How accurate is the period prediction?

Predictions are estimates based on an assumed regular cycle. Actual dates can vary due to stress, illness, hormonal changes, and other factors. The more consistent your cycle, the more accurate the prediction.

When does ovulation occur?

Ovulation typically occurs 14 days before the start of the next period, regardless of cycle length. For a 28-day cycle, that is around day 14. For a 35-day cycle, it is around day 21.

Can I use this calculator for birth control?

No. This calculator is for informational purposes only and is not a reliable form of contraception. Natural family planning methods require careful tracking over many cycles and guidance from a healthcare professional.

What if my cycle is irregular?

This calculator assumes a regular cycle length. If your periods are irregular, predictions will be less reliable. Track your cycle over several months to identify patterns, and consult a gynecologist if irregularity is a concern.

Period & Menstrual Cycle Tracker

What is a Menstrual Cycle Tracker?

A menstrual cycle tracker predicts when your next period will arrive, when ovulation is expected, and when your fertile window falls. Understanding your cycle helps you plan for pregnancy, recognize irregularities, and track your reproductive health.

A typical menstrual cycle lasts between 21 and 35 days, with 28 days being the commonly cited average. The cycle begins on the first day of your period and ends the day before your next period starts.

Key dates this calculator predicts:

Next period date: Last period date + cycle length
Ovulation date: Next period date − 14 days (based on the standard 14-day luteal phase)
Fertile window start: Ovulation date − 5 days (sperm can survive up to 5 days)
Fertile window end: Ovulation date + 1 day (egg is viable for ~24 hours)

The 14-day luteal phase is the phase between ovulation and menstruation. This phase is relatively constant across women, which is why ovulation is calculated backward from the expected next period rather than forward from the last period.

How to Use the Menstrual Cycle Calculator

Enter the first day of your last period: Input the date your most recent period started.
Enter your cycle length: The number of days from the first day of one period to the first day of the next. If unsure, 28 is a good starting point.
Enter your period length: How many days bleeding typically lasts (2–10 days).
Review your results: The calculator shows your next period date, ovulation date, fertile window, and days until your next period.

For the most accurate predictions, track your cycle for at least 3 months to establish a personal average cycle length.

Examples

Example 1: Standard 28-Day Cycle

Last period: January 1, 2024
Cycle length: 28 days
Next period: January 29, 2024 (January 1 + 28 days)
Ovulation: January 15, 2024 (January 29 − 14 days)
Fertile window: January 10–16, 2024 (January 15 ± 5/+1 days)

Example 2: 35-Day Cycle

Last period: January 1, 2024
Cycle length: 35 days
Next period: February 5, 2024 (January 1 + 35 days)
Ovulation: January 22, 2024 (February 5 − 14 days)
Fertile window: January 17–23, 2024 (January 22 −5 to +1)

Example 3: 21-Day Short Cycle

Last period: March 1, 2024
Cycle length: 21 days
Next period: March 22, 2024 (March 1 + 21 days)
Ovulation: March 8, 2024 (March 22 − 14 days)
Fertile window: March 3–9, 2024 (March 8 −5 to +1)

FAQ

How many days is the fertile window? The fertile window spans 6 days: the 5 days before ovulation and the day of ovulation itself. This accounts for the fact that sperm can survive in the female reproductive tract for up to 5 days, while the egg can be fertilized for only about 12–24 hours after ovulation.

Can I use this calculator to avoid pregnancy? No. This calculator is not a contraceptive tool. Natural cycle-based methods require much more detailed tracking, confirmatory signs (basal body temperature, cervical mucus), and professional guidance. Always use reliable contraception.

What factors make my cycle irregular? Many factors can affect cycle length including: significant weight changes, intensive exercise, stress, thyroid disorders, polycystic ovary syndrome (PCOS), perimenopause, certain medications, and travel across time zones. If your cycle is consistently irregular, consult a healthcare provider.

Why does ovulation happen 14 days before the next period? The luteal phase — from ovulation to the next menstruation — is typically 14 days in length and is relatively constant. In contrast, the follicular phase (from menstruation to ovulation) varies in length and is responsible for most cycle-to-cycle differences.

Is this calculator accurate for cycle lengths other than 28 days? Yes. The calculator adjusts for any cycle length between 21 and 35 days. For a 35-day cycle, ovulation occurs around day 21 (not day 14 as in a standard cycle). The luteal phase remains fixed at 14 days regardless of total cycle length.