Start testing based on your shortest cycle length minus 17 (for a 28-day cycle, start on day 11). Test between 10 AM and 8 PM, not with first morning urine. A positive result means the test line is as dark or darker than the control line. Have sex the day of the positive and the next two days. Cheapie strips (Easy@Home, Wondfo) at $0.25 each are just as accurate as $3 digital tests for most women.
When to Start Testing
Take your shortest cycle length and subtract 17. That's your start day. For a 28-day cycle: 28 − 17 = day 11. For a 30-day cycle: day 13. For a 26-day cycle: day 9. Starting early ensures you don't miss the LH surge, which can be brief (sometimes less than 12 hours).
How to Test
- Test between 10 AM and 8 PM. Unlike pregnancy tests, OPKs are more accurate later in the day because the LH surge typically begins in the early morning and takes several hours to appear in urine at detectable levels.
- Reduce liquid intake for about 2 hours before testing for a more concentrated sample.
- Dip the strip in a urine cup for 5–10 seconds (don't dip past the MAX line).
- Lay flat and read at 5 minutes (not before, not after 10).
Reading Results
Positive: Test line is as dark as or darker than the control line. This means your LH surge is happening and ovulation is likely 24–36 hours away.
Negative: Test line is lighter than the control line, even slightly. “Almost positive” is still negative. The LH surge produces a dramatic, unambiguous darkening.
No control line: Invalid test. Retest with a new strip.
Common OPK mistakes
- Testing with first morning urine: LH is not reliably concentrated in FMU the way hCG is
- Only testing once a day: If you have a short surge (<12 hours), you might miss it. Test twice daily (morning and evening) once you start seeing the line darken.
- Stopping sex after the positive: A positive OPK means ovulation is coming, not that it happened. Have sex the day of the positive and the next 2 days.
- Comparing today's strip to yesterday's dried strip: Dried strips darken over time. Always compare to the control line on the same strip, read at 5 minutes.
Which Brand to Choose
| Type | Examples | Cost Per Test | Best For |
|---|---|---|---|
| Cheapie strips | Easy@Home, Wondfo, Pregmate | $0.20–$0.40 | Most women — accurate, affordable, test as often as you want. Easy@Home on Amazon |
| Digital (smiley face) | Clearblue Digital | $2–$4 | Women who want no ambiguity in reading results; binary yes/no |
| Advanced digital | Clearblue Advanced | $3–$5 | Detects estrogen rise (early fertile days) + LH surge (peak); wider window |
Build Your Complete Tracking System
OPKs are just one tool. See how they fit with BBT, CM, and apps in our full comparison.
Read the Tracking Comparison