dB, dBm, dBµV and Free-Space Path Loss Calculator
Ratio ↔ dB (power and voltage), dBm ↔ W ↔ dBµV, free-space path loss.
Ratio ↔ dB
Absolute level
Free-space path loss
Power ratio: dB = 10·log₁₀(P2/P1); voltage or current ratio: 20·log₁₀(V2/V1). dBm = 10·log₁₀(P / 1 mW); dBµV = 20·log₁₀(V / 1 µV). FSPL = 20·log₁₀(d) + 20·log₁₀(f) + 32.44 with d in km and f in MHz (ideal line-of-sight, no antennas gains or obstacles).
Frequently asked questions
When do I use 10·log and when 20·log?
Use 10·log10 for power ratios and 20·log10 for voltage or current ratios (into the same impedance). Doubling the power is +3 dB (3.01 dB); doubling the voltage is +6 dB (6.02 dB).
For preliminary assessment only — the final decision must follow the manufacturer’s datasheet and the applicable NEC / IEC / UL standards. Results are calculated from the standard formulas stated on this page and are for design estimates only. They are not a rating of any specific part — always confirm current, voltage, temperature and environment limits against the manufacturer’s datasheet for the part you intend to use.
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