How to use Ovulation & Fertility Estimate

1. Prepare the required inputs

Estimate ovulation and the fertile window from the first period day and usual cycle length. Use the units and options beside each field. Results update as inputs change; reset returns to example inputs.

First day of last period: calendar date
Usual cycle length (days): minimum 21, maximum 35

2. Try a worked example

A cycle starting 1 January 2026 with 28 days gives estimated ovulation 15 January and next period 29 January.

3. Understand the calculation

Estimated next period = start + cycle length. Estimated ovulation = next period − 14 days. Approximate fertile window: five days before through one day after.

4. Read the assumptions

Luteal phases and cycles vary. These calendar estimates cannot identify safe days for contraception or diagnose fertility.

5. Check and compare results

Match entered values to their labels and units. Review output labels alongside the assumptions, then change one input at a time. A currency selector changes display labels without converting amounts. Use the available save or copy actions and retain the input values so the estimate can be reproduced.

6. Fix an invalid input

Complete required fields and respect the listed bounds. Check date order, age relationships and mode-specific requirements where relevant; the error message identifies invalid combinations. Reset to example inputs to recover. A valid calculation applies to its stated model and does not verify a real-world outcome.