Call *3001#12345#* from your iPhone.

The Field Test screen will appear. The signal icon in the upper left of the screen will now display the signal in a decibel (dB) reading instead of dots or bars (in older iPhones). A full signal is -51 and -105 is effectively no signal. The number is negative and represents the distance between the phone and tower antennas. It is the amount of power (and battery) your phone needs to send its signal.

The Field Test screen gives information about your cellular environment. Regions of the world use different frequency bands for cellular services. You can see the 3G band in use by selecting UMTS Cell Environment, then UMTS RR Info. The screen displayed will list the uplink (UL) and downlink (DL) channel numbers.

When Would Field Test Mode Be Helpful?

The numerical readout is precise and will give you a better idea of the strength of your signal. When you travel, you can find out which frequency bands are in use in Field Test Mode. You may also live or work in an area where the cellular signal is weak. You can purchase the right cell phone signal booster with the information available in Field Test Mode.

Always Show Signal Number Instead of Dots

The dB reading will revert to signal dots when you exit the Field Test screen. If you want your iPhone to continue to display the precise numerical signal strength instead of the less accurate signal dots, hard-reset the phone when in Field Test mode (by holding down the power and home buttons at the same time until the Apple logo appears). The numerical signal strength now remains on your phone. Tap the signal number to switch to signal dots, and vice versa. To exit this permanent mode, enter Field Test Mode again by calling the number above, tap the signal number to switch to signal dots, and exit Field Test.

Update, 11/13/17

  • iOS 11 removed the ability to see the signal number in Field Test (and replaced the signal bars with dots).
  • The information above pertains to 3G networks but the principles apply to all generations of cellular networks.
  • Current iPhones have multiple antennas within them that can connect to the frequency bands of many networks around the world; it is rarely necessary to change smartphones when traveling (changing SIM cards is still necessary).
  • Field Test is still useful to learn about local cellular network conditions.