Formula Used
- Power ratio: dB = 10 × log10(P₂ ÷ P₁).
- Amplitude ratio: dB = 20 × log10(A₂ ÷ A₁).
- dBm: dBm = 10 × log10(P ÷ 1 mW).
- dBW: dBW = 10 × log10(P ÷ 1 W).
- dBV: dBV = 20 × log10(V ÷ 1 V).
- dBu: dBu = 20 × log10(V ÷ 0.774596 V).
- dBµV: dBµV = 20 × log10(V ÷ 1 µV).
- dB SPL: 20 × log10(p ÷ 20 µPa).
How to Use This Calculator
- Select the required decibel conversion mode.
- Enter the source value and applicable unit.
- Add impedance, references, gains, or losses when requested.
- Choose precision and optional scientific notation.
- Review the live estimate before submitting.
- Select Calculate for validated server-side results.
- Copy, print, or export the finished result.
Common Decibel Comparisons
+3 dB
Approximately twice the power.
−3 dB
Approximately half the power.
+6.02 dB
Approximately twice the voltage.
+10 dB
Ten times the power.
Example Data
| Conversion | Input | Result |
|---|---|---|
| Power ratio to dB | 2:1 | 3.0103 dB |
| Amplitude ratio to dB | 2:1 | 6.0206 dB |
| dBm to power | 30 dBm | 1 W |
| dBW to power | 10 dBW | 10 W |
| dBV to voltage | 6.0206 dBV | 2 V RMS |
| dBu to voltage | 0 dBu | 0.774596 V RMS |
| dBµV to voltage | 120 dBµV | 1 V RMS |
| dB SPL to pressure | 94 dB SPL | About 1 Pa |
Frequently Asked Questions
What does a decibel measure?
It expresses a logarithmic ratio between two quantities. The quantities may describe power, voltage, sound, or signal levels. Always confirm the reference before interpreting any decibel value.
Why use 10 for power ratios?
Power quantities use ten times the base-ten logarithm. This convention directly represents ratios between measured power levels. It also keeps cascaded gains easy to add together.
Why use 20 for voltage ratios?
Voltage and amplitude quantities often square into power. Therefore, their logarithmic coefficient becomes twenty instead of ten. Use equal impedances when comparing voltage-derived power ratios accurately.
What is the difference between dBm and dBW?
dBm references one milliwatt of electrical power. dBW references one watt of electrical power. The numerical difference is always exactly thirty decibels.
What does zero dBV represent?
Zero dBV represents one volt RMS. Positive values exceed that reference voltage. Negative values remain below the one-volt reference level.
What does zero dBu represent?
Zero dBu represents approximately 0.774596 volts RMS. The reference originated from professional audio power conventions. It remains useful without requiring a specific load impedance.
What does dB SPL reference?
dB SPL references twenty micropascals in air. That pressure approximates a typical human hearing threshold. Actual hearing sensitivity varies by frequency and person.
Can decibel values be negative?
Yes, negative values indicate levels below their reference. They do not necessarily mean negative physical power. Confirm the chosen reference before judging signal strength.
Why is impedance required for dBm voltage?
Voltage alone does not uniquely determine electrical power. The load impedance connects RMS voltage with delivered power. Incorrect impedance produces an incorrect dBm conversion result.
How are multiple decibel stages combined?
Add gains and subtract positive loss magnitudes. Logarithmic values combine without multiplying each linear stage. This method simplifies amplifiers, cables, filters, and connectors.
Are the three-decibel rules exact?
Three decibels is only an approximate doubling. The exact power doubling value is about 3.0103 decibels. Six-point-zero-two decibels approximately doubles an amplitude quantity.