How accurate is calendar tracking?

Useful for general patterns but not reliable for contraception. Combine with basal body temperature, cervical mucus, or ovulation kits for accuracy.

Period Calculator & Menstrual Cycle

Q: What is the average menstrual cycle length?

Typical range is 21-35 days with 28 days as the textbook average. Length varies by individual, age, stress, and hormonal contraception.

Q: When does ovulation occur?

About 14 days before the next period. The fertile window spans 5 days before ovulation through ovulation day.

Last Period Start Date

Average Cycle Length (days)

Period Duration (days)

Number of Cycles to Predict

About the Period Calculator

This calculator predicts future menstrual period dates, ovulation days, and fertile windows based on your last period start date and average cycle length. It is a practical tool for menstrual health tracking, family planning awareness, and identifying patterns in your cycle. The predictions are generated using established cycle arithmetic and the standard luteal-phase model.

How to Use

Enter the start date of your most recent period.
Input your average cycle length (typically 21–35 days; 28 is default).
Set your typical period duration in days.
Choose how many future cycles to predict and click "Calculate".

Formula / Methodology

Next Period = Last Period Start + Cycle Length

Ovulation ≈ Period Start + (Cycle Length − 14)
(Luteal phase is ~14 days before next period)

Fertile Window:
Start = Ovulation − 5 days
End = Ovulation + 1 day

The luteal phase (from ovulation to next period) is relatively fixed at around 14 days for most women, making it possible to back-calculate ovulation from cycle length. The fertile window accounts for sperm survival (up to 5 days) before ovulation.

Understanding Your Results

Next Period Date Predicted start date of your next menstruation, calculated by adding your cycle length to the last period start date. The table extends this for multiple future cycles.

Ovulation Day Estimated day of egg release — the point of peak fertility. Conception is most likely on the ovulation day itself and the two days immediately before it.

Fertile Window The 6-day window (5 days before ovulation to 1 day after) when conception is biologically possible. Highlighted in green in the prediction table.

Frequently Asked Questions

What is the average menstrual cycle length?

A typical cycle ranges from 21 to 35 days, with 28 days as the textbook average. Cycle length naturally varies between individuals and can shift with age, stress, exercise, weight changes and hormonal contraception.

When does ovulation occur?

Roughly 14 days before your next period, regardless of total cycle length. In a 28-day cycle that's day 14; in a 32-day cycle, day 18. The fertile window spans 5 days before ovulation through ovulation day, due to sperm survival.

When are irregular cycles a concern?

Occasional variation is normal. See a gynaecologist if cycles regularly fall outside 21–35 days, are missing for 3+ months without pregnancy, last longer than 7 days, or are unusually heavy. PCOS, thyroid issues, or stress are common causes.

How accurate is calendar-based tracking?

Calendar tracking is useful for predicting general patterns but not reliable for contraception or precise fertility timing. Combine with basal body temperature and cervical mucus tracking, or use ovulation predictor kits, for higher accuracy.

Not a Contraception Tool

Cycle lengths vary due to stress, illness, hormonal changes and other factors. Predictions are estimates and cannot account for individual variation. This calculator is not a reliable method of contraception. Consult a gynaecologist for fertility treatment, irregular cycles, or contraception guidance.

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