Ovulation Calculator

Estimate ovulation date and fertile window based on your last period and cycle length. See 3 cycles ahead.

Last updated: April 2026 · Source: NHS — How to get pregnant

Disclaimer

This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.

How It Works

Ovulation typically occurs 12–16 days before the start of your next period. For a 28-day cycle, this means around day 14. The fertile window spans approximately 6 days: the 5 days before ovulation (sperm can survive up to 5 days) plus the day of ovulation itself (the egg survives 12–24 hours).

This calculator estimates your ovulation day and fertile window based on your cycle length and the first day of your last period. For regular cycles, it counts back 14 days from the expected next period. For irregular cycles, it provides a broader estimated range and recommends additional tracking methods.

The NHS advises that the most accurate way to confirm ovulation is with ovulation predictor kits (OPKs), which detect the LH surge 24–36 hours before ovulation. Basal body temperature tracking and cervical mucus monitoring can also help. If you have been trying to conceive for 12 months (or 6 months if over 35), the NHS recommends speaking to your GP.

Example: Last period 1 April 2026, 30-day cycle

  1. Expected next period: 1 May 2026
  2. Estimated ovulation: 1 May − 14 days = 17 April 2026
  3. Fertile window: 12–17 April 2026
  4. Most fertile days: 15–17 April 2026

Source: NHS — How to get pregnant

Frequently Asked Questions

What does the Ovulation Calculator do?
Estimate ovulation date and fertile window based on your last period and cycle length. See 3 cycles ahead. All calculations are performed in your browser using official UK rates and thresholds.
Is this medical advice?
No. This calculator is for informational purposes only and does not constitute medical advice. Always consult your GP or an appropriate healthcare professional for medical guidance.
Are the reference values from the NHS?
Where applicable, this calculator uses reference values and guidelines from the NHS and other UK health authorities. Individual needs may vary based on personal health circumstances.