Total Harmonic Distortion (THD) Calculator

Last updated: October 18, 2026 · Written by the Pitch Detectors Editorial Team

Measure Total Harmonic Distortion to grade audio clarity. Enter the fundamental signal voltage and the harmonic voltages — get THD as a percentage, a distortion grade, and what the number means for speakers, amplifiers, and full audio systems.

Comma-separated (2nd, 3rd, 4th harmonics)

Total Harmonic Distortion

0.00 %

Distortion Grade
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Signal Purity
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Best Use Case
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A THD calculator measures how much unwanted harmonic distortion audio gear adds to the signal. Enter the fundamental voltage and each harmonic voltage; the calculator returns total harmonic distortion as a percentage, plus a distortion grade, signal-purity rating, and best use case.

Key Takeaways

  • THD % = √(V₂² + V₃² + …) ÷ V₁ × 100 — all harmonic voltages root-summed, divided by the fundamental.
  • 1% THD = −40 dB; 0.1% = −60 dB; 0.01% = −80 dB — each extra zero is another −20 dB.
  • Even harmonics (2nd, 4th) sound warm; odd harmonics (3rd, 5th) sound harsh — character beats quantity.
  • Below ~0.1%, THD is inaudible in music — speakers and room matter more than another zero.
  • Clipping is the #1 real-world THD source; amplifier headroom is the cure.

What Is Total Harmonic Distortion?

Definition: Total harmonic distortion (THD) is the ratio of all unwanted harmonic frequencies a device adds to the original (fundamental) signal, expressed as a percentage. When a 1 kHz tone passes through an amplifier and emerges with traces of 2 kHz, 3 kHz, and 4 kHz riding along, THD quantifies those uninvited guests. Lower THD means the output is a truer copy of the input.

Harmonics are integer multiples of the fundamental: the 2nd harmonic is an octave up, the 3rd an octave plus a fifth, and so on. Every amplifier, speaker, and DAC creates some — the only questions are how much, and which orders dominate.

How This Calculator Works

This calculator applies the standard THD definition directly to your numbers. It takes the RMS voltage of the fundamental signal and the RMS voltages of each harmonic component, combines the harmonics as a root-sum-of-squares, and divides by the fundamental.

Methodology: THD % = √(V₂² + V₃² + V₄² + …) ÷ V₁ × 100, where V₁ is the fundamental and V₂…Vₙ are harmonic RMS voltages. The result is graded against fixed thresholds — under 0.1% = Reference Grade, under 0.5% = High Fidelity, under 1% = Acceptable, 1% and above = High Distortion. We verified this calculator's output against hand calculations using 5 sample input combinations; every result matched to 0.01%. Last reviewed: October 2026.

THD Formula and Worked Example

THD % = √(V₂² + V₃² + V₄² + …) ÷ V₁ × 100

Worked example: 1 V RMS fundamental with harmonics of 0.02 V, 0.015 V, and 0.01 V:

  1. Square each harmonic: 0.02² = 0.0004; 0.015² = 0.000225; 0.01² = 0.0001
  2. Sum: 0.0004 + 0.000225 + 0.0001 = 0.000725
  3. Square root: √0.000725 = 0.0269 V of distortion
  4. Divide by fundamental: 0.0269 ÷ 1 × 100 = 2.69% THD — high distortion, likely clipping

Now the dB conversion spec sheets love: THD in dB = 20 × log₁₀(THD ÷ 100).

THD %THD in dBTypical of
10%−20 dBHeavily clipping amplifier
1%−40 dBAudibility threshold for pure tones
0.1%−60 dBGood hi-fi amplifier
0.01%−80 dBExcellent modern amplifier/DAC
0.001%−100 dBState-of-the-art (inaudible in any music)

How to Use This Calculator

  1. Fundamental signal: enter the RMS voltage of the original test tone (default 1 V).
  2. Harmonic RMS voltages: enter comma-separated values for the 2nd, 3rd, 4th… harmonics (e.g., 0.02,0.015,0.01).
  3. Read THD % instantly — it updates as you type.
  4. Check the distortion grade, signal purity, and best use case ratings.
  5. Compare against the audibility table below to judge whether the number matters.

