Work out ovulation date instantly with clear inputs, formula shown and shareable results.
The luteal phase is the most consistent part of the cycle at around fourteen days, so ovulation is estimated by counting back from the expected next period rather than forward from the last one. A 28 day cycle from 20 August 2026 puts ovulation near 3 September, cycle day 15, with a fertile window from five days before to one day after.
Ovulation day
day = cycle length - luteal phase length
Fertile window
from five days before ovulation to one day after
Ovulation estimates from cycle averages are approximate and vary considerably between cycles. They are not reliable for contraception. For fertility or contraceptive advice, consult a healthcare professional, who may recommend ovulation testing rather than calendar estimation.
Because the luteal phase is relatively fixed while the follicular phase varies. Counting forward from the last period is less reliable in irregular cycles.
Up to five days in favourable cervical mucus, which is why the fertile window opens well before ovulation itself.