If you don’t know, the bars in the top-left of your iPhone don’t actually represent the exact signal, but instead represent a signal range.  There’s a “secret” (or at least it was a secret before I posted this blog) number that you can dial to get the exact signal in real-time.  Here’s how to do it:

Get the exact signal value by entering Field Test Mode:

1) DIAL *3001#12345#* and then press CALL

2) Your phone will enter Field Test Mode and your signal strength will change to a new number! Like this:

3) Hold down the power/lock button until the power down display is on the screen.  Then PRESS AND HOLD the home button for about 6 seconds:

Your phone will go back to the Homescreen and the new number will stay.  Tap the number in the top-left to switch back and forth with the bars. The higher the number the stronger the signal.  Since the numbers are negative, a range between -40 and -77 is a good five-bar signal.  A one bar signal would be about -102 to -120.

-40 to -77 = GOOD 

-102 to -120 = BAD

If you want to turn this mode off, enter the secret number (code from Step 1) again and it will be disabled.