OPK to Ovulation Calculator
OPK to Ovulation Calculator
Turn your ovulation predictor kit results into an estimated ovulation window, fertile window, and best days to try — or figure out when to start testing in the first place.
Not sure when to start testing?
Estimate a testing start date from your last period and typical cycle length.
Based on cycle length alone, before any test results — treat it as a starting point, not a guarantee. Once you get a positive, use the calculator below instead.
Got a positive? Calculate your window
Enter the date and time of your first positive (or peak) result.
This fills in automatically once you log a positive result in the table below — or enter it directly here.
Test log & timeline
Log each day’s result to see your whole testing window at a glance — the surge is usually easy to spot once it’s plotted.
| Date & time tested | Result |
|---|
Getting a clean read on your OPK
| Event | Typical timing |
|---|---|
| LH surge to ovulation | 24 – 36 hours (range 12 – 48h) |
| Egg viability after release | 12 – 24 hours |
| Sperm survival in fertile mucus | Up to 5 days |
| Best window to start testing | ~4 days before predicted ovulation |
- Test around the same time each day — many people find mid-morning to early evening works better than first-morning urine, since overnight urine can dilute LH.
- Cut back on fluids for about two hours before testing so the sample isn’t overly diluted.
- Once you see a clear positive, you can usually stop testing that cycle — the surge has already happened.
- If you test daily for a full 10 days without ever seeing a positive, that’s worth mentioning to your provider.
About the numbers: ovulation window is calculated as 24–36 hours after the logged surge time. Fertile window spans from 5 days before the ovulation estimate through 1 day after it, reflecting typical sperm and egg survival times.
Nothing entered here is saved, transmitted, or stored — all calculations run locally in your browser.