Decibels (dB) calculator

Compare two power levels, or two voltage levels, and see the change in decibels. Or turn a dB figure back into a ratio.

1Type your numbersUse the white boxes. Tap an example chip if you want a starting point.
2Read the answerThe green panel updates as you type. You cannot edit it.
3See whyThe steps below show the formula with your numbers filled in.

TRY AN EXAMPLE

YOUR NUMBERS · TYPE IN THE WHITE BOXES

COMPARING

Starting power (before)

Ending power (after)

THE ANSWER · WORKED OUT FOR YOU

Gain

+10dB

Ratio (after ÷ before)

10×

OR TURN A DB FIGURE INTO A RATIO

Decibels

Power: 1.995×
Voltage: 1.413×

HOW IT WAS WORKED OUT

dB = 10 × log₁₀(after ÷ before)

dB = 10 × log₁₀(100 ÷ 10) = 10 dB

Power uses 10 × log.

DECIBEL SHORTCUTS WORTH MEMORIZING

Change	Power ratio	Voltage ratio
-10 dB	0.1×	0.316×
-6 dB	0.251×	0.501×
-3 dB	0.501×	0.708×
+3 dB	2×	1.41×
+6 dB	3.98×	2×
+10 dB	10×	3.16×
+20 dB	100×	10×

+3 dB is about double the power, +6 dB about four times, +10 dB exactly ten times. Negative values are the same ratios flipped.

What this means

Decibels describe a change as a ratio on a logarithmic scale, so big changes become small numbers. Doubling power is about +3 dB. Cutting it in half is about -3 dB. A ten-fold increase is +10 dB.

For voltage, the same change counts double: doubling voltage is about +6 dB, because power goes up with the square of voltage.

dB (Power) = 10 log₁₀(P₂ / P₁) | dB (Voltage) = 20 log₁₀(V₂ / V₁)

ON THE EXAM

Memorize three anchors: 3 dB is double or half, 6 dB is four times or a quarter, 10 dB is ten times or a tenth. Most exam questions need nothing more.

55 questions in the official pools involve this topic:

Common questions

Why is power 10 log and voltage 20 log?

Power is proportional to voltage squared. Taking the logarithm of a square doubles it, so the voltage formula uses 20 instead of 10.

What does a negative dB mean?

A loss. The output is lower than the input, as in a long run of coax cable.

Can I add dB values?

Yes. Gains and losses in a chain add up. A 100 W transmitter, 3 dB of cable loss and 6 dB of antenna gain nets +3 dB overall.

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