Accuracy depends on the method and the child's age. For newborns and small babies, a digital rectal thermometer gives the most reliable reading, though many parents use the under-the-arm (axillary) method with a digital thermometer for convenience. Ear (tympanic) thermometers can be accurate in older babies and children but need correct positioning. Forehead (temporal) scanners are quick but can be affected by sweat or a cold room.
Whatever method you choose, follow the device instructions, and be aware that different methods can give slightly different numbers. Do not use old-style glass mercury thermometers, which are unsafe if broken, or forehead strip thermometers, which are not accurate.
Measure when your child is calm rather than crying, and avoid taking a temperature right after a warm bath or being bundled in blankets. If a reading concerns you, recheck and consider how your child looks overall. For any fever in a baby under 3 months, contact a doctor.