Total Harmonic Distortion Calculator

Total harmonic distortion, usually abbreviated THD, measures how much energy in a periodic signal appears in harmonics rather than in the intended fundamental frequency. A pure sine wave contains only its fundamental and has zero percent THD. Real amplifiers, speakers, power supplies, converters, generators, and electrical loads introduce nonlinear behavior that creates components at integer multiples of the fundamental frequency. THD summarizes those unwanted components as one percentage that is easier to compare than a long spectrum. This calculator accepts the RMS amplitude of the fundamental plus the second, third, fourth, and fifth harmonics. It squares each harmonic amplitude, adds those squares, and takes the square root to obtain their combined root-sum-square amplitude. Dividing that value by the fundamental amplitude and multiplying by one hundred gives THD percent. If a 100 volt fundamental has one 10 volt harmonic, THD is 10 percent. If two harmonics measure 3 and 4 volts, their combined amplitude is 5 volts and THD is 5 percent against a 100 volt fundamental. All entries must use the same measurement basis and unit. Do not mix peak voltage with RMS voltage, or volts with millivolts, unless you first convert them. Spectrum analyzers may report harmonic levels in decibels relative to the carrier. Those values must be converted to linear amplitude before entry. Also ensure that harmonic readings represent amplitudes rather than powers, since power ratios use a different conversion. Lower THD generally means a waveform more closely resembles the desired sinusoid, but acceptable levels depend on the application. High-fidelity audio equipment may target fractions of a percent, while power-system standards specify limits based on voltage level, load, and harmonic order. A low aggregate number can still conceal one problematic harmonic, so engineering diagnosis should inspect the complete spectrum in addition to THD. This calculation includes only the four entered harmonics. A laboratory THD measurement may include dozens of orders and use bandwidth, weighting, windowing, or noise rejection rules defined by a standard. THD plus noise, written THD+N, is also different because it includes broadband noise in the numerator. Use this result for quick design checks, educational examples, and comparison of known harmonic components. For compliance testing, follow the applicable measurement standard and calibrated instrument procedure.