Decibel (dB) Calculator

The decibel (dB) is a logarithmic unit used in audio, electronics, and telecommunications to express signal strength, gain, or power ratios on a compact, human-readable scale. The Decibel Calculator converts between ratios and dB, or between power (watts) and dBm. Select your Calculation Type and Quantity Type, then enter your power ratio, voltage ratio, dB value, or power in watts to get the converted result alongside power gain (dB), voltage gain (dB), power ratio, voltage ratio, dBm, and watts.

Quantity Type *

dB
watts
dBm

Results

Result

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Power Gain (dB)

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Voltage Gain (dB)

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Power Ratio

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Voltage Ratio

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dBm

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Power

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Frequently Asked Questions

What is the difference between the 10 log and 20 log formulas?

The 10 log formula (dB = 10 log₁₀(P₁/P₂)) is used for power quantities like watts, while the 20 log formula (dB = 20 log₁₀(V₁/V₂)) is used for field quantities like voltage. This is because power is proportional to the square of voltage.

How do you convert dB to a ratio?

To convert dB to a ratio, use the antilog function: For power ratios, ratio = 10^(dB/10). For voltage ratios, ratio = 10^(dB/20). For example, 10 dB equals a power ratio of 10:1 and a voltage ratio of approximately 3.16:1.

What does dBm mean and how is it calculated?

dBm is a unit of power measurement referenced to 1 milliwatt (0.001 watts). It's calculated using the formula: dBm = 10 log₁₀(P/0.001), where P is the power in watts. For example, 1 watt equals 30 dBm.

Can you add and subtract dB values directly?

No, you cannot add or subtract dB values directly like regular numbers because decibels use a logarithmic scale. You must first convert them back to linear ratios, perform the arithmetic operation, then convert back to dB.

What is a 3 dB increase in power terms?

A 3 dB increase represents approximately double the power (actually 10^(3/10) = 1.995 times). Similarly, a 6 dB increase is about 4 times the power, and a 10 dB increase is exactly 10 times the power.

How do voltage and power gains relate in dB?

When the same signal experiences both voltage and power changes, the power gain in dB is twice the voltage gain in dB. This is because power is proportional to voltage squared, so dB_power = 2 × dB_voltage.

What are common dB reference levels?

Common references include dBm (1 milliwatt), dBW (1 watt), dBV (1 volt), and dBμV (1 microvolt). Each uses the same logarithmic principles but with different reference points for measurement.