OPK Ovulation Calculator

Start testing 3-4 days before your expected ovulation. For a 28-day cycle (ovulation ~day 14), start testing on day 10-11. For a 30-day cycle, start on day 12-13. Test once daily with afternoon urine.

OPKs detect luteinizing hormone (LH) in urine. LH surges 24-36 hours before ovulation. A positive OPK (test line as dark or darker than control) means ovulation is likely within 24-36 hours — have intercourse that day and the next.

Rarely, but possible with PCOS (multiple LH surges without ovulation), certain medications (hCG, clomiphene), and perimenopause. Some women have naturally high baseline LH that makes interpretation difficult. Digital OPKs reduce ambiguity.