Field Test Mode is a hidden feature on the iPhone which allows you to see technical details of the device, the most useful of which is the true cell signal strength displayed as a number rather than the traditional signal bars or dots. We’ll show you exactly how to enter Field Test Mode, as well as how to enable Field Test Mode all the time to be able to see the real cellular signal from your iPhone represented as numbers in the upper left corner. Of course, we’ll also show you how to read the numbers so that you can understand what a good cell signal looks like versus a bad signal reception. It only takes a minute or two to complete the process, and it’s easy to revert back to normal signal indicators if you decide you’d rather not see the numbers anymore, so even though it’s a bit of iPhone geekiness, try it out!

Entering Field Test Mode on iPhone

This will work on any iPhone model and any version of iOS, except the very original:

Open the Phone app as if you were going to make a regular phone call

From the iPhone keypad, dial *3001#12345#* and hit the “Call” button

You will immediately see the signal numbers in the upper left corner, and you can tap around the menus to discover other random features and information that is generally meaningless outside of cell technicians and field operators. If you hit the Home button you’ll quit out of Field Test and the signal indicator will return to dots or bars rather than the signal numbers, but it’s easy to always see the numbers too as described below.

Enabling Signal Number as Reception Indicator Rather Than Signal Bars / Dots

To always see the signal numbers rather than the signal bars or dots, you’ll use the Force Quit app function to kill Field Test when it’s open:

Dial *3001#12345#* and hit “Call” if you haven’t done so already to launch Field Test

Now 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

Tap the signal bars or signal numbers to switch between the two

To remove the tap-to-switch signal indicator ability, you can either reboot the iPhone or go back into Field Test and close out of it as usual.

In the newest versions of iOS, Field Test Mode transforms the signal ‘dots’ into the signal numbers instead, otherwise the feature is exactly the same:

How to Read the Field Test Signal Indicator Numbers

The numbers don’t follow a scale that makes much sense to normal people, but the lower the number (in other words, the more negative) the worse the signal, and the higher the number (less negative) the better.

Anything above -80 is good, and would be considered full bars

Anything below -110 is bad, and would be considered few bars

For example, a signal number of -105 is considerably worse than a signal of -70. You’ll generally find that anything approaching -105 or lower is fairly bad reception, while anything above -80 is usually good, and if you tap the number signal it’s usually shown as full bars. The full range of the signal numbers extends from -40 to -120, with -130 being a nearly impossible number to see because it means no reception, and -40 would be about the strength you’d get being right alongside a cell tower. Technically, the number goes all the way to -140, but you will almost never see that because it basically means there is no signal to speak of, and most users will see -120 or -130 before it switches over to the “No Service” indicator instead.

Once you get the hang of reading the numbers, you’ll find it’s much more accurate, and it becomes easier to predict when you may drop a call or start to get a bad signal or connection, which creates the weird artifacts and sounds on phone calls, often before it starts to cut out or even drop completely. That typically starts happening around -110 or so, before dropping the connection or call completely if it hits -120 to -130.

If you’re having any problems getting this to work, or you want to see how to do this yourself before jumping in, watch the video below:

This is actually a fairly old hidden feature that works on any iPhone running iOS 4.1 or later, and yes that includes iOS 7.1.1 and beyond, but we’ve had a lot of questions about it recently due to several recent iPhone tip screenshots showing the signal numbers. Basically all of us here at OSXDaily have these signal numbers shown full time on our phones for a variety of reasons, and so you will commonly see them in articles around here.

You can click the image below to detail instructions further, this demonstrates the feature being enabled in iOS 7.1 on an iPhone 5s:

For those who use Personal Hotspot often, or just for the geekier folks among us, it can be fun to test the mobile download speeds using apps like Speed Test with these signal numbers visible, because it can help to find optimal signal locations and device placement for the best possible speeds.