THD Calculator

Summary

Enter the fundamental and up to 8 harmonic amplitudes to get THD as a percentage and in dB instantly.

How it works

1. Enter the amplitude of the fundamental frequency (H1) — this is the reference signal, typically the desired output.
2. Enter the amplitudes of the 2nd through 8th harmonics (H2–H8) present in the signal. Leave unused harmonics at 0.
3. THD% = (sqrt(H2² + H3² + … + Hn²) / H1) × 100.
4. THD (dB) = 20 × log10(THD% / 100) — a lower (more negative) dB value means less distortion.
5. The results panel shows overall THD% and dB, plus a bar chart of each harmonic's relative level.
6. Use any consistent amplitude unit (volts, dBu, mV) — only the ratios matter for THD.

Use cases

• Measure the distortion performance of an audio amplifier from spectrum analyzer readings.
• Verify that a power supply output meets THD specifications for sensitive equipment.
• Compare two amplifier designs by entering their harmonic profiles side by side.
• Convert a THD percentage figure to dBc for datasheet comparison.
• Analyze which harmonic order dominates the distortion in an audio signal chain.
• Quickly estimate whether an output stage meets a < 0.1% THD target.
• Cross-check THD results from a simulation tool against a hand calculation.
• Evaluate the effect of adding a harmonic filter on overall THD.

Frequently Asked Questions