dB Calculator

Calculate decibel (dB) values from power or voltage ratios — or convert back from dB to ratios. Enter your Power Gain (P1/P2) or Voltage Gain (V1/V2) to get the equivalent dB level, or enter a dB value to find the corresponding gain ratios. Also includes a dBm calculator for converting absolute power to decibels relative to 1 milliwatt. Whether you're working with audio engineering, electronics, or acoustics, this tool covers field quantity (20×log) and energy quantity (10×log) conversions in one place.

Ratio of output power to reference power (e.g. 10 means 10× more power)

Ratio of output voltage to reference voltage

dB

Enter a dB value to convert back to power and voltage ratios

W

Absolute power in watts for dBm calculation (reference = 1 mW)

Results

Decibel Value (dB)

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Power Gain (P1/P2)

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Voltage Gain (V1/V2)

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dBm Value

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Formula Applied

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dB Scale: Power vs Voltage Gain

Results Table

Frequently Asked Questions

What is a decibel (dB) and why is it logarithmic?

A decibel is a logarithmic unit used to express the ratio of two quantities — typically power or amplitude (voltage, sound pressure). Because the human ear perceives loudness on a logarithmic scale, and because signal ranges in electronics span many orders of magnitude, the dB scale makes comparisons far more manageable. A 10 dB increase represents a 10× increase in power and roughly a 3.16× increase in voltage.

What is the difference between the power formula and the voltage formula for dB?

For power (energy) quantities, the formula is dB = 10 × log₁₀(P1/P2). For voltage or sound pressure (field) quantities, the formula is dB = 20 × log₁₀(V1/V2). The factor of 20 arises because power is proportional to the square of voltage, and log(x²) = 2·log(x). Always use the correct formula for your quantity type.

How do I add or subtract decibel values?

You cannot simply add or subtract dB values arithmetically because they are logarithmic. You must first convert each dB value back to a linear power ratio using P = 10^(dB/10), sum (or subtract) the linear values, and then convert back to dB using 10 × log₁₀(result). For example, 94 dB + 94 dB combined = 97 dB, not 188 dB.

What does dBm mean?

dBm is a unit of absolute power measured in decibels relative to 1 milliwatt (0.001 W). The formula is dBm = 10 × log₁₀(P / 0.001). It is widely used in radio frequency (RF) engineering, telecommunications, and audio to express signal strength in absolute terms rather than as a ratio.

What is 0 dB?

0 dB means the ratio between the two quantities is exactly 1:1 — there is neither gain nor loss. It does not mean there is no signal; it means the output equals the reference level. In the context of dBm, 0 dBm equals exactly 1 milliwatt of power.

How accurate is the decibel calculator compared to professional tools?

This calculator uses the standard IEEE/IEC logarithmic formulas (10·log for power, 20·log for voltage) and is mathematically exact for the given inputs. Practical measurement accuracy depends on your instrumentation, but the calculation itself is identical to what professional audio analyzers and RF tools use internally.

When would I use dB in real-world applications?

Decibels are used extensively in audio engineering (microphone sensitivity, amplifier gain, speaker output), telecommunications (signal-to-noise ratio, cable loss, antenna gain), acoustics (sound pressure levels, noise regulations), and electronics (filter attenuation, impedance matching). Any time you need to express very large or very small ratios compactly, dB is the standard unit.

What is the relationship between sound pressure level (SPL) and dB?

Sound Pressure Level in dB SPL is calculated as 20 × log₁₀(P / P₀), where P₀ is the reference sound pressure of 20 μPa (the threshold of human hearing). Because sound pressure is a field quantity, the factor of 20 applies. A whisper is about 30 dB SPL, normal conversation around 60 dB SPL, and the threshold of pain near 120–130 dB SPL.

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