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Ovulation & Period Calculator

Explore approximate cycle dates and a calendar fertile window for an entered regular cycle length.

Your numbers

Make it your own
Updates as you type
Approximate ovulation date
2026-01-14

Calculated from your inputs · rounded for display

Approximate fertile window2026-01-09 to 2026-01-14
Estimated next period2026-01-29
Entered period end2026-01-05

Calendar approximation for regular cycles; actual ovulation can vary substantially. Not contraception, a fertility diagnosis or a probability of pregnancy. Date inputs stay in this browser unless you explicitly share them.

Your calculation, step by step

No black boxes
  1. Cycle day 1 is 2026-01-01.
  2. Calendar convention: ovulation cycle day = 28 − 14; date offset is cycle day − 1.
  3. Approximate fertile window is the five days before ovulation plus ovulation day.

How to use this calculator

  1. Enter your own values in the labeled fields. Check the units and assumptions before comparing results. Only the fields needed for your selected calculation mode are shown.
  2. Your answer updates automatically. Review the calculation steps to see exactly how it was calculated.
  3. Copy, share, save, or download your result. Saved inputs stay on this device.

How the calculation works

Ovulation cycle day≈cycle length−14; date offset=cycle day−1

The first day of bleeding is cycle day one. The calendar method assumes ovulation around cycle day length minus 14, and a six-day fertile window ending on that day. Actual timing varies. This tool is not contraception, a fertility diagnosis or a pregnancy probability estimate. Review the units, period and assumptions before comparing scenarios. Displayed values are rounded, while the underlying calculation retains its available precision.

A worked example

For January 1, 2026 as cycle day one and a 28-day cycle: approximate ovulation is January 14, fertile window January 9–14 and next period January 29.

Reading your result

Use the main result together with its units and supporting figures. Display values are rounded; calculations keep their underlying precision. A change in inputs or assumptions can change the answer. Compare the step-by-step working with your intended use before relying on an estimate.

Frequently asked questions

What assumptions does this calculation use?

The first day of bleeding is cycle day one. The calendar method assumes ovulation around cycle day length minus 14, and a six-day fertile window ending on that day. Actual timing varies. This tool is not contraception, a fertility diagnosis or a pregnancy probability estimate.

Can I save or check a scenario?

The result includes the working and supporting values. Save and Restore keep inputs on this device; Copy, Download and Print export the result. Sharing a scenario explicitly includes the inputs in its link.

Sources & methodology

ASRM: natural fertility and calendar limitations ↗

Formula review: October 24, 2026. Read our methodology.