Total Harmonic Distortion Calculator
Total harmonic distortion quantifies how faithfully a system reproduces a signal by comparing the energy in unwanted harmonics with the energy in the fundamental tone. It is a key figure of merit for audio amplifiers, power supplies, and electrical-supply quality. Enter the amplitude of the fundamental and of the second through fifth harmonics in the same units; this calculator returns the THD as a percentage, the THD as a plain ratio, the combined harmonic amplitude, and the equivalent value in decibels.
THD formula
Combined harmonics = sqrt(V2^2 + V3^2 + V4^2 + V5^2)
THD ratio = combined harmonics / V1
THD % = THD ratio * 100
THD dB = 20 * log10(THD ratio)
V1 is the fundamental amplitude and V2 through V5 are the harmonic amplitudes, all in the same units. This is the THD-F definition, which references the harmonics to the fundamental.
Harmonic distortion context
- THD-F divides the combined harmonic amplitude by the fundamental; lower values mean a cleaner signal.
- The harmonics combine as the root of the sum of their squares because they are at different frequencies.
- High-fidelity audio amplifiers often achieve THD below 0.1 percent.
- Power-quality standards limit supply THD to the low single-digit percent range.
- Expressed in decibels, a 1 percent THD is -40 dB; more negative is cleaner.
Total harmonic distortion: frequently asked questions
What is total harmonic distortion?
Total harmonic distortion, or THD, measures how much a signal is distorted by harmonics added to its fundamental frequency. It is the ratio of the combined amplitude of all the harmonics to the amplitude of the fundamental, usually expressed as a percentage.
How is THD calculated?
Take the square root of the sum of the squares of the harmonic amplitudes, then divide by the fundamental amplitude. Multiplying by 100 gives THD as a percentage. This calculator uses the second through fifth harmonics you enter relative to the fundamental.
What is the difference between THD-F and THD-R?
THD-F references the harmonics to the fundamental amplitude, while THD-R references them to the total RMS signal including the fundamental. THD-F is the more common engineering definition and is what this calculator reports as the percentage.
What is a good THD value?
Lower is better. High-fidelity audio amplifiers often achieve THD well below 0.1 percent, while power-quality standards for electrical supplies set limits in the low single-digit percent. Acceptable THD depends entirely on the application.
How does THD convert to decibels?
THD in decibels is 20 times the base-10 logarithm of the THD expressed as a ratio (not a percentage). A THD of 1 percent, a ratio of 0.01, equals -40 decibels. More negative decibel values mean lower distortion.
Official sources
- NIST: Physical Measurement Laboratory (waveform metrology).
- NIST: The decibel and logarithmic ratios (SP 811).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.