What is the difference between THD-F and THD-R?

THD-F (distortion relative to the Fundamental) divides the harmonic RMS by the fundamental amplitude. THD-R (distortion relative to the total RMS) divides by the full signal RMS including the fundamental. THD-F is more common in power electronics; THD-R appears in some audio standards. THD-F is always ≥ THD-R.

What units should I enter?

Any consistent linear amplitude unit works — volts (V), millivolts (mV), or arbitrary FFT magnitude bins. The result is a dimensionless percentage, so mixed units will give wrong answers. Do not enter dBV values directly.

How many harmonics do I need?

IEEE 519 typically evaluates up to the 50th harmonic for power systems, but most energy is in the lower harmonics. In practice, capturing the 2nd through 10th harmonics covers the dominant distortion components for most waveforms.

What is a good THD value?

For utility power, IEEE 519 limits voltage THD to 5% at most points of common coupling and current THD varies by system. For high-fidelity audio, amplifier THD below 0.1% is excellent. Industrial drives often target below 5% THD-I to avoid equipment heating.

Can I enter zero for unused harmonics?

Yes. Leaving a field blank or entering 0 is equivalent — that harmonic contributes nothing to the distortion sum.

Harmonic Distortion THD Calculator

Enter the fundamental and harmonic amplitudes to compute THD% instantly.

Harmonic Amplitudes

Enter amplitudes in any consistent linear unit (V, mV, etc.).

Fundamental (H1) V

2nd harmonic (H2) V

3rd harmonic (H3) V

4th harmonic (H4) V

5th harmonic (H5) V

6th harmonic (H6) V

7th harmonic (H7) V

8th harmonic (H8) V

9th harmonic (H9) V

10th harmonic (H10) V

Enter harmonic values and click Calculate THD

THD-F

—

relative to fundamental

THD-R

—

relative to total RMS

Harmonic Breakdown

Harmonic	Amplitude	% of Fund.	% of Total

Signal Composition

Fundamental RMS: —

Harmonic RMS: —

Total RMS: —

Active harmonics: —

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Summary

Total Harmonic Distortion (THD) quantifies how much a signal deviates from a pure sine wave due to harmonic content. This calculator accepts the fundamental amplitude and up to nine higher harmonics, then computes both THD-F (relative to the fundamental) and THD-R (relative to the total signal RMS) according to standard formulas. It is useful for power quality assessment, audio amplifier evaluation, and inverter waveform analysis.

How it works

Enter the amplitude of the fundamental frequency (1st harmonic).
Enter the amplitudes of any higher harmonics (2nd, 3rd … up to 10th).
The calculator squares each harmonic, sums them, and takes the square root.
THD-F divides that RMS by the fundamental; THD-R divides by the total RMS signal.
Click Calculate THD or press Enter to view THD-F, THD-R, and the harmonic breakdown.

Use cases

Measure power quality on AC mains with a spectrum analyzer reading.
Evaluate audio amplifier distortion from FFT output data.
Verify inverter output quality meets IEEE 519 harmonic limits.
Analyze motor drive waveforms for harmonic content.
Compare THD before and after adding a harmonic filter.
Convert between THD-F and THD-R definitions for different standards.

Frequently Asked Questions

Last updated: 2026-06-19 · Reviewed by Nham Vu