Fertile Window Calculator
Estimate fertile window from your measurements.
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What this tool does
Estimate fertile window from your measurements. Calendar estimate, not contraception, confirmed pregnancy dating or fertility diagnosis. Irregular cycles and variable luteal phases reduce accuracy.
How to use the Fertile Window Calculator
- Enter or select first day of last menstrual period (yyyy-mm-dd).
- Enter or select typical cycle length (days).
- Read the calculated result; change any measurement to compare alternatives.
Formula
due date=LMP+280+(cycle−28) days; next period=LMP+cycle; estimated ovulation=next period−14 days; fertile window=ovulation−5 through ovulation+1
- date
- First day of last menstrual period (YYYY-MM-DD)
- cycle
- Typical cycle length (days)
Calendar estimate, not contraception, confirmed pregnancy dating or fertility diagnosis. Irregular cycles and variable luteal phases reduce accuracy.
Worked example
For fertile window calculator, the following measurements illustrate the exact method: First day of last menstrual period (YYYY-MM-DD): 2026-10-01; Typical cycle length (days): 28.
Inputs
- First day of last menstrual period (YYYY-MM-DD)2026-10-01
- Typical cycle length (days)28
Result
- Estimated fertile window2026-10-10 through 2026-10-16
Results explained
- Estimated fertile window
- Estimated fertile window from the formula above. Calendar estimate, not contraception, confirmed pregnancy dating or fertility diagnosis. Irregular cycles and variable luteal phases reduce accuracy.
Frequently asked questions
due date=LMP+280+(cycle−28) days; next period=LMP+cycle; estimated ovulation=next period−14 days; fertile window=ovulation−5 through ovulation+1
Calendar estimate, not contraception, confirmed pregnancy dating or fertility diagnosis. Irregular cycles and variable luteal phases reduce accuracy.
No. These are general-information estimates, not diagnosis, treatment, fertility advice or exercise clearance. Consult a qualified health professional.
Individual physiology, measurement technique and health conditions may cause large differences from model estimates.
No. BAC estimates never establish that driving is safe or legal; do not drive after drinking.