Decibel Calculator
Convert power ratios (10 log) and voltage ratios (20 log) to dB and back, switch between dBm, dBW, watts, volts, dBu and dBV, and add sound or signal levels.
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About this toolWhat is the Decibel Calculator?
The decibel compresses huge ratios into small numbers: a million-fold power gain is just 60 dB. It also confuses people more than any other unit in electronics, because there are two formulas (10 log for power, 20 log for voltage) and a dozen suffixes that each name a different reference. This decibel calculator handles the three jobs that come up most: converting a ratio to dB and back, converting an absolute level between dBm, dBW, watts, volts, dBV, dBu and dBµV, and adding several levels together.
Pick power or voltage, type a ratio or a dB figure, and the other side updates with the matching ratio of the other kind. For absolute levels, choose the system impedance (50 Ω for RF, 75 Ω for video, 600 Ω for telephone and legacy audio, 8 Ω for speakers, or your own) and every unit is shown at once, including peak and peak-to-peak voltage for a sine wave. The adding panel gives both the incoherent total, right for independent noise sources and machines, and the in-phase total for identical signals.
- Power and voltage ratios to dB and back
- dBm, dBW, W, mW, V rms, dBV, dBu and dBµV in one view
- Selectable impedance: 50, 75, 600, 8 Ω or custom
- Incoherent and in-phase addition of levels
- Quick reference table of common dB values
- Runs in your browser with nothing uploaded
How to use the Decibel Calculator
- In the ratio panel choose Power (10 log) or Voltage or current (20 log), then the direction.
- Type the ratio (output over input) or the dB value. Read the converted figure and the matching ratio of the other kind.
- In the absolute levels panel enter a value, pick its unit and the system impedance; all other units appear together.
- To combine levels, type them separated by commas or spaces, all in the same unit, and read the uncorrelated and in-phase totals.
- Use the quick reference table for the common round numbers, and copy the results when you are done.
Worked examples
Doubling the power
Input
Power ratio 2
Output
+3.01 dB; matching voltage ratio 1.414
Doubling power is 3 dB, but doubling voltage across the same load is 6 dB, because power goes as voltage squared.
0 dBm on a 50 Ω RF system
Input
0 dBm, 50 Ω
Output
1 mW, −30 dBW, 223.6 mV rms, −13.01 dBV, −10.79 dBu, 106.99 dBµV
RF signal generators and spectrum analysers work in dBm; the volts depend on the 50 Ω impedance.
Pro and consumer line level
Input
+4 dBu, then −10 dBV
Output
1.228 V rms (+1.78 dBV); 0.316 V rms (−7.78 dBu)
The two standards differ by 11.78 dB, which is why consumer gear driving a pro input sounds quiet.
Two machines in a workshop
Input
Levels 90, 80 dB
Output
90.41 dB (uncorrelated)
A source 10 dB quieter adds less than half a decibel. Three machines at 85 dB each combine to 89.77 dB.
Background and insight
The bel was introduced by Bell Telephone Laboratories in the 1920s as the base-10 logarithm of a power ratio, and the decibel is a tenth of it: dB = 10 log10(P2 ÷ P1). Because power across a resistance goes as voltage squared, the same ratio expressed in voltage or current is dB = 20 log10(V2 ÷ V1). Both give the same number of decibels for the same change, provided the two voltages are measured across equal impedances. That condition is often forgotten: a 20 log voltage ratio between a 600 Ω source and a 10 kΩ input is a voltage gain, not a power gain.
A suffix turns a ratio into an absolute level by fixing the reference. dBm is relative to 1 mW and dBW to 1 W, so 0 dBm = −30 dBW and 30 dBm = 1 W. dBV is relative to 1 V rms. dBu is relative to 0.7746 V rms, the voltage that delivers 1 mW into 600 Ω, so 0 dBu equals 0 dBm only in a 600 Ω system. The two common audio line levels are +4 dBu (1.228 V, professional) and −10 dBV (0.316 V, consumer), about 11.8 dB apart. Converting dBm to volts needs the impedance: 0 dBm is 223.6 mV in 50 Ω but 774.6 mV in 600 Ω.
Adding decibel levels is not adding numbers. Independent (uncorrelated) sources add as powers: L = 10 log10(Σ 10^(Li/10)), so two equal sources make 3.01 dB more than one and ten make 10 dB more. Identical signals in phase add as voltages, giving 6.02 dB for two. Acoustic levels in dB SPL follow the same power rule for separate sources, which is how noise surveys combine machines.
Common use cases
RF link budgets
Move between transmitter power in watts and dBm, and add gains and losses in dB.
Audio levels and gain staging
Convert between dBu, dBV and volts when connecting consumer and professional equipment.
Filter and amplifier specs
Turn a datasheet attenuation or gain in dB into a ratio, or a measured ratio into dB.
Noise assessments
Combine sound levels from several sources, or check how much quieter the room gets when one is switched off.
Tips and limitations
- Use 20 log only for amplitude quantities (voltage, current, pressure, field strength) and 10 log for power, energy and intensity.
- Decibel gains and losses in a chain simply add, which is why link budgets and gain staging are done in dB.
- dBm figures need the system impedance to become volts. Most RF work assumes 50 Ω and most video 75 Ω.
- A 1 dB change is about 26% in power and 12% in voltage; 0.1 dB is roughly 1% in voltage.
- Sound pressure level, dB(A) weighting and loudness are separate topics. This calculator does the arithmetic of levels, not hearing perception.
Frequently asked questions
When do I use 10 log versus 20 log for decibels?
Use 10 log10 for power ratios and 20 log10 for voltage or current ratios. They give the same dB for the same change because power is proportional to voltage squared across a fixed impedance.
How do I convert dBm to watts?
P (W) = 10^((dBm − 30) ÷ 10). 30 dBm is 1 W, 20 dBm is 100 mW, 0 dBm is 1 mW and −30 dBm is 1 µW.
What is the difference between dBu and dBV?
dBV is referenced to 1 V rms and dBu to 0.7746 V rms. A level in dBu is always 2.218 dB higher than the same voltage in dBV.
How many volts is 0 dBm?
It depends on the impedance: 0.2236 V rms in 50 Ω, 0.2739 V in 75 Ω and 0.7746 V in 600 Ω. dBm is a power level, so volts need P = V² ÷ R.
How do you add two decibel levels together?
Convert each to a power ratio, add, and convert back: L = 10 log10(10^(L1/10) + 10^(L2/10)). Two equal levels give 3 dB more, and 90 dB plus 80 dB gives 90.4 dB.
Is 3 dB double the power?
Yes, almost exactly: 10 log10(2) = 3.0103 dB. For voltage, doubling is 6.02 dB. A 10 dB increase is ten times the power.
What does a negative decibel value mean?
The ratio is less than one: a loss or attenuation. −3 dB is half the power, −20 dB is a hundredth of the power or a tenth of the voltage. In absolute units such as dBm it means below the reference, so −30 dBm is 1 µW.
Is there an in-depth guide to the Decibel Calculator?
Yes. “Decibels Explained: 10 log, 20 log, dBm, dBu and Adding Levels” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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