BCBetter Calculators

Period Due Date Calculator

Calculate your next period date, ovulation window, and days until your next period.

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Enter your values and click Calculate

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How It Works

The calculator adds your average cycle length to the first day of your last period to project your next period start date. Ovulation is estimated by subtracting 14 from your cycle length (the luteal phase constant): ovulation day = cycle length − 14. The fertile window spans cycle days (ovulation day − 5) through (ovulation day + 1), accounting for sperm lifespan and egg viability. Days until next period is the difference in days between today and the projected next period date. All calculations assume a consistent cycle; individual cycles naturally vary by a few days each month.

Examples

Standard 28-Day Cycle
Last period started April 1, 2025 with a 28-day cycle.
Result: Next period: April 29, 2025. Ovulation window: days 9–15 (peak day 14).
Longer 32-Day Cycle
Last period started April 1, 2025 with a 32-day cycle.
Result: Next period: May 3, 2025. Ovulation window: days 13–19 (peak day 18).
Shorter 25-Day Cycle
Last period started April 10, 2025 with a 25-day cycle.
Result: Next period: May 5, 2025. Ovulation window: days 6–12 (peak day 11).

Frequently Asked Questions

How accurate is this period due date calculator?
The accuracy depends on how regular your cycles are. If your cycle length varies by more than a few days each month, the prediction will be less precise. Tracking your period start dates over several cycles and using the average length gives the most reliable estimate. Stress, illness, significant weight changes, and hormonal factors can all cause your period to arrive earlier or later than predicted.
What counts as day 1 of my cycle?
Day 1 of your menstrual cycle is the first day of full menstrual flow — not spotting. Light spotting the day before your period officially starts does not count as day 1. Counting from true day 1 is important for accurate cycle tracking, ovulation estimation, and fertility planning.
Can my period arrive earlier or later than predicted?
Yes. Even with regular cycles, periods can vary by 2–5 days in either direction. Late periods are often caused by stress, illness, intense exercise, or hormonal fluctuations. A period more than 7 days late warrants a pregnancy test if applicable, and consistently irregular cycles are worth discussing with a healthcare provider.
How is the ovulation window calculated?
Ovulation is estimated using the luteal phase constant: it occurs approximately 14 days before the next period. Ovulation day = cycle length − 14. The fertile window spans 5 days before ovulation through 1 day after, as sperm can survive up to 5 days in fertile cervical mucus and the egg remains viable for 12–24 hours after release. These are statistical averages; confirm ovulation with LH test strips for precision.

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