dBm and mW — decibel-milliwatts across radio links
A 20 dB path loss cuts your signal by 100×. Adding losses in dB is just adding numbers. That is why RF engineers never went back to linear power.
Radio signals span 15 orders of magnitude from transmitter (watts) to receiver (femtowatts). Linear units get unreadable. The decibel-milliwatt (dBm) compresses that range into a comfortable ±100 window and turns cascaded loss calculations into addition.
The formula
P(dBm) = 10 × log₁₀(P/1 mW) > 0 dBm = 1 mW > +10 dBm = 10 mW > +20 dBm = 100 mW > +30 dBm = 1 W > −30 dBm = 1 µW > −60 dBm = 1 nW > −90 dBm = 1 pW
Why the log
A link budget adds transmitter power, subtracts path loss, adds antenna gain, subtracts cable loss. In linear terms these are multiplications. In dB they are additions. A 100-page cellular design document works in dB throughout because signed arithmetic beats scientific notation every time.
Familiar numbers
WiFi transmit power: +20 dBm = 100 mW. Cell phone tx: +23 dBm = 200 mW. WiFi receiver sensitivity: −80 dBm = 10 pW. LTE minimum received power: −120 dBm = 1 fW. Bluetooth Class 2: +4 dBm = 2.5 mW. FM broadcast transmitter: +90 dBm = 1 MW.
Worked example — a WiFi link budget
Router transmits at +20 dBm (100 mW). Router antenna gain: +3 dBi. Free-space path loss at 5 m, 2.4 GHz: ~54 dB. Two walls: −10 dB. Client antenna: +0 dBi. Client sensitivity: −72 dBm. Received power = 20 + 3 − 54 − 10 + 0 = −41 dBm. Margin above sensitivity: 31 dB — comfortable. At 20 m with 4 walls: 20 + 3 − 66 − 20 = −63 dBm, margin 9 dB — marginal, packet loss likely.
dBi, dBd, dBc — the other decibels
dBi = gain relative to isotropic radiator. dBd = gain relative to dipole (add 2.15 dB for dBi). dBc = level relative to a carrier. Always read the suffix — a '10 dB antenna' with no suffix is ambiguous.
The five-second recap
0 dBm = 1 mW. Each +10 dB = 10× power. Each +3 dB ≈ 2×. Add dB values through a link; don't multiply linear watts.
Try the converters mentioned in this article
FAQ
Why is dB always divided by 10?+
Because a bel is 10 dB. The bel was a large unit; the deci-bel is more useful for everyday electronics. The name honours Alexander Graham Bell.
Is dBm the same as dB?+
No. dB is dimensionless (a ratio). dBm is dB referenced to 1 mW — an absolute power. Similarly dBW is referenced to 1 W (0 dBW = 30 dBm).
What is a good WiFi RSSI?+
Above −60 dBm: excellent. −60 to −70: good. −70 to −80: fair. Below −80: unreliable. Below −90: no connection.
This article was written by The Turbo Unit Converter engineering desk (RF & telecommunications) and last reviewed on 2026-06-30 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.