Period Calculator

1. Enter your last period start date. Use the first day of your most recent period, meaning the first day of actual bleeding, not spotting.
2. Enter your cycle length. This is the number of days from the first day of one period to the first day of the next. The average is 28 days, but 21 to 35 days is considered normal. Track a few cycles to find your average.
3. Enter your period duration. How many days does your period typically last? The average is 5 days, with 2 to 7 days being normal.
4. Read your results. The table shows your next 6 period start dates, end dates, fertile windows, and estimated ovulation days.

The calculator applies simple cycle math to project forward from your last known period:

Next period start = Last period start + (Cycle length × N)
Period end        = Period start + (Duration - 1 days)
Ovulation day     = Period start + (Cycle length - 14 - 1 days)
Fertile window    = Ovulation day - 5 days to Ovulation day + 1 day

Why cycle minus 14? The luteal phase (from ovulation to menstruation) is fairly constant at 14 days across most women. The follicular phase (from period to ovulation) varies. So ovulation is estimated by counting back 14 days from the expected next period.

Worked example with a 28-day cycle:

• Last period: April 1
• Next period: April 1 + 28 = April 29
• Ovulation: April 29 - 14 - 1 = April 14
• Fertile window: April 9 to April 15

For a 32-day cycle: Ovulation shifts to day 18 (32 - 14 = 18), so the fertile window moves later compared to a 28-day cycle.