Estimate the likely conception window and fertile days from your last period.
How this calculator arrives at its answer
Enter your last period and cycle length to estimate your fertile window and likely conception timing. Cycles vary, so treat it as a general estimate.
Step by step
- Enter your figures in the fields provided.
- The result updates as you type.
- Review the breakdown below the main number.
The formula
ovulation ≈ last period + (cycle length − 14) days
Try it with these numbers
As an example, with a cycle length of 28, the calculator returns a estimated ovulation of Aug 15. Underneath, Fertile window comes out at Aug 10 – Aug 16 and Next period at Aug 29. Change any field and every figure updates as you type.
Questions people ask about this
When does conception happen?
Usually around ovulation, roughly 14 days before the next period, within the fertile window.
Is this reliable?
It’s an estimate based on average timing. Tracking methods give a clearer picture.