By Cell Surveys · Published
Short answer
Open the Phone app, dial *3001#12345#*, and tap Call to open Field Test Mode, then look for RSRP: your iPhone's signal strength in dBm. For LTE and 5G, better than −90 dBm is good, −100 to −110 dBm is poor, and below −110 dBm is effectively no usable service. No App Store app can read this number, because iOS does not give apps the measured signal; iPhone apps that show dBm are estimating.
Bars Are Not the Number
The bars on the status bar are Apple's simplified summary of the signal. They do not say how strong it is in absolute terms, and they leave out interference, which is why full bars can still come with a page that will not load. The number behind them is the signal strength in dBm, and on an iPhone it lives in Field Test Mode.
Open Field Test Mode
- Open the Phone app and go to the keypad.
- Dial *3001#12345#*, with both asterisks and both pound signs, and tap Call. Field Test Mode opens instead of a call.
- Find RSRP. That is the signal strength on LTE and 5G, in dBm. RSRQ and SINR (or SNR) next to it show how clean the signal is.
Apple does not document Field Test Mode and rearranges it between iOS versions, so look for those labels rather than a fixed menu path. What it shows can also vary with the iPhone model and the carrier.
What the Number Means
RSRP is a negative number, and the closer to zero, the stronger: −85 dBm is stronger than −105 dBm.
| RSRP | Rating |
|---|---|
| Better than −80 dBm | Strong |
| −80 to −90 dBm | Good |
| −90 to −100 dBm | Fair |
| −100 to −110 dBm | Poor |
| Below −110 dBm | No usable service |
Strength is half the story. A strong RSRP with a SINR below 0 dB means interference is drowning the signal, the classic "full bars, nothing loads." The full scale, with RSRQ and SINR.
Get a Reading You Can Trust
- Wait and watch it change. The values are not timestamped and do not always refresh. Take a few steps and check that the number moves before trusting it.
- Note whether you are on LTE or 5G. Compare like with like; the bands behave differently.
- Check outside, then inside. The difference between the reading at the curb and the reading in a room is what the building costs you.
- Write it down. Field Test Mode keeps no history and draws no map: close it and the reading is gone.
Why No App Can Show It
You might expect an App Store app to do this better. None can: iOS does not let third-party apps read the measured signal, its quality, or the ID of the cell. Asked about exactly these values on Apple's developer forums in 2026, an Apple engineer answered: "No public API provides the level of information you're describing." Any iPhone app that shows dBm is estimating it; here is what iPhone signal apps actually show.
Mapping Signal Across a House or a Drive
Field Test Mode answers one question: how strong is the signal right here, right now. To see which rooms are weak, or where a drive loses service, you need many readings with their locations, on a map.
Map it with your iPhone. The free Cell Surveys app for iPhone logs your signal as you walk or drive and maps it as a heatmap. On iPhone the level is an estimate, which is enough to find dead spots and weak rooms; turn on Everyone's coverage to see your carrier's average from all users' readings. For measured dBm at every step, use the Android app.
