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Read THD-F (IEEE) next to THD-R (IEC 60268) from H1 and H2-H8 amplitudes, in percent, ppm and dB. SINAD and ENOB share the panel for ADC and DAC checks.

📘 How to Use

  1. Type the fundamental H1 and the H2-H8 harmonic amplitudes into the input rows
  2. Switch the input mode between linear amplitude and dBc reference
  3. Read the THD-F, THD-R, SINAD and ENOB results

Total Harmonic Distortion (THD) Calculator

Enter the fundamental H1 and seven harmonic amplitudes (H2-H8) read off an FFT.

Enter amplitudes in volts or any linear unit (negative values clamp to 0).

THD-F (IEEE: harmonics RMS / fundamental)
1.118%
11180 ppm · -39.0 dB
THD-R (IEC: harmonics RMS / total RMS)
1.117%
11173 ppm · -39.0 dB
SINAD (equivalent)
39.0 dB
ENOB (full-scale sine)
6.18 bit

※ THD-F = sqrt(H2^2 + H3^2 + ... + H8^2) / H1. This is the IEEE / ITU-T definition.

※ THD-R = sqrt(sum of harmonic squares) / sqrt(sum of all squares). This is the IEC 60268 family definition.

※ ENOB = (SINAD - 1.76) / 6.02 for an ideal full-scale sine. No noise term is given, so SINAD here equals SNDR with N = 0.

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Total Harmonic Distortion (THD) Calculator | THD-F, THD-R, SINAD and ENOB on One Panel

Enter the fundamental H1 and harmonics H2-H8 and read the IEEE-style THD-F next to the IEC-style THD-R in percent, ppm and dB. SINAD and ENOB for ADC/DAC evaluation land on the same screen.

💡 Why two THD numbers stop a spec comparison from going sideways

"THD" hides two different definitions that disagree on what sits in the denominator. THD-F (IEEE / ITU-T) divides the harmonic RMS by the fundamental; THD-R (IEC 60268) divides it by the RMS of the whole signal. Below a percent or so they track each other, but once distortion climbs the gap is real. Compare two amplifiers without checking which definition each datasheet used and an otherwise equal part can look worse than its rival.

This calculator takes whatever you can read off an FFT and prints both definitions side by side so you never mix up the standard. Type magnitudes in volts (linear) or, if your analyzer reports relative levels, drop into dBc with the fundamental pinned at 0 dB — the math normalizes to a linear ratio either way. It also derives SINAD = -THD-R(dB) and ENOB = (SINAD − 1.76) / 6.02, so you can cross-check a vendor's THD, SINAD and effective-bits claim straight from a spectrum screenshot.

🧐 Frequently Asked Questions

Which do I report, THD-F or THD-R? Match the datasheet you are comparing against. Audio amplifier and ITU-T work usually quotes THD-F; IEC 60268 instrument measurements lean on THD-R. Both are shown, so just read the one that matches the other side.

How do I enter the fundamental in dBc mode? The fundamental H1 is always treated as the 0 dB reference. Enter each harmonic as a relative level (e.g. −60 dBc). The more negative the value, the lower the distortion contribution.

What assumptions sit behind SINAD and ENOB here? There is no noise-floor input field, so SINAD is computed with the noise term set to zero — effectively an SNDR derived from harmonics alone. ENOB then uses the ideal full-scale sine formula (SINAD − 1.76) / 6.02. Treat both as harmonic-only figures, not a bench-measured value that folds in real noise.

Why only up to H8? The tool covers the 2nd through 8th harmonics — seven rows. In audio and data-converter work the low-order harmonics dominate, so H8 captures the practically meaningful THD without chasing diminishing tails.

What happens to a negative value in linear mode? Amplitude is a magnitude, so negatives clamp to 0. Paste the FFT peak magnitudes as-is.

📚 Even vs odd harmonics: the fingerprint a single THD hides

The "flavor" of distortion lives in how the harmonics line up. Tube amps get called warm because they lean on 2nd-order (even) harmonics, which sit an octave above the fundamental and read as consonant rather than gritty. Transistor and digital clipping tends to throw strong 3rd and 5th (odd) harmonics, perceived as a harder edge. A lone THD number erases that — it tells you how much but not which orders dominate. That is exactly why the H2-H8 rows are separate: scan the per-harmonic amplitudes and two parts with identical THD reveal whether even or odd orders rule, giving you a qualitative read on the circuit's distortion character.