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Ovulation & Fertility Calculator

Estimate your likely ovulation day and most fertile window from the first day of your last period and your average cycle length.

FreeNo sign-upPrivate — runs in your browser, nothing is uploaded

Enter the first day of your last period to estimate your fertile window.

Estimates assume a regular cycle and ovulation about 14 days before the next period. Real cycles vary, so this is general information for planning or awareness, not contraception or medical advice. Computed in your browser.

Estimate only. This tool is for general information only and is not medical advice or a diagnosis. Results are estimates based on the formula shown. Consult a qualified healthcare professional for guidance about your own situation.

Quick answer

Ovulation usually happens about 14 days before your next period starts. From the first day of your last period (LMP) and your cycle length, the estimated ovulation day is LMP + (cycle length − 14). The most fertile window is roughly the five days before ovulation through the day after. For a 28-day cycle starting June 1, ovulation is about June 15 and the fertile window is about June 10–16. These are estimates and assume a regular cycle.

Formula & method

The tool uses the common calendar method: ovulation ≈ LMP + (cycle length − 14 days), based on a luteal phase of about 14 days. The fertile window spans the five days before ovulation plus ovulation day and the day after, because sperm can survive several days and the egg about a day. Your next period is estimated as LMP + cycle length.

Examples

Example 1: 28-day cycle
Input
LMP June 1, cycle 28
Result
Ovulation ~June 15
Why
28 − 14 = 14, so ovulation is about 14 days after the period starts.
Example 2: Fertile window
Input
Same inputs
Result
~June 10 – June 16
Why
Five days before ovulation through the day after.
Example 3: Longer cycle
Input
LMP June 1, cycle 32
Result
Ovulation ~June 19
Why
32 − 14 = 18 days after the LMP.

When to use this tool

  • Estimating your fertile window when trying to conceive.
  • Understanding roughly when ovulation occurs in your cycle.
  • Planning around your next expected period.

Common mistakes

  • Assuming ovulation is always day 14 — it's about 14 days before the next period, which shifts with cycle length.
  • Relying on this for birth control; the calendar method is not reliable contraception.
  • Expecting precision from an irregular cycle, where calendar estimates are much less accurate.

Frequently asked questions

When do I ovulate?

Typically about 14 days before your next period. For a 28-day cycle that's around day 14; for longer or shorter cycles it shifts accordingly (cycle length minus 14).

What is the fertile window?

The roughly six days ending on ovulation day: the five days before (sperm can survive several days) plus ovulation and about a day after, when the egg is viable.

Can I use this as birth control?

No. Calendar-based estimates are not reliable contraception, especially with irregular cycles. Use a proven method and consult a professional.

What if my cycle is irregular?

Estimates become much less accurate with irregular cycles. Ovulation predictor kits or tracking basal body temperature give better signals.

Is my data uploaded?

No. The dates are calculated entirely in your browser; nothing you enter is sent or stored.

Is this medical advice?

No. It's general information for planning and awareness. For fertility or contraception decisions, consult a healthcare professional.

Sources & references

External references open in a new tab. We are independent and not affiliated with these organizations.

Disclaimer

This tool is for general information only and is not medical advice or a diagnosis. Results are estimates based on the formula shown. Consult a qualified healthcare professional for guidance about your own situation.

  • ✓ Free to use
  • ✓ No sign-up required
  • ✓ Runs entirely in your browser — nothing is uploaded.
  • ✓ Formula and method shown above
Last updated: How we calculateReport a correctionDisclaimer

Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.

Built and reviewed by HIFreeTools against the formula shown above and any authoritative references cited on this page. See our methodology and editorial standards.

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