Ovulation Calculator
Find your estimated ovulation day, most fertile window and next period from the first day of your last period and your cycle length.
How it works
ovulation ≈ last period + (cycle length − 14) · fertile window = ovulation − 5 days to ovulation + 1 day · next period = last period + cycle length
The luteal phase — from ovulation to the next period — is fairly constant at about 14 days, so ovulation is estimated by counting back 14 days from the next expected period. Because sperm can survive up to about five days, the fertile window opens several days before ovulation and closes about a day after, when the egg is no longer viable. This assumes regular cycles; ovulation predictor kits or basal body temperature tracking pin the day more precisely.
Worked example
A last period on June 1, 2026 with a 28-day cycle: ovulation is estimated around June 15 (14 days in), the fertile window runs June 10 to June 16, and the next period is expected around June 29.
Frequently asked questions
Is the fertile window always about six days?
Roughly, yes. Because sperm can live about five days and the egg around a day, only about six days each cycle carry a real chance of pregnancy — the five days before ovulation and the day of ovulation itself.
What if my cycles are irregular?
Then calendar estimates are less reliable, since ovulation day shifts with cycle length. Ovulation predictor kits, tracking basal body temperature, or advice from your doctor give better timing.
Does ovulation always happen on day 14?
No — day 14 is only the average for a 28-day cycle. Ovulation typically occurs about 14 days before the next period, so it lands later in longer cycles and earlier in shorter ones.
Related calculators
Estimates based on standard formulas and average cycles — every pregnancy is different. This is not medical advice; your doctor or midwife, with an ultrasound, gives the accurate dates and guidance.