Common Applications

• Cycle tracking — predict upcoming period dates, ovulation, and fertile windows up to 3 months in advance for personal awareness and planning
• Fertility awareness — identify the 6-day fertile window for conception planning, with the peak fertility days being the 2 days before ovulation
• Health monitoring — track cycle regularity and identify potential concerns (cycles shorter than 21 or longer than 35 days) that may warrant medical investigation
• Lifestyle planning — plan around predicted menstrual phases for travel, events, athletic training, and symptom management including PMS and cramps

Ovulation occurs ~14 days before the next period; the fertile window spans the 6 days ending on ovulation

Understanding the Concept

The menstrual cycle consists of four phases: menstruation (Days 1–5) where the uterine lining sheds, the follicular phase (Days 1–13) where estrogen rises and an egg matures in the ovary, ovulation (~Day 14 in a 28-day cycle) when the mature egg is released, and the luteal phase (Days 15–28) where progesterone prepares the uterus for potential implantation. Ovulation — the release of a mature egg — occurs ~14 days before your next period, not 14 days after your last period. This is a common misconception. The egg survives 12–24 hours, while sperm can survive up to 5 days in fertile cervical mucus, creating a 6-day fertile window. The luteal phase is hormonally driven by progesterone and is relatively constant at 12–16 days across cycles for each individual, making it the most reliable predictor of ovulation timing. For example, if your cycle is 33 days and your luteal phase is consistently 14 days, ovulation occurs around day 19 (33 − 14 = 19), not day 14. Calendar-based tracking is most accurate for people with regular cycles (±2–3 days). For those with irregular cycles (varying by 7+ days), combining calendar methods with ovulation predictor kits (which detect the LH surge 24–36 hours before ovulation) or basal body temperature tracking (which rises 0.5–1°F after ovulation) improves accuracy. Common cycle variations can indicate health issues: cycles shorter than 21 days or longer than 35 days may warrant investigation for conditions like PCOS, thyroid disorders, or perimenopause.

Frequently Asked Questions

Sources & References