Radio engineers, EMC testers and audio technicians all measure signal level, but in different units. A spectrum analyser reads dBm, an EMI receiver reads dBµV, an oscilloscope shows volts peak-to-peak and a power meter shows watts. Converting between them requires the impedance the signal is measured in, because power and voltage are linked by P = V² ÷ Z.
In a 50 Ω system, 0 dBm (1 mW) corresponds to 0.2236 V RMS, or 107 dBµV; in a 75 Ω cable-TV system the same voltage is a different power. Decibel units compress huge ranges into manageable numbers: each 10 dB is a factor of 10 in power, each 20 dB a factor of 10 in voltage, and 3 dB doubles power. Because the reference values differ, dBµV = dBm + 107 in 50 Ω (and + 108.8 in 75 Ω).
Signal generators add another subtlety. Their level may be quoted as the voltage across a matched load (potential difference, PD) or as the open-circuit source voltage (EMF), which is twice as large – 6 dB higher. Choose which one your value refers to, and the converter shows both. Peak and peak-to-peak values assume a sine wave.
How to use the RF Unit Converter
- System impedance
- Unit of the value you know
- Value
- Voltage values refer to
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the system impedance, pick the unit of the value you know and enter it. The calculator converts everything to the RMS voltage across the load, then derives power, current and all the decibel units from it.
Worked example
A +10 dBm signal in 50 Ω carries 10 mW. Its RMS voltage is √(0.01 × 50) = 0.7071 V, which is 1.000 V peak, 2.000 V peak-to-peak, −3.01 dBV and 116.99 dBµV. The current is 14.14 mA. If 0.7071 V is the generator’s EMF instead, only 0.3536 V reaches the load and the power is 3.98 dBm. A 0 dBm signal (1 mW) is 0.2236 V RMS, or 106.99 dBµV.
Quick reference (50 Ω)
| dBm | Power | V RMS | dBµV |
|---|---|---|---|
| +30 | 1 W | 7.07 V | 137 |
| +10 | 10 mW | 0.707 V | 117 |
| 0 | 1 mW | 0.224 V | 107 |
| −30 | 1 µW | 7.07 mV | 77 |
| −60 | 1 nW | 224 µV | 47 |
Inputs explained
| Field | What to enter |
|---|---|
| System impedance | In Ω |
| Unit of the value you know | Choose one: V (RMS), V (peak), V (peak-to-peak), mV (RMS) … |
| Value | A number |
| Voltage values refer to | Choose one: Voltage across the load (PD), Open-circuit source voltage (EMF), matched source |
What each result means
- Power in dBm = 10×log10([V RMS across the load]^2÷[System impedance]×1000)
- Power = [V RMS across the load]^2÷[System impedance]
- Power in dBW = 10×log10([V RMS across the load]^2÷[System impedance])
- Power in dBµW = 10×log10([V RMS across the load]^2÷[System impedance]×1e6)
- Voltage (RMS) across the load = [V RMS across the load]
- Voltage (peak) = [V RMS across the load]×sqrt(2)
- Voltage (peak-to-peak) = [V RMS across the load]×2×sqrt(2)
- Voltage in dBV = 20×log10([V RMS across the load])
- Voltage in dBmV = 20×log10([V RMS across the load]×1000)
- Voltage in dBµV = 20×log10([V RMS across the load]×1e6)
- Source EMF (open-circuit, matched source) = 2×[V RMS across the load]
- EMF in dBµV = 20×log10(2×[V RMS across the load]×1e6)
- Current (RMS) = [V RMS across the load]÷[System impedance]
- Current in dBµA = 20×log10([V RMS across the load]÷[System impedance]×1e6)
Formula
P = V_rms² ÷ Z · dBm = 10 log₁₀(P ÷ 1 mW) · dBµV = 20 log₁₀(V_rms ÷ 1 µV) · V_peak = √2 × V_rms · V_pp = 2√2 × V_rms · EMF = 2 × V_load (matched) · I = V_rms ÷ Z
Worked example
Inputs — System impedance: 50 Ω; Unit of the value you know: dBm; Value: 10.
Results — Power in dBm = 10 dBm; Power = 10 mW; Power in dBW = -20 dBW; Power in dBµW = 40 dBµW; Voltage (RMS) across the load = 0.707107 V; Voltage (peak) = 1 V; Voltage (peak-to-peak) = 2 V; Voltage in dBV = -3.0103 dBV; Voltage in dBmV = 56.9897 dBmV; Voltage in dBµV = 116.9897 dBµV; Source EMF (open-circuit, matched source) = 1.414214 V; EMF in dBµV = 123.0103 dBµV; Current (RMS) = 14.142136 mA; Current in dBµA = 83.0103 dBµA.
Key terms
- dBm
- Power relative to 1 milliwatt, in decibels.
- dBµV
- Voltage relative to 1 microvolt RMS, in decibels.
- RMS
- Root mean square – the DC-equivalent value of an AC voltage.
- EMF
- Electromotive force – the open-circuit voltage of a source.
Frequently asked questions
What voltage is 0 dBm in 50 Ω?
0.2236 V RMS (0.632 V peak-to-peak), because 1 mW = V² ÷ 50 Ω.
How do I convert dBm to dBµV?
In 50 Ω add 107 dB (dBµV = dBm + 107); in 75 Ω add 108.75 dB.
How do I convert dBm to watts?
P (W) = 10^((dBm − 30) ÷ 10): 30 dBm = 1 W, 20 dBm = 100 mW, −30 dBm = 1 µW.
What is the difference between EMF and PD?
EMF is the open-circuit voltage of the source; across a matched load only half of it (PD) appears, 6 dB lower.
Are peak values correct for any waveform?
No – peak and peak-to-peak conversions assume a sine wave; for other waveforms use the crest factor.
Why does the impedance matter?
Power depends on voltage squared divided by impedance, so the same voltage carries less power in 75 Ω than in 50 Ω.