Signal strength is measured in decibels relative to one milliwatt, written as dBm. This is the universal unit for both cellular and WiFi signals, and it runs on a negative scale: -30 dBm is a powerful signal, -90 dBm is weak, and -110 dBm is barely usable. The bars on your phone are a rough translation of this number, but checking the actual dBm value gives you a precise, comparable reading you can use to troubleshoot problems or find the best spot in your house for a router.
Why dBm Matters More Than Signal Bars
Signal bars look simple, but they’re unreliable for comparison. There is no industry standard for how many dBm each bar represents. Your carrier and phone manufacturer each use their own algorithm to convert the raw signal into those one-to-five icons. One phone might show three bars at -95 dBm while another shows two bars at the same reading. This makes bars useful as a quick glance but useless for diagnosing a weak connection, comparing locations, or deciding where to place equipment.
The dBm scale is logarithmic, which means small number changes represent big real-world differences. Every 3 dB drop cuts the actual power roughly in half. So -83 dBm isn’t just “a little worse” than -80 dBm; it’s about half the power. A reading of 0 dBm equals exactly 1 milliwatt of power, and the further into negative territory you go, the weaker the signal. At -60 dBm, the power reaching your device is just 1 millionth of a watt. At -90 dBm, it’s a billionth. Your radio hardware is remarkably sensitive, but there are limits.
What Good and Bad Signal Looks Like
WiFi Ranges
WiFi signal strength is reported as RSSI (received signal strength indicator), measured in dBm. Here’s what the numbers mean in practice:
- -30 to -50 dBm: Excellent. You’re very close to the router. Streaming, video calls, and large downloads will work without interruption.
- -50 to -60 dBm: Very good. Still full bars on most devices. You won’t notice any performance issues.
- -60 to -65 dBm: Good. Reliable for everyday browsing, streaming, and calls.
- -65 to -75 dBm: Fair. Web pages load, but video quality may dip and latency increases.
- -75 dBm and below: Weak. Expect dropped connections, buffering, and slow speeds.
A reading above -30 dBm can actually cause problems. Signal saturation occurs when a device is too close to a high-power access point, which can degrade performance rather than improve it.
Cellular (5G and LTE) Ranges
Cellular signals use a metric called RSRP (reference signal received power) instead of plain RSSI. RSRP isolates just the cell tower’s reference signal from background noise, giving a cleaner picture of your actual connection quality. For 5G specifically:
- -80 dBm or higher: Excellent. Fast, consistent data speeds.
- -80 to -90 dBm: Good. Reliable for most tasks.
- -90 to -100 dBm: Fair. Usable but you may notice slower speeds.
- Below -100 dBm: Poor. Dropped calls, failed uploads, and very slow data.
There’s a second number worth knowing: SINR (signal to interference plus noise ratio), measured in plain dB. This tells you how clean your signal is relative to interference from other devices and towers. Above 15 dB is excellent; below 5 dB is poor. You can have decent RSRP but terrible SINR in a crowded area, which explains why your phone shows bars but data barely works at a concert or stadium.
How to Check Signal on an iPhone
Your iPhone has a hidden diagnostic screen called Field Test Mode. To access it, turn off WiFi first, then open the Phone app and dial *3001#12345#* followed by the call button. This opens a menu with raw signal data your phone normally hides behind those bars.
On iOS 15 and earlier, tap “Serving Cell Meas” to see your RSRP, RSRQ (signal quality), and SINR values. On iOS 16 and later, the same dialer code works, but Apple has limited some of the detailed metrics, especially for 5G connections. You’ll still see RSRP in most cases, which is the most important number for gauging your cellular signal strength.
For WiFi signal on an iPhone, your options are more limited. The Settings app shows the network name but not the dBm value. Third-party apps like AirPort Utility (with its WiFi scanner toggle enabled in settings) can display RSSI, or you can use a dedicated WiFi analyzer app from the App Store.
