Estimates ovulation date and fertile window from the start of your last period and your average cycle length.
How it works
Ovulation is estimated by counting back 14 days (the average luteal phase length) from the expected start of the next period.
What this does not include
This is an estimate based on average cycle patterns — individual cycles vary, and this does not replace tracking methods like basal body temperature or ovulation test kits for more precise timing.
How to use this calculator
- Enter the first day of your last period and your average cycle length.
A worked example
Last period started September 1, average cycle length 28 days.
Next period is expected September 1 + 28 days = September 29. Ovulation is estimated 14 days before that: September 29 − 14 days = September 15.
The fertile window spans 5 days before ovulation through 1 day after: September 10 to September 16.
What the variables mean
| Variable | Meaning |
|---|---|
| Last period start date | The first day of your most recent period |
| Average cycle length | Days from the start of one period to the start of the next |
| Luteal phase (14 days, fixed) | The relatively consistent stretch from ovulation to the next period |
Edge cases worth knowing
Irregular cycles make this estimate less reliable. The 14-day luteal-phase assumption holds best for fairly regular cycles — cycle length that varies a lot month to month widens the real uncertainty around this date.
This estimates, it doesn’t confirm. Basal body temperature tracking or ovulation predictor kits detect what’s actually happening in a given cycle, rather than projecting from an average.
Frequently asked questions
Why 14 days specifically?
The luteal phase (from ovulation to the next period) is much more consistent across cycles than the first half, averaging close to 14 days regardless of total cycle length.
Why does a longer cycle push ovulation later?
Because the extra days in a longer cycle occur before ovulation, not after — the luteal phase stays roughly the same length either way.
How accurate is this estimate?
It’s a reasonable starting estimate for average, regular cycles — actual timing can vary cycle to cycle, so tracking methods provide more precision for those relying on exact timing.
This calculator is for general informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional for guidance specific to your situation.
