Fertile Window Calculator
See your fertile window, ovulation day and peak days from your last period, cycle length and luteal phase.
How it works
ovulation ≈ last period + (cycle length − luteal phase) · fertile window = the 5 days before ovulation through 1 day after
The fertile window is the span of days when intercourse can lead to pregnancy: the five days before ovulation (sperm can survive that long) plus the day of ovulation and about a day after (while the egg is viable). Ovulation itself is placed by subtracting your luteal phase from your cycle length, so a shorter luteal phase moves ovulation later in the cycle. Fertility is highest in the two days before and the day of ovulation.
Worked example
A last period on June 1, 2026, a 28-day cycle and a 14-day luteal phase put ovulation around June 15, so the fertile window runs June 10 to June 16, with peak days June 14–15.
Frequently asked questions
When in the window is pregnancy most likely?
In the two to three days ending on ovulation day. The days just before ovulation tend to be the most fertile because sperm are already present when the egg is released.
How do I find my luteal phase length?
It’s the number of days from ovulation to the first day of your next period — most often 12 to 14 days. Ovulation kits or basal body temperature tracking over a few cycles can reveal yours; if unsure, leave it at 14 or ask your provider.
Can I rely on this to avoid pregnancy?
No. The window shifts with ovulation, which is hard to predict exactly, so calendar timing is not reliable contraception. Speak to a clinician about effective options.
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.