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THD Percentage & Decibel Level

Convert a total harmonic distortion amplitude ratio between percentage and decibels over the captured 0.001%–100% range.

Formula & worked example
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How to use this calculator

Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

A THD amplitude ratio of 1% corresponds to −40 dB: 20 × log10(0.01). A 0.1% ratio corresponds to −60 dB.

THD level (dB) = 20 × log10(THD percent / 100)

Worked example

Enter these known values and leave the other values blank.

THD amplitude ratio: 1 %

THD level relative to fundamental: -40 dB

Assumptions and limitations

The ratio is the combined RMS harmonic amplitude relative to the RMS fundamental amplitude. This is an amplitude conversion using 20 log10, not a power-ratio conversion using 10 log10.
The captured domain is 0.001% through 100%, corresponding to −100 through 0 dB. Zero distortion has no finite logarithmic level and is outside this tool.
The tool converts an already computed ratio; it does not analyze audio, identify harmonics, apply a frequency bandwidth or include noise in THD+N.
Some specifications report a positive suppression magnitude. This calculator reports the signed harmonic-to-fundamental level, which is negative below 100%.

Common questions

Does −60 dB mean 0.1% THD?

Yes under this amplitude-ratio convention: 100 × 10^(−60/20) = 0.1%.

Can I enter THD+N?

The arithmetic can convert an amplitude ratio, but THD+N includes noise and is a different measurement. Do not treat the two metrics as interchangeable.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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