Decibel Calculator
Convert between decibels and power or voltage ratios.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Select power ratio or voltage ratio mode. Enter the measured value and the reference value. The calculator computes the decibel level instantly.
About Decibel
The decibel is a relative unit of measurement widely used in acoustics, electronics, and signal processing. It expresses the ratio of one value to another on a logarithmic scale. For power, dB = 10 x log10(P1/P2). For voltage or amplitude, dB = 20 x log10(V1/V2). Common references include the threshold of hearing (0 dB SPL), milliwatt (dBm), and volt (dBV). The logarithmic nature means that every 3 dB doubling of power, and every 6 dB doubling of voltage.
Frequently Asked Questions
What is a decibel?
A decibel (dB) is a logarithmic unit used to express the ratio of two values, typically power or intensity. It is one-tenth of a bel, named after Alexander Graham Bell.
Why use a logarithmic scale?
Sound intensity, signal power, and many physical quantities span enormous ranges. Logarithmic scales compress these ranges into manageable numbers. A 10x power increase is just 10 dB.
What is the difference between power and voltage dB?
Power dB uses 10 x log10(P1/P2). Voltage dB uses 20 x log10(V1/V2). The factor of 20 arises because power is proportional to voltage squared.