Cell Signal Strength Explained: dBm, RSRP and What's Good
Understand cell signal strength in dBm: what RSRP, RSRQ and SINR mean, which readings are good or bad, and how to check the real numbers on your iPhone.
Cell signal strength is measured in dBm — decibel-milliwatts — on a scale that runs from roughly -50 dBm (outstanding) down to about -120 dBm (no service). Closer to zero means stronger: -85 dBm is a comfortable LTE connection, while -110 dBm is dropped-call territory. If you’ve ever opened a field test screen and wondered whether -103 is a crisis, this guide is for you.
Below: why the scale is negative and logarithmic, what RSRP, RSRQ and SINR mean in plain English, a good/bad reference table for LTE and 5G, how to read the real numbers on an iPhone, and how to use them when you’re repositioning a phone, router or antenna.
Why cell signal strength is measured in dBm
dBm expresses power relative to one milliwatt: 0 dBm equals exactly 1 mW, and everything below it goes negative. Received cell signals are absurdly small — after a tower’s transmission has crossed a few miles of air, trees and drywall, your phone is catching something like 0.0000000001 milliwatts. That’s -100 dBm. Nobody wants to count those zeros, so engineers use a logarithmic scale instead.
The log part is what trips people up, and there are only three rules you need:
- Every 3 dB doubles (or halves) the power. -87 dBm is twice the power of -90 dBm.
- Every 10 dB is a 10× change. -90 dBm is ten times stronger than -100 dBm.
- 20 dB is 100×. The gap between -80 (great) and -100 (meh) is a factor of one hundred.
This is why chasing even a few dB is worth the effort. Move a router to a better window, gain 6 dB, and you’ve quadrupled the received power — the kind of change you can feel on a video call.
One more mental adjustment: because the numbers are negative, a “higher” signal is a smaller-looking number. -70 beats -90. Think “closer to zero wins” and you’ll never misread it.
Turn dB into direction: Those hard-won decibels come fastest when you know which way your tower lies — Cell Tower Finder & Locator is free to download and shows the bearing and distance to every site around you.
RSSI vs RSRP, RSRQ and SINR — in plain English
Open a field test screen and you won’t find one tidy “signal” number; you’ll find alphabet soup. Here’s what each metric actually tells you:
| Metric | What it really measures | Rule of thumb |
|---|---|---|
| RSSI | Total power the phone hears on the channel — your tower plus neighbor cells plus noise | Blunt instrument; the main metric only on 2G/3G |
| RSRP | Power of your serving tower’s reference signal alone — the honest “how loud is my tower” number on LTE (SS-RSRP on 5G) | The one to watch; use the table below |
| RSRQ | Quality: roughly how your tower’s signal stacks up against everything else on the channel | -10 dB or better is healthy; -15 is struggling; -20 is bad |
| SINR | Signal versus interference plus noise — the best predictor of real-world speed | 20+ dB excellent; 13–20 good; 0–13 usable; below 0 the network is shouting over static |
Two practical notes. First, RSRP typically reads about 20 dB lower than RSSI on the same connection, because it measures one narrow reference slice rather than the whole channel — don’t panic when you switch metrics and the number “drops.” Second, strength and quality are different axes: you can sit at a strong -85 dBm RSRP and still crawl because SINR is poor or the cell is congested. That mismatch is a big part of why signal bars mislead people — bars mostly track strength and ignore quality entirely.
What’s a good cell signal strength? dBm ranges for LTE and 5G
For LTE and sub-6 GHz 5G, judge RSRP against this table:
| RSRP (dBm) | Rating | What it feels like |
|---|---|---|
| -80 or better | Excellent | Full speed, rock-solid calls, works deep indoors |
| -80 to -90 | Good | Everything just works; you never think about signal |
| -90 to -100 | Fair | Fine outdoors; slower at busy hours; weak rooms suffer |
| -100 to -110 | Poor | Sluggish data, calls drop when you move, battery drains faster |
| -110 to -120 | Very poor | Barely holding on; texts eventually, data mostly theoretical |
Somewhere past -120 dBm RSRP, phones give up and drop the cell. 5G’s SS-RSRP is read on the same scale, and low-band 5G often hangs on a hair deeper than LTE, but the ranges above won’t steer you wrong.
