How can you tell you re ovulating by temp?

BBT works OPK better: Basal body temperature needs a special 0.1 degree f sensitive thermometer: your temperature rises 0.5 to 1 degree f after you ovulate. Test before you get out of bed in the A.M. Ovulation predictor kits are more useful when ttc as they go positive 1-1.5 days before you ovulate, so you know your most fertile days. If you have very irregular periods don't do bbt, see an OB or a fertility md for help.