Your phone can tell you exactly how strong its signal is, in decibels, by entering Field Test Mode. How to enter Field Test Mode varies from phone to phone, but it is very straightforward and doesn’t require jailbreaking or in any way damaging your phone!
Most phone screens display signal strength in “bars” at the top of the screen. But “bars” aren’t a very precise measurement. Some phones (like the iPhone) have 5 bars, others (like the Nexus One) have 4. One phone might display a signal strength as 3 bars, another might only call it 2. To assess the level of signal an amplifier will be working with, we need to be a lot more specific. We need to determine exactly what the signal strength is in decibels (dB). Decibels are a very precise unit of measurement, where lower values means strong signal (so -60 signifies stronger signal than -80, -80 is stronger than -90, etc).
iOS 11 update: Unfortunately, as of September 2017, iOS 11 does not displays decibel values in Field Test Mode. You can still access Field Test Mode as described below, but the signal strength will show in bars (not dots or decibel values). This is a really disappointing development, and we're hopeful that an updated version of iOS 11 will bring back the decibel readings.
iPhone 7 update: At present, Field Test Mode is not working on some iPhone 7 and iPhone 7 Plus models. Apple used two different modem chip options in the new iPhones - some use Qualcomm modems, like all previous iPhones did, and some use new Intel chips.
The new Intel modem chips (in addition to having a lot of slow performance issues) do not support Field Test Mode right now.
There are two easy ways to check if you have an Intel-powered iPhone 7:
Field Test Mode works for any other models of iPhone and any version of iOS, except the very original. Older versions of iOS display signal in “bars”, while more recent versions from iOS 7 and beyond display it in “dots”.
If you tap the number in the top left corner, it will toggle between displaying signal in decibels, and displaying it in “dots”.
To exit Field Test Mode and go back to your phone as normal, just press the Home button.
If you want to exit Field Test Mode, but keep the signal display in decibels, not dots, that’s easy. It’s particularly convenient if you are in the middle of a site survey and you want to move around from room to room to find the strongest signal. While in Field Test Mode, hold down the Power button until the “Slide to Power Off” message appears, then release the Power button and hold the Home button until Field Test quits. You can now tap the signal dots or signal numbers to switch between the two. To exit this entirely and go back to normal, go back into Field Test Mode, then quit it normally by pressing the Home button.
Enter Settings — About Phone. Your signal strength in decibels (dB) will be displayed under “Network” or “Status”.
An alternate navigation sequence for some Android phones is Settings – More Options or More Settings – About Phone – Mobile Networks – Signal Strength.
Older phone models may require specific codes. We’ve laid them all out in this PDF guide.
As a general rule, -80dB or higher (ie, -70, -60, -50) is considered strong signal. -110dB or lower (ie, -120, -130) is very weak.