Ovulation Calculator
Predict your ovulation date, fertile window and next period from your last period and cycle length.
Last updated · View algorithm, constants and default inputs (JSON)
Results
About this calculator
Estimate a possible ovulation date and fertile window from a cycle model; actual timing can vary.
Worked example
calculated with the default values below
First day of last period 2026-10-17 · Average cycle length 28 days · Luteal phase length 14 days Estimated ovulation date 2026-10-31 · Fertile window starts 2026-10-26 · Fertile window ends 2026-11-01 · Next period expected 2026-11-14 · Days until ovulation 0 days
General background references
Disclaimer
These calculators are for general information only and are not a medical device, a diagnosis or a treatment plan. Reference ranges describe populations, not individuals, and do not account for pregnancy, medication, disability or athletic training. Discuss your own situation with a qualified healthcare professional.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
D_ov = D_LMP + (C − L); fertile = D_ov − 5 -> D_ov + 1; D_next = D_LMP + C
- D_LMP
- First day of last period
- C
- Cycle length (days)
- L
- Luteal phase length (days)
- D_ov
- Ovulation date
Estimated ovulation day = cycle length − user-entered luteal length after the last-period start. Fertile window is modeled as the five preceding days through one day after; combinations that place it before cycle day 1 are rejected. This calendar estimate cannot confirm ovulation or prevent pregnancy.
Frequently asked questions
Why can some cycle and luteal-length combinations be rejected?
The model estimates ovulation as cycle length minus luteal-phase length and places the fertile window five days before ovulation. If that would put the window before cycle day 1, the inputs are inconsistent with this simplified cycle model. Calendar estimates cannot confirm ovulation or prevent pregnancy.
Can this calendar estimate confirm ovulation or prevent pregnancy?
This is a calendar estimate from the cycle and luteal-phase lengths entered. Ovulation timing can vary and the result cannot confirm ovulation or provide reliable contraception. ACOG describes fertility-awareness approaches as requiring careful tracking; do not rely on this estimate alone to prevent pregnancy.