Antenna gain and RF power — dBi, dBd, dBm
dB is a ratio. Reference matters. dBi vs dBd differs by 2.15 dB forever.
Antenna gain and RF power sit on decibel scales that reference different quantities. Getting the reference right is what separates a working link from a dead one.
The exact relationships
dB = 10 log₁₀(P₂/P₁) > dBi = gain over isotropic radiator > dBd = gain over half-wave dipole > dBi − dBd = 2.15 dB (a dipole's own gain over isotropic) > dBm = 10 log₁₀(P / 1 mW); dBW = 10 log₁₀(P / 1 W); dBm = dBW + 30
Common values
1 W = 30 dBm = 0 dBW. 100 mW = 20 dBm. 1 µW = −30 dBm. Home Wi-Fi router transmit: 15–20 dBm. 5 dBi antenna = 2.85 dBd. 24 dBi parabolic dish ≈ 22 dBd.
Worked example — link budget
TX 20 dBm, antenna 5 dBi, cable −2 dB, path loss −90 dB, RX antenna 5 dBi, RX sensitivity −80 dBm. Received power = 20 + 5 − 2 − 90 + 5 = −62 dBm. Margin = −62 − (−80) = 18 dB. Read the TX antenna spec as 5 dBd instead: add 2.15 dB, redo — 20 dB margin.
Recap
Reference matters. dBi > dBd by 2.15. dBm = dBW + 30.
Try the converters mentioned in this article
FAQ
Is dB the same as dBA?+
No — dBA is A-weighted sound pressure, unrelated to RF.
How do I convert dBm to watts?+
P (W) = 10^((dBm − 30)/10). 30 dBm = 1 W, 20 dBm = 100 mW, 0 dBm = 1 mW.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.