Period Calculator

What Is a Period Calculator?

Tracking your cycle takes the guesswork out of planning — whether you're trying to conceive, avoid pregnancy naturally, or simply know when to expect your period. Most menstrual cycles run between 21 and 35 days, with 28 days being the most common average. But average doesn't mean universal, and your personal cycle length is what actually matters.

The fertile window — the days when conception is most likely — typically runs from around day 10 to day 17 of a 28-day cycle, with peak fertility at ovulation on approximately day 14. Sperm can survive for up to 5 days, so the window starts before ovulation. In longer or shorter cycles, these dates shift accordingly — a 35-day cycle has ovulation around day 21, not day 14.

Enter the first day of your last period, your typical cycle length, and how long your period usually lasts. The calculator projects your next three cycles with predicted start and end dates, fertile windows, and estimated ovulation dates for each.

How Do You Use This Period Calculator?

Enter the first day of your last period, your average cycle length in days, and your average period duration. Click Calculate to see your next three predicted periods, fertile windows, and ovulation dates.

1. Enter the first day of your most recent period using the date picker.
2. Set your average cycle length in days (21–45, with 28 as the default).
3. Enter your typical period duration in days.
4. Click Calculate to see the next three cycle predictions.
5. Each cycle shows the period start and end, fertile window, and ovulation estimate.
6. Track your actual dates to refine your average cycle length over time.

How Does the Period Calculator Formula Work?

The formula used: Next period = last period start + cycle length. Ovulation ≈ cycle length − 14 days before next period. Fertile window ≈ days 10–17 of cycle.

What Are Some Example Calculations?

Last period: 1 March. Cycle length: 28 days. Period length: 5 days. Next period: 29 March (ends 2 April). Fertile window: 10–17 March. Ovulation: ~14 March.

When Should You Use a Period Calculator?

When planning around your cycle — for travel, important events, or athletic performance. Hormonal fluctuations affect energy levels, mood, and recovery throughout the month, and many women plan training blocks around their cycle phases.

For natural family planning. This calculator gives estimates based on cycle averages — if you're relying on natural methods for contraception, use it alongside daily basal body temperature tracking or ovulation tests for greater accuracy. Cycle length varies, and a predicted ovulation date can shift by several days.

What Do These Terms Mean?

What Are the Best Tips to Know?

• Track your actual period start dates for 3–6 months to get a reliable average cycle length.
• Stress, illness, travel, and significant weight changes can all shift your cycle temporarily.
• Ovulation predictor kits (OPKs) confirm ovulation more accurately than date calculations.
• The luteal phase (ovulation to next period) is consistently around 14 days — it's the follicular phase that varies.

What Mistakes Should You Avoid?

• Using the end of your last period rather than the start as the reference date.
• Assuming a 28-day cycle when yours is consistently longer or shorter.
• Treating predicted dates as exact — they are estimates based on averages.