Fertility & Ovulation Calculator

How to calculate Fertility & Ovulation calculator?

This calculator uses your last period date, average cycle length, and luteal phase length to estimate ovulation, the fertile window, and the next period date.

Key formula: ovulation_day = last_period_start_date + (average_cycle_length - luteal_phase_length)

Using the Fertility & Ovulation calculator calculator: an example

Example inputs: last period start = 2025-07-01, average cycle = 30 days, luteal phase = 14 days.

Step-by-step calculation:

1. Calculate ovulation day: 2025-07-01 + (30 − 14) = 2025-07-17 (ovulation day).
2. Determine fertile window: ovulation ± 4 days → 2025-07-13 to 2025-07-21.
3. Plan intercourse during the fertile window; peak probability around 1–2 days before ovulation (e.g., 2025-07-15–2025-07-17).
4. Estimate next period: last period start + average cycle → 2025-07-01 + 30 days = 2025-07-31.

Frequently Asked Questions

How accurate are these predictions?

These are estimates based on average cycle values. Individual variation is common, so use results as a guide rather than a definitive medical prediction.

What if my cycles are irregular?

If cycle length varies significantly, predictions are less reliable. Track multiple cycles or use ovulation tests for better timing and consult a healthcare provider if irregularities persist.

Can I conceive outside the calculated fertile window?

Conception is less likely outside the fertile window but possible; sperm can survive up to five days and ovulation timing can vary, so occasional conception outside the window can occur.

Should I use ovulation tests alongside this calculator?

Yes. LH ovulation tests and tracking basal body temperature or cervical mucus can provide more precise timing and complement the calculator's estimates.