mW and dBm Explained
Wi-Fi power levels are expressed in two units that measure the same thing differently:
| Unit | Type | How It Works | Example |
|---|---|---|---|
| mW (milliwatts) | Linear | Direct power measurement — doubling mW doubles power | 100 mW = twice the power of 50 mW |
| dBm | Logarithmic | Power relative to 1 mW on a log scale — each +10 dBm = 10× power | 20 dBm = 100 mW; 17 dBm = 50 mW |
dBm to mW Conversion Table
| dBm | mW | Typical Context |
|---|---|---|
| 30 dBm | 1000 mW (1 W) | High-end outdoor AP; above most regulatory limits for indoor |
| 27 dBm | 500 mW | High-power indoor AP |
| 24 dBm | 250 mW | Upper range of home router transmit power |
| 20 dBm | 100 mW | Common regulatory limit for Wi-Fi in many countries |
| 17 dBm | 50 mW | Mid-range home router setting |
| 14 dBm | 25 mW | Low transmit power setting |
| 10 dBm | 10 mW | Low — used in small cells or high-density deployments |
| 0 dBm | 1 mW | Reference point; very low — IoT sensor level |
Key dB Math Rules
| Change | Effect on Power | Plain English |
|---|---|---|
| +3 dB | Approximately 2× power | Double the milliwatts |
| -3 dB | Approximately 0.5× power | Half the milliwatts |
| +10 dB | Exactly 10× power | Ten times the milliwatts |
| -10 dB | Exactly 0.1× power | One tenth the milliwatts |
The critical insight: doubling transmit power (+3 dB) does not double range. Because signal spreads in three dimensions, you need approximately 4× the power (+6 dB) to double range in open space. In a real home with walls and floors, the actual gain in coverage from increasing power is even smaller relative to placement changes.
Received Signal Strength (RSSI) Reference
dBm is also used to express the received signal strength at your device — this is RSSI (Received Signal Strength Indicator). Unlike transmit power (always a positive dBm value), received signal is always a negative number — because the received power is a tiny fraction of 1 mW.
| RSSI (received) | Signal Quality | What to Expect |
|---|---|---|
| -30 to -50 dBm | Excellent | Maximum throughput; same room as AP |
| -50 to -65 dBm | Good | Fast and reliable; 1–2 rooms away |
| -65 to -75 dBm | Fair | Usable but noticeably slower; video may buffer at higher quality |
| -75 to -85 dBm | Poor | Unreliable; frequent retransmissions; video calls may drop |
| Below -85 dBm | Very poor | Connected in name only; throughput near zero |
Why Maximum Transmit Power Can Hurt
- Asymmetric links: Phones and laptops transmit at 15–20 dBm (30–100 mW). A router at maximum power (24–27 dBm) can reach the device easily, but the device cannot talk back with equal strength. The router hears the device poorly and must retransmit more often.
- Sticky client problem: High power extends the AP's audible range farther than optimal. Devices hold a connection to a distant AP they can barely hear rather than roaming to a closer one, because the RSSI threshold for forcing a roam decision is never met.
- Increased interference for neighbours and mesh nodes: Your high-power AP raises the noise floor for nearby networks. In a multi-AP home, the APs interfere with each other more at higher power, reducing overall capacity.
- Power does not fix placement problems: A wall that absorbs 15 dB absorbs the same whether you are transmitting at 17 dBm or 24 dBm. The signal still does not penetrate — it is just louder on the transmitter side.
Regulatory Limits
Wi-Fi transmit power is regulated by spectrum authorities (FCC in the US, ETSI in Europe, OFCOM in the UK). For standard indoor 2.4 GHz and 5 GHz Wi-Fi, limits are typically 20–30 dBm EIRP (effective isotropic radiated power, which accounts for antenna gain). Consumer routers are designed to operate within these limits. Aftermarket firmware options that allow "boosting" beyond regulatory limits are illegal in most jurisdictions and can cause interference that affects neighbours' networks.
When to Adjust Power
Home users should almost always leave transmit power on auto or the default setting. Adjusting power downward makes sense in specific multi-AP scenarios:
- You have two or more APs covering overlapping areas and want to define cleaner cell boundaries for better roaming
- A professional Wi-Fi site survey has identified that a specific AP is covering too large an area and causing sticky-client problems
- Devices are frequently choosing a far AP over a close one due to high transmit power attracting them from a distance
If your home has dead zones or weak coverage in specific rooms, the fix is almost always a better-placed access point, not a power increase on the existing router.
Frequently Asked Questions
What is dBm in Wi-Fi?
dBm is a logarithmic measurement of power referenced to one milliwatt (0 dBm = 1 mW). It is used for both transmit power (always positive for Wi-Fi routers, typically 14–24 dBm) and received signal strength (always negative at the client, typically -30 to -90 dBm). The log scale means a 10 dBm increase represents exactly 10 times the power.
Is higher Wi-Fi transmit power always better?
No. Higher transmit power helps the router reach farther, but does nothing for the device's ability to talk back. It also creates asymmetric links, causes sticky-client roaming problems in multi-AP homes, increases interference for neighbours, and does not help when signal loss is caused by wall absorption rather than distance alone.
Should I turn my router to maximum power?
Leave it on auto or default for almost all home scenarios. Maximum power is rarely beneficial and often counterproductive in homes with multiple APs. If coverage is the issue, add a properly placed access point rather than increasing power on an existing one.
How do I check my device's received signal strength?
On macOS: Option-click the Wi-Fi menu bar icon and read RSSI. On Windows: netsh wlan show interfaces in Command Prompt. On Android: Wi-Fi analyser apps from the Play Store. On iPhone: field test mode (*3001#12345#*). Target -65 dBm or better for reliable high-speed connections.