How to See the Precise Signal Strength on iPhone
Step 1. Open the Phone app.
Step 2. Dial *3001#12345#* and tap the green call button to put your iPhone into a secret Field Test Mode.
Step 3. Hold the Sleep/Wake button until the Slide to Power Off screen appears.
Step 4. Release the Sleep/Wake button and ignore the Slide to Power Off prompt.
Step 5. Hold the Home button until you go back to the Home screen.
You should now see a negative number instead of dots in the upper left, like -102. This is your exact signal strength, measured in decibel-milliwatts, called the Received Signal Strength Indication (or RSSI if you want to impress technical support). The higher the number, the better, but note that these are negative numbers, so -1 would be an outstanding signal, while -1000 would be beyond poor. In the real world, you’ll probably see signal strengths somewhere between -40 (a five-bar signal) and -120 (a one-bar signal).
To go back to dots permanently, restart your iPhone by holding the Sleep/Wake button, swiping the Slide to Power Off Switch to the right, and then pressing the Sleep/Wake button again to turn it back on. Then, to get rid of the numbers for good, go back into Field Test Mode and exit it by pressing the Home button.