Voltage to dB Calculator

Calculator

Example data table

These examples assume RMS values and common references.

Formula used

For voltage ratios (gain), decibels are computed as: dB = 20 · log10(Vout / Vin).

For absolute voltage levels with a reference voltage Vref: dB = 20 · log10(V / Vref).

When you enter peak or peak-to-peak values, the calculator converts them to RMS: Vrms = Vpeak/√2 and Vrms = Vpp/(2√2).

How to use this calculator

1. Select Absolute voltage → dB for dBV/dBu/dBmV style outputs.
2. Enter your voltage magnitude, choose its unit, and choose RMS/Peak/Vpp.
3. Pick a reference preset or enter a custom reference voltage.
4. For gain, switch to Voltage ratio and enter Vout and Vin.
5. Press Calculate to see results above the form.
6. Use Download CSV or Download PDF to export the latest result.

Voltage to dB guide

1) What decibels mean for voltage

Decibels express how large a voltage is compared with a reference. Because voltage is a field quantity, the conversion uses a factor of 20, not 10. A positive value means your voltage exceeds the reference; a negative value means it is below the reference.

2) Absolute levels: dBV, dBu, and dBmV

dBV uses 1.000 V RMS as its reference, so 1 V equals 0 dBV. dBu uses 0.775 V RMS (historically tied to 1 mW in 600 Ω systems), so 0.775 V equals 0 dBu. dBmV uses 1.000 mV RMS, which is useful for small audio and sensor signals.

3) Ratio gain: Vout/Vin in dB

For amplification or attenuation, you often only need a ratio. A doubling of voltage corresponds to about +6.02 dB, while halving corresponds to about −6.02 dB. Ratios are convenient because they remain meaningful across different absolute signal levels.

4) RMS vs peak vs peak-to-peak

Many instruments report RMS for AC measurements, but oscilloscopes frequently show peak or peak-to-peak. This calculator converts peak to RMS using Vrms = Vpeak/√2 and converts Vpp to RMS using Vrms = Vpp/(2√2). Using consistent RMS values prevents misleading dB readings.

5) Why impedance assumptions matter

The 20·log10 relationship is correct for voltage ratios when the measurement impedance is the same on both sides, or when you are comparing like-for-like RMS levels. If you switch impedances, power transfer changes, and a power-based reference may be more appropriate.

6) Interpreting common numbers

Here are practical reference points: 2 V RMS is about +6.02 dBV; 0.2 V RMS is about −13.98 dBV. A 10× voltage ratio is +20 dB, while a 0.1× ratio is −20 dB. These anchors help you sanity-check results quickly.

7) Custom references for calibration

Custom Vref is helpful when you have a lab calibration voltage, a sensor full-scale output, or a system-specific “0 dB” level. Set Vref to your baseline, then any measurement becomes a relative dB reading against that baseline without changing your workflow.

8) Exporting results for reports

After calculating, you can download a CSV for spreadsheets or a PDF summary for sharing. Exports reflect the most recent calculation stored in your session, including mode, inputs, and computed outputs. This makes documentation and repeat testing much faster.

FAQs

1) Is dBV the same as dBu?

No. dBV references 1 V RMS, while dBu references 0.775 V RMS. The same physical voltage will produce different numerical values because the reference levels differ.

2) Why does voltage use 20·log10 instead of 10·log10?

Power ratios use 10·log10. Voltage ratios map to power through a square relationship, so the formula becomes 20·log10 when comparing voltages under consistent impedance conditions.

3) What happens if I enter 0 volts?

Decibels require a logarithm of a positive ratio. Zero or negative voltages cannot be converted, so the calculator will show an error and ask for a value greater than zero.

4) Can I use peak values directly in the formula?

You can, but the reference is typically defined in RMS. This tool converts peak and peak-to-peak into RMS first, then computes dB, which matches most audio and test standards.

5) Is +6 dB always double the voltage?

Approximately, yes. Exact doubling is +6.0206 dB because 20·log10(2) equals 6.0206. Small rounding differences are normal when displaying a limited number of decimals.

6) When should I use “ratio” mode instead of “absolute”?

Use ratio mode when you care about gain or attenuation between two voltages. Use absolute mode when you want a level relative to a fixed reference like 1 V (dBV) or 0.775 V (dBu).

7) Does this calculator compute dBm?

No. dBm is power relative to 1 mW and needs impedance to convert from voltage. This calculator focuses on voltage-based decibel measures and voltage gain ratios.