Fertility Window by Cycle Length
See which days of your cycle are fertile from your average cycle length alone — as cycle-day numbers, no dates needed.
How it works
ovulation day = cycle length − luteal phase · fertile window = the 5 days before ovulation through 1 day after · day 1 = first day of your period.
You can find your fertile days from cycle length alone, without any calendar dates, by working in cycle-day numbers where day 1 is the first day of your period. Ovulation is placed by subtracting the luteal phase (about 14 days) from the cycle length, and the fertile window spans the five days before ovulation — because sperm can survive that long — through about a day after, while the egg is viable. A longer cycle pushes ovulation to a later day number; a shorter luteal phase moves it earlier.
Worked example
On a 28-day cycle with a 14-day luteal phase, ovulation is day 14, so days 9 through 15 are fertile, peaking on days 13–14. On a 32-day cycle it shifts to ovulation around day 18, with fertile days 13–19.
Frequently asked questions
How do I count the cycle days?
Day 1 is the first day of full bleeding of your period. Every day after adds one, up to the day before your next period starts, which ends the cycle.
What if I don’t know my luteal phase?
It is usually 12 to 14 days; leaving it at 14 is a reasonable default. Ovulation kits or basal body temperature tracking over a few cycles can reveal yours.
Is this reliable if my cycles vary?
Less so. The method assumes a steady cycle length, so if yours swings by many days the fertile window shifts too. Treat it as a guide and confirm timing with ovulation tests or your provider.
Related calculators
Educational estimates from standard formulas and population averages — every pregnancy and baby is different. This is not medical advice; your doctor, midwife or pediatrician, with an examination or ultrasound, gives the accurate guidance.