Enter the first day of your last period, average cycle length, and luteal phase length to estimate ovulation day, fertile window, and next period.
Estimated ovulation—
Fertile window starts—
Fertile window ends—
Next period (estimate)—
Days until ovulation—
Fertility status today—
Due date if conceived this cycle—
How this calculation works
Ovulation is counted backward from the next period because the luteal phase is usually stable (~14 days) while the follicular phase varies.
Fertile window = 5 days before ovulation (sperm survival) through 1 day after (egg survival). Peak fertility is ovulation day.
This is a calendar estimate for planning awareness only — it is not contraception, diagnosis, or medical advice.
Worked example
Last period March 1, 28-day cycle, 14-day luteal → ovulation March 15, fertile March 10–16, next period March 29.
How this estimate works
The tool subtracts your luteal length from the projected next period to place ovulation, then expands a fertile window around that day.
When to see a clinician
If cycles are very irregular, you are trying to conceive for 6–12 months without success, or you have pain/bleeding concerns, get personalized medical advice.
Frequently asked questions
How accurate is the calendar method?
Useful for planning when cycles are regular. Irregular cycles, illness, stress, and PCOS reduce accuracy. Track basal temperature or LH strips for tighter timing.
Can I use this as birth control?
No. Calendar estimates alone are not reliable contraception. Talk to a clinician about evidence-based methods.
What is luteal length?
Days from ovulation to the next period. Most people are near 12–14 days. If you know yours from past charts, enter it; otherwise leave 14.