How to Check Signal on Android
Android makes this easier than iOS. Go to Settings, then Network & Internet (or Connections, depending on your manufacturer), then tap your cellular network and look for “Signal strength” or “Network details.” Many Android phones display the dBm value directly on this screen alongside the network type.
Some Android devices also support dialer codes similar to the iPhone’s Field Test Mode. Dialing *#*#4636#*#* on many models opens a hidden “Testing” menu with phone information, including signal strength in dBm. This code doesn’t work universally, though. Samsung, Pixel, and OnePlus devices each handle it differently, and some carriers disable it entirely.
For more detailed measurement, apps like Network Cell Info Lite or SignalCheck can log your RSRP, RSRQ, and SINR over time, which is useful if you’re trying to find the best spot in your home for a cellular signal booster or fixed wireless router.
How to Check WiFi Signal on a Computer
Windows
Open Command Prompt as an administrator by searching for “Command prompt” in the taskbar, right-clicking it, and selecting “Run as administrator.” Then type netsh wlan show interfaces and press Enter. This displays your current WiFi connection details, including signal strength as a percentage. To convert that percentage to a rough dBm estimate: 100% is approximately -30 dBm, 50% is around -70 dBm, and 1% is about -100 dBm.
For a more detailed report, type netsh wlan show wlanreport in the same administrator Command Prompt. Windows generates an HTML file with a full history of your WiFi connections, disconnections, and signal quality over the past three days. The file path is displayed after the command runs; just paste it into a browser to view.
macOS
Hold the Option key and click the WiFi icon in your menu bar. Instead of the normal dropdown, you’ll see a detailed panel showing your RSSI value in dBm, the noise floor, transmit rate, channel, frequency band, and security type. No apps or terminal commands needed.
The noise floor is a useful bonus here. It tells you how much background radio interference exists in your environment. Subtract the noise floor from your RSSI to get your signal-to-noise ratio. For example, an RSSI of -55 dBm with a noise floor of -90 dBm gives you a 35 dB margin, which is comfortable. If that margin drops below 20 dB, you’ll start noticing performance problems even if your RSSI looks decent.
Mapping Signal Across a Space
A single reading tells you what the signal is like right where you’re standing, right now. To actually solve a coverage problem, you need readings from multiple spots. Walk through your home or office, pausing every few feet to note the dBm value. Pay attention to areas near walls, floors, and large metal objects like refrigerators or filing cabinets, all of which absorb or reflect radio waves.
Dedicated tools make this process faster. On Windows and macOS, apps like NetSpot or Ekahau let you upload a floor plan and tap your location as you walk, automatically building a color-coded heat map of signal coverage. For cellular, apps like OpenSignal or Network Cell Info can log readings by GPS coordinate.
The goal of mapping isn’t just finding dead zones. It also reveals whether your problem is signal strength (low RSRP or RSSI) or interference (low SINR or high noise floor). These two problems have different solutions. Low signal strength means you need a closer access point, a repeater, or a signal booster. High interference means you need to change your WiFi channel, move to the 5 GHz or 6 GHz band, or physically relocate the source of interference.
Tips for Getting Accurate Readings
Signal fluctuates constantly. A single snapshot can be misleading, especially for cellular, where your connection hands off between towers and adjusts power levels dozens of times per minute. Take at least three readings at each location, spaced 10 to 15 seconds apart, and use the average. If you’re comparing two spots in your home, test them within the same few minutes to minimize the effect of network congestion changes.
Hold your phone the way you normally would during a call or while browsing. Your hand absorbs some signal, and the difference between holding a phone and setting it on a table can be 5 to 10 dBm. Since each 3 dB change doubles or halves the power, that’s a significant gap. You want your measurement to reflect real-world use, not ideal lab conditions.
For WiFi, make sure you’re measuring on the correct frequency band. Most modern routers broadcast on both 2.4 GHz and 5 GHz (and some on 6 GHz). The 5 GHz band is faster but weaker at distance, so your dBm reading will differ depending on which band your device connects to. Check which band you’re on before comparing readings between locations.