dB / dBm / Watt Converter — RF Power & Ratio Calculator

Formulas Used

• dBm ↔ Watts: \( P(\mathrm{W}) = 10^{\frac{\mathrm{dBm}}{10}} / 1000 \),   \( \mathrm{dBm} = 10 \log_{10}\big(P(\mathrm{mW})\big) \)
• dBW ↔ Watts: \( P(\mathrm{W}) = 10^{\frac{\mathrm{dBW}}{10}} \),   \( \mathrm{dBW} = 10 \log_{10}\big(P(\mathrm{W})\big) \)
• Ratio (power): \( \Delta\mathrm{dB} = 10 \log_{10}\!\left(\dfrac{P_2}{P_1}\right) \),   \( P_2 = P_1 \cdot 10^{\Delta\mathrm{dB}/10} \)
• Voltage/Current (with impedance \(R\)): \( V_{\mathrm{rms}} = \sqrt{P R} \),   \( I_{\mathrm{rms}} = \sqrt{\dfrac{P}{R}} \),   for sine \( V_{\mathrm{pp}} = 2\sqrt{2}\,V_{\mathrm{rms}} \)

Impedance only affects the derived Vrms/Vpp/Irms. dBm and dBW are absolute power levels independent of impedance.

dB, dBm, and dBW — What They Mean and When to Use Them

dB (decibel) is a ratio, not an absolute unit. It tells you how much bigger or smaller one power (or amplitude) is compared with another. Because dB is logarithmic, it’s perfect for RF and audio where values span many orders of magnitude. For power ratios, use 10·log10(P2/P1). For voltage or current ratios across the same impedance, use 20·log10(V2/V1).

dBm is an absolute power level referenced to 1 milliwatt. It answers “how many dB above (or below) 1 mW is this signal?” Likewise, dBW is referenced to 1 watt. Conversions are straightforward: dBm = 10·log10(P[mW]), dBW = 10·log10(P[W]). A quick bridge between the two: dBm = dBW + 30 (since 1 W = 1000 mW).

Quick Reference (handy mental math)

• 0 dBm = 1 mW
• 10 dBm ≈ 10 mW
• 20 dBm ≈ 100 mW
• 30 dBm = 1 W
• 40 dBm = 10 W
• -3 dB ≈ half the power; +3 dB ≈ double the power
• +10 dB = ×10 power; −10 dB = ÷10 power

Impedance and Voltage/Current Readouts

Power in dBm or dBW does not depend on impedance. However, if you want the equivalent Vrms, Vpp (sine), or Irms, you must assume (or measure) a load, commonly 50 Ω in RF systems (and other values in audio and instrumentation). With an impedance R, the relationships are Vrms = √(P·R), Irms = √(P/R), and for a sine wave Vpp = 2√2·Vrms. If your system isn’t 50 Ω, set your actual impedance in the tool for accurate voltage/current numbers.

Common Use Cases

• RF links (Wi-Fi, LoRa, cellular, amateur radio): Transmit power is often listed in dBm. Antenna gain (dBi) and cable/connector loss (dB) are added/subtracted to estimate received signal strength.
• Lab measurements: Spectrum analyzers and power meters typically display dBm. Converting to Watts helps size attenuators, terminations, and amplifiers safely.
• System budgeting: Use dB for cascaded gains and losses; switch to dBm/W when you need absolute power at a stage.

Typical Pitfalls (and how to avoid them)

• Mixing dB (ratio) with dBm/dBW (absolute): Keep them straight—dB modifies, dBm/dBW states.
• Using 20·log10 for power ratios: For power, always use 10·log10. Reserve 20·log10 for voltage/current ratios at constant impedance.
• Forgetting impedance: dBm ↔ W needs no impedance, but Vrms/Irms/Vpp do.
• Assuming Vpp = 2·Vrms: That’s incorrect for sine waves—use Vpp = 2√2·Vrms.

Worked Example

You measure 30 dBm. That’s 1 W. Into 50 Ω, Vrms = √(1·50) ≈ 7.071 V, Vpp ≈ 2√2·7.071 ≈ 20.0 V, and Irms = √(1/50) ≈ 0.141 A. If you insert a 6 dB attenuator, output power drops by a factor of 4: to 24 dBm ≈ 0.25 W.