Reading Your Result

THDGradeWhat it meansSafe next step
Under 0.1%ReferenceExtremely clean; beyond audibilityStop chasing numbers — look at speakers and room instead.
0.1–0.5%High fidelityClean for any home or car systemYou're fine; check amplifier headroom if pushing limits.
0.5–1%AcceptableFine for casual listeningAudible only on test tones; don't upgrade for this alone.
Over 1%High distortionLikely audible harshnessReduce gain / add headroom — you're probably clipping.

Reading Manufacturer THD Specs Honestly

One THD number is meaningless without its test conditions. Manufacturers quote the most flattering figure; before comparing two specs, demand answers to these 4 questions:

  • At what level? Distortion explodes near maximum output (clipping). A 0.01% figure at 1 watt says nothing about behavior at full power.
  • At what frequency? Most audio circuits distort more at high frequencies — which is exactly why specs are measured at 1 kHz, the flattering middle.
  • Into what load? A harder load (lower impedance, steeper phase) raises distortion. "8 Ω resistive" is the easy test.
  • With or without noise? Pure THD excludes the noise floor; THD+N includes it. A tiny THD figure can hide a noisy device.

Even vs. Odd Harmonics: Why "Warm" and "Harsh" Exist

Which harmonics dominate matters as much as how much distortion there is. Even-order harmonics (2nd, 4th, 6th) are octaves and consonant intervals of the fundamental; they sound "warm," "full," or "euphonic." This is the famous tube-amplifier sound: single-ended triodes produce predominantly 2nd-harmonic distortion.

Odd-order harmonics (3rd, 5th, 7th) create dissonant intervals — the 3rd harmonic sits a twelfth above the fundamental, clashing with the musical harmony. Transistor clipping generates strong odd-order products, which is why a clipping solid-state amp sounds "harsh" or "grainy" while a softly saturating tube amp at the same THD % sounds pleasant. Two devices at 1% THD can sound completely different — the percentage alone never tells the whole story.

even-order harmonics sound consonant, odd-order harmonics sound dissonant
Diagram: even-order harmonics sound consonant, odd-order harmonics sound dissonant

When Can You Actually Hear THD?

Under laboratory conditions with pure test tones, trained listeners detect distortion around 1% — and even then mostly in the midrange where hearing is most sensitive. But music is not a test tone: complex signals mask distortion, pushing audibility thresholds several times higher.

Practical takeaways: below 0.1%, THD is inaudible in any real music — the difference between a 0.01% and 0.001% amplifier exists only on the spec sheet. Between 0.5–2%, you might catch harshness on loud, simple passages (solo piano, female vocals). Above ~3–5%, clipping harshness is obvious to anyone. Speaker drivers themselves routinely produce 1–5% THD in the bass at high output — which is why speaker distortion, not amplifier distortion, usually dominates what you hear.

THD vs. THD+N vs. SINAD: The Decoder

Spec sheets use three related numbers for one underlying idea. Here's how they connect:

MetricWhat it includesRelation
THDHarmonic distortion onlyThe classic percentage
THD+NHarmonics + noise floorAlways ≥ THD; the honest real-world number
SINADSignal ÷ (noise + distortion), in dBSINAD (dB) = −20 × log₁₀(THD+N as a ratio)

A DAC rated "0.0005% THD+N" equals 106 dB SINAD — superb. When comparing gear, match like with like: THD against THD, SINAD against SINAD. And remember noise dominates THD+N at low signal levels, while distortion dominates near maximum output.

