Most iPhones can measure elevation changes using a built-in pressure sensor

Most iPhones since the iPhone 6 have included a barometer, which helps estimate changes in elevation by measuring air pressure. The sensor matters because it supplies data used by features such as stair and step tracking, and can support fitness and outdoor apps, though it cannot distinguish a climb from changing weather on its own.

Apple does not provide a dedicated barometer app, and its Weather app gets pressure readings from external sources rather than the phone’s sensor. To see the built-in sensor’s measurements, use an app that explicitly supports it; examples include Barometer & Altimeter & Thermo and My Altitude, which can also draw on GPS or weather data for calibration. The first-generation iPhone SE is an exception to the otherwise broad availability of the sensor.