Amplitude to dB Calculator

Example Data Table

Formula Used

For amplitude-like quantities (voltage, current, pressure), use dB = 20·log10(A/Aref). For power, use dB = 10·log10(P/Pref). Absolute scales use a fixed reference: dBV uses 1 V RMS, dBu uses 0.775 V RMS, and dBm uses 1 mW. With impedance, power is P = V²/Z.

How to Use This Calculator

1. Select the conversion type that matches your problem.
2. Pick the correct formula: 20·log10 for amplitude, 10·log10 for power.
3. Enter your values carefully (use RMS voltage for audio/RF work).
4. Press Calculate to see outputs and step-by-step math.
5. Use Download CSV or Download PDF to save results.

Amplitude to dB Guide

1) Why decibels are used

Decibels compress huge ranges into manageable numbers. Audio, vibration, RF, and instrumentation often span ratios from millionths to thousands. A logarithmic scale turns multiplication into addition, so cascaded gains and losses add cleanly. This calculator handles both ratios and absolute reference levels.

2) The two core equations

The most common confusion is choosing 20 or 10. Use 20·log10 when your “amplitude” squared is proportional to power (voltage, current, acoustic pressure). Use 10·log10 when you already have power. Example: a 2× voltage ratio is +6.02 dB, while a 2× power ratio is +3.01 dB.

3) Reading typical reference points

Absolute units anchor the scale. 0 dBV = 1.000 V RMS. 0 dBu = 0.775 V RMS (historically linked to 600 Ω systems). 0 dBm = 1 mW. These let you compare equipment levels, not just ratios. The form above converts both directions.

4) Practical audio levels (with data)

Many pro audio devices use +4 dBu as nominal line level, which is about 1.228 V RMS. Consumer line levels often sit near -10 dBV, roughly 0.316 V RMS. If a device output changes from -10 dBV to +4 dBu, that is a large real-world jump you can quantify instantly.

5) Power and dBm quick checks

With dBm, remember it is power-based. +10 dBm = 10 mW, +20 dBm = 100 mW, and +30 dBm = 1 W. Negative values are common: -30 dBm = 1 µW. The calculator accepts µW, mW, W, and kW for fast conversions.

6) Impedance matters for voltage to dBm

To convert voltage into dBm, you must know impedance because P = V²/Z. In a 50 Ω system, 0 dBm corresponds to about 0.2236 V RMS. In a 600 Ω system, the voltage for the same power is higher. Always match Z to your instrument or load.

7) Avoiding common mistakes

Use positive ratios only: logarithms require inputs greater than zero. For wave measurements, keep units consistent and prefer RMS values when references are RMS. If you see results off by about 6 dB, you likely used 10 instead of 20 (or vice versa). The “Use Formula” selector helps prevent this.

8) When this calculator is most useful

Use it when comparing microphone sensitivities, amplifier gains, filter attenuation, RF link budgets, and test bench readings. It is also handy for documenting measurements: export a CSV for spreadsheets or a PDF for reports. Pair the results with the example table above to sanity-check your workflow.

FAQs

1) Why do I get -6.02 dB when the ratio is 0.5?

A half amplitude means the output is smaller than the reference. Using 20·log10(0.5) gives -6.02 dB, which represents a 2× decrease in amplitude.

2) When should I use 10·log10 instead of 20·log10?

Use 10·log10 when your input is power (watts, milliwatts). Use 20·log10 when your input is an amplitude like voltage, current, or pressure, where power scales with the square.

3) What is the difference between dBV and dBu?

dBV references 1 V RMS, while dBu references 0.775 V RMS. They are both voltage-based, so the conversion uses 20·log10, but the reference level changes the number.

4) Is dBm always tied to 50 ohms?

No. dBm is defined as power relative to 1 mW, independent of impedance. Impedance only matters when you convert from voltage to power (or back) using P = V²/Z.

5) Can dB values be negative?

Yes. Negative dB means the measured quantity is below the reference. Examples include attenuation, losses, or signals below 1 V (dBV) or 1 mW (dBm).

6) Why does the calculator reject zero or negative inputs?

Logarithms require positive numbers. Ratios, voltages, and powers must be greater than zero for log10. If your signal crosses zero, use a magnitude or RMS value.

7) How do I export results for my report?

After calculating, use “Download CSV” for spreadsheets or “Download PDF” to save a neat summary. You can also print directly to a PDF using your browser’s print dialog.