Three caveats worth knowing:
- Your body is part of the antenna environment. A hand wrapped around the phone or a thick case can cost several dB.
- Readings bounce. Reflections off buildings and terrain make RSRP wobble ±5 dB while you stand still, so watch it for 20–30 seconds before judging.
- A bad number is a symptom, not a diagnosis. Distance, building materials and terrain each leave different fingerprints — here’s how to figure out why your signal is bad before you spend money on a fix.
How to read dBm on an iPhone (Field Test Mode)
First, an uncomfortable truth: no App Store app can show your live signal strength — ours included. Apple doesn’t expose the modem’s radio metrics to third-party apps, full stop. The one built-in tool is Field Test Mode:
- Open the Phone app, dial
*3001#12345#*and press call. - The Field Test app opens. On recent iOS versions you land on a dashboard — look for RSRP, RSRQ and SINR.
- For more detail, dig into the menus: LTE → Serving Cell Meas on an LTE connection, or the NR entries when you’re on 5G.
- Tap a metric to pin it to the dashboard so it’s front and center next time.
Quirks to expect: menu names shuffle between iOS versions and modem generations, values update lazily (give each reading a few seconds), and occasionally the app opens blank — close it and dial again. It’s clunky, but the numbers come straight from the modem.
On Android it’s easier — usually Settings → About phone → SIM status shows dBm directly. iPhone users are entitled to a little jealousy here.
Using dBm when you position an antenna (or just your phone)
Numbers beat vibes when you’re trying to improve a connection. The workflow that works:
- Find your tower first. Readings only make sense when you know what you’re measuring against. The map in Cell Tower Finder & Locator shows nearby sites with carrier, network generation, distance and compass bearing, and its radar view points you at a specific tower. (Mapped positions are close estimates aggregated from public and crowdsourced data — not surveyed coordinates — which is exactly what you need for aiming.)
- Take a baseline. With Field Test Mode open, note RSRP where the phone or router normally lives.
- Change one thing. A different window, a higher shelf, an antenna rotated ten degrees. One variable at a time.
- Wait, then re-read. Let the number settle, and write it down — memory lies.
- Confirm with a speed test. RSRP can rise while speeds stay flat (congestion, interference), so verify the change helped where it counts. Our app has a built-in speed test for exactly this before/after check.
A real gain of 3 dB doubles your received power; 6–10 dB transforms a marginal link. If you’re aiming a directional antenna or a booster’s donor antenna, the full step-by-step lives in our guide to pointing an antenna at a cell tower.
FAQ
What is a good dBm for LTE?
An RSRP of -80 dBm or better is excellent, -80 to -90 is good, and anything down to about -100 is workable. Past -105 you’ll feel it in speeds and call reliability.
Why is signal strength a negative number?
Because received power is a tiny fraction of the 1-milliwatt reference point (0 dBm). Every 10 dB below zero is another 10× smaller — -100 dBm is one ten-billionth of a milliwatt.
Is -110 dBm bad?
Yes — that’s the “very poor” zone. Calls may hold outdoors, but expect slow data, indoor drops and faster battery drain as your phone transmits at maximum power just to be heard.
Do the same numbers apply to 5G?
Broadly, yes. 5G’s SS-RSRP uses the same scale, and the LTE table is a sound guide for low- and mid-band 5G. mmWave 5G is a different animal — huge speeds, tiny range — and deserves its own judgment.
Want the numbers and the map behind them? Download Cell Tower Finder & Locator on the App Store — it’s free to try, and pairing tower bearings with a before/after speed test is the fastest way to turn dBm readings into a genuinely better connection.