Breakdown
How it works
Ovulation occurs about 14 days before the next period (the luteal phase is fairly constant), so for a 28-day cycle it is day 14, for 32 days it is day 18. Sperm survive up to five days, the egg about one, so the fertile window is the five days before ovulation and the day itself.
Formula
Ovulation = LMP + cycle length − luteal phase
Fertile window = ovulation − 5 days to ovulation + 1
Next period = LMP + cycle length
Worked example
LMP 1 October, 28-day cycle: ovulation 15 October; fertile 10–16 October; next period 29 October.
Frequently asked questions
My cycles are irregular — can I use this?
Use your average, but expect wide error. Ovulation predictor kits (LH strips) are more reliable for irregular cycles.
When can I test for pregnancy?
From the day of the missed period for a reliable result; some tests detect 4–5 days earlier with more false negatives.
Does this work as contraception?
No. Calendar methods alone have a high failure rate.