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Decibel Calculator

Decibel Calculator is part of Toolspire and is ready as soon as this page loads. This is the power form, the 10 log form. Amplitude ratios use 20 log. A voltage that doubles is about 6 dB on the amplitude rule and is not what this page computes.

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What you can do here

Ten, not twenty

The factor 10 is for power. The detail says so, because the 20-log amplitude form is the usual mix-up.

Either power can be larger

A smaller output is a negative decibel. Half the power is about −3.01 dB.

Same unit on both powers

Watts over watts, or milliwatts over milliwatts. A mix is a fake ratio.

The sequence

How to use the Decibel Calculator

  1. 1

    Use two powers, not two voltages

    If you have voltages, this 10-log page is the wrong factor.

  2. 2

    Check 20

    100 against 1.

  3. 3

    Try equal powers

    The result must be 0 dB.

Worth knowing

  • Decibel Calculator on Toolspire: dB = 10 × log10(power ÷ reference).
  • 100 ÷ 1 is 20 dB. A hundredfold power is twenty decibels.
  • A ratio of 1 is 0 dB. The powers match.
  • A power or reference of 0 or less is rejected. The log of a non-positive number is not real.

A hundredfold is twenty, not a hundred

The bel is a log of a ratio, and the decibel is a tenth of that, which is why the formula multiplies by 10. log10(100) is 2, and 10 × 2 is 20. People add a second 10 when the ratio is an amplitude, because power goes with amplitude squared and the square becomes that extra factor. This page does not square anything. It logs the power ratio you type.

FAQ

Questions about Decibel Calculator

Where does Decibel Calculator run?

Decibel Calculator runs in this tab on Toolspire. What you type or drop in is not sent to us and is not stored on a server.

How many decibels is a power ratio of 100?

20 dB.

What is a ratio of 1?

0 dB.

Why not 20 log?

20 log is for amplitude ratios. Power already includes the square, so the factor stays 10.

Can the result be negative?

Yes. A power below the reference is a negative decibel.

Keep going

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