Reference chartLow code sensitivityLast reviewed June 7, 2026
Electrical reference chart
Decibel Reference Chart
Use this decibel reference chart after the calculator result to document the reference value, ratio type, gain or loss direction, and impedance basis.
Quick reference table
Use this decibel reference chart to remember that 3 dB is about 2x power, 6 dB is about 2x voltage, 10 dB is 10x power, and -3 dB is about half power. Decibels are ratios tied to a reference, so record the power or voltage basis before using the calculator result.
Common decibel ratios
| dB | Power ratio | Voltage ratio | Reference use |
|---|---|---|---|
| 3 dB | about 2x | about 1.41x | Approximate doubling of power |
| 6 dB | about 4x | about 2x | Approximate doubling of voltage |
| 10 dB | 10x | about 3.16x | One decade of power ratio |
| 20 dB | 100x | 10x | One decade of voltage ratio |
| -3 dB | about 0.5x | about 0.71x | Approximate half-power point |
Scroll horizontally to view the full table.
Decibel result context checks
| Calculator result is | Record next | Why it matters |
|---|---|---|
| Power gain or loss | P1, P2, measurement point, and reference condition | A dB value has no meaning without the ratio behind it |
| Voltage gain or loss | V1, V2, and whether impedance basis is the same | The 20 log formula depends on that assumption |
| Negative dB value | Whether the result represents attenuation, loss, or lower measured level | Sign communicates direction, not a separate unit |
| Power-quality note | Meter type, measurement window, and reference value | Measurement setup can change interpretation |
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Decibel chart to calculator handoff
| Search or worksheet need | Use this chart for | Open the calculator when |
|---|---|---|
| dB power ratio | Choosing the 10 log formula and checking gain or loss direction | Actual P1 and P2 values need a calculated dB result |
| dB voltage ratio | Checking the 20 log relationship on the same impedance basis | V1, V2, and the measurement basis must be recorded |
| Negative dB value | Confirming that the compared value is below the reference | The sign, reference value, and attenuation note need a worksheet record |
| Power-quality or signal note | Keeping ratio math separate from measurement interpretation | Meter setup, measurement window, and reference values change the conclusion |
Scroll horizontally to view the full table.
Formula basis
Power ratio: dB = 10 log10(P2 / P1). Voltage ratio: dB = 20 log10(V2 / V1) when impedance basis is the same.
- P2 and P1 are compared power values.
- V2 and V1 are compared voltage values.
- dB is the logarithmic ratio between the two values.
Worked examples
Assumptions. Review the assumptions listed for this chart before applying its values to your case.
- Voltage-ratio dB assumes the same impedance basis for the compared voltages.
- The chart is a ratio reference and does not define a measurement standard by itself.
- Measurement equipment, reference values, bandwidth, and power-quality context must be documented before drawing conclusions.
Code and standard notes. Planning limits that should be checked before final equipment selection.
- Use appropriate meters, reference values, and measurement procedures before drawing power-quality conclusions from dB values.
How to use this chart
Worksheet checklist. Record source basis, review gaps, and assumptions before using the chart result.
- Record measurement typeWrite whether the result is a power ratio, voltage ratio, gain, loss, attenuation, or power-quality measurement note.
- Record reference valuesAdd the baseline and comparison values so the dB result can be traced back to the measurement.
- Check formula familyUse the power-ratio formula for power comparisons and the voltage-ratio formula only when the voltage impedance basis is appropriate.
Common mistakes to avoid. Review these before turning chart current into an equipment decision.
- Treating a dB value as a standalone unit without recording the reference or ratio behind it.
- Using the voltage-ratio formula for a power comparison or the power-ratio formula for a voltage comparison.
- Ignoring the sign of the result and losing whether the value represents gain, loss, attenuation, or reduction.
Frequently asked questions
These answers explain how to use the chart without turning a quick reference into a final design decision.
Why are power and voltage dB formulas different?
Power uses 10 log because dB is defined around power ratio. Voltage uses 20 log when impedance is the same because power is proportional to voltage squared.
Is dB a standalone electrical unit?
No. A dB value describes a ratio or a value relative to a reference, so the reference must be documented.
Why can a dB value be negative?
A negative dB value means the compared value is lower than the reference value. It usually represents loss, attenuation, or reduction.
When should I use the decibel calculator instead of the chart?
Use the calculator when the actual power or voltage values must be entered, when the reference value must be documented, or when the sign of gain or loss matters to the worksheet.