Limitations

  • Percentage hides character. 1% of warm 2nd-harmonic and 1% of harsh 3rd-harmonic get the same number — your ears won't.
  • Single-frequency snapshot. Real THD varies with frequency and level; one number can't describe an amplifier's full behavior.
  • No noise included. This is pure THD; real measurements (THD+N) add the noise floor.
  • Ignores intermodulation. IMD (distortion from two interacting tones) often correlates better with perceived harshness than THD.
  • Speaker distortion dominates. Chasing 0.001% amplifier THD is pointless if your speakers produce 2% in the bass — fix the biggest offender first.

Reducing Distortion: What Actually Works

ProblemFixWhy it works
Amplifier clippingAdd headroom — use the amplifier power calculatorClipping is the #1 THD source; unused power never distorts
Speaker breakup at high volumeHigh-pass filter / smaller drivers for highsKeeps drivers in their linear excursion range
Gain staging too hotLower source gain, raise amp gain lastDistortion multiplies through every hot stage
Underpowered amp strainingMore clean watts via the wattage calculatorAn amp at 50% never clips; an amp at 110% always does
[Infographic: clean sine wave → softly clipped (warm) → hard clipped (harsh) with harmonic spectra]
Infographic: how clipping shape determines whether distortion sounds warm or harsh

Frequently Asked Questions

**Under 0.1% is excellent and inaudible in music; under 0.5% is clean for any home or car system.** Most modern solid-state amplifiers measure 0.01–0.05%. Below 0.1%, stop comparing — frequency response, noise, and your speakers matter far more than another zero.

**THD % = √(V₂² + V₃² + V₄² + …) ÷ V₁ × 100**, where V₁ is the fundamental RMS voltage and V₂… are harmonic RMS voltages. Square each harmonic, sum them, take the square root, and divide by the fundamental. Enter your values above — the calculator does it instantly.

**Not necessarily — character beats quantity.** A tube amp at 1% THD (mostly warm 2nd harmonic) often sounds more pleasant than a transistor amp at 0.5% THD (mostly harsh 3rd harmonic). Below ~0.1%, differences are inaudible regardless. The percentage alone never tells the whole story.

**THD counts only harmonic distortion products; THD+N adds the device's noise floor.** THD+N is always equal or higher, and it's the more honest real-world figure — especially at low volumes where noise dominates. SINAD is the same information expressed in dB: higher is better.

**Yes — on pure test tones, trained listeners can; around 1% is the classic threshold.** On real music, masking hides it well; most people won't notice 1–2% on complex material. Above ~3–5%, clipping harshness becomes obvious to anyone. Context matters more than the raw number.

**Tube amplifiers sound warm because single-ended designs produce mostly 2nd-harmonic distortion** — an octave above the fundamental, which the ear hears as warmth and fullness rather than harshness. A tube amp at 2% THD can sound cleaner than a clipping transistor amp at 1% THD dominated by odd harmonics.

**Amplifier clipping is the #1 cause — running out of clean power.** Next come speaker nonlinearities at high excursion (the motor and suspension leaving their linear range), overdriven gain stages, and underpowered amplifiers straining. Fix clipping first with proper headroom; it dwarfs every other source.

**To convert THD percentage to dB, use dB = 20 × log₁₀(THD ÷ 100).** So 1% = −40 dB, 0.1% = −60 dB, 0.01% = −80 dB — each extra zero after the decimal is another −20 dB. SINAD is this number with the sign flipped and noise included: 0.001% THD+N ≈ 100 dB SINAD.

**Speakers contribute far more distortion than amplifiers — often a hundred times more.** A good amplifier holds 0.01–0.1% THD while a typical woofer produces 1–5% in the bass at high output. If your system sounds distorted, suspect the speakers (or clipping) long before blaming the amplifier's THD spec.

**Look at independent measurements — most speaker makers don't publish THD.** Under 1% through the midrange at 90 dB is good; under 3% in the bass at high output is respectable for the price. Prioritize smooth frequency response first; it predicts sound quality far better than a THD number.

Note: this calculator is an educational tool for understanding distortion math. For certified equipment measurements, use a calibrated audio analyzer (e.g., Audio Precision) in controlled conditions — this page is not professional test-lab advice.