THD Calculator (Total Harmonic Distortion)
By Santaji GadepatilinUpdated October 27, 2026
Drives, UPSs, LED drivers and computers draw distorted current that adds harmonics to the supply. Total harmonic distortion (THD) sums them into one figure you can compare with IEEE 519 limits. Read power harmonics explained and how to do a THD calculation for power systems.
How to use the THD calculator
Enter the fundamental and harmonic magnitudes from a power quality analyser — either in volts, amps or as percentages of the fundamental. You get the THD and the true RMS value. Enter 0 for harmonics you do not have.
For more background, read Active Harmonic Filter Working and Sizing and Neutral Conductor Sizing with Harmonics.
THD formula
THD = √(V₂² + V₃² + V₄² + …) ÷ V₁
True RMS = √(V₁² + V₂² + V₃² + …) = V₁ × √(1 + THD²)
Example
Fundamental 100; harmonics 2, 4, 3, 1.5 and 1 (3rd, 5th, 7th, 11th, 13th):
THD = √(4 + 16 + 9 + 2.25 + 1) ÷ 100 = 5.7%
IEEE 519-2014 voltage limits
| Bus voltage at PCC | Individual harmonic | THD |
|---|---|---|
| ≤ 1 kV | 5.0% | 8.0% |
| 1 kV – 69 kV | 3.0% | 5.0% |
| 69 kV – 161 kV | 1.5% | 2.5% |
| > 161 kV | 1.0% | 1.5% |
Current limits (TDD) depend on the ratio of short-circuit current to load current at the point of common coupling.
Typical current THD
| Load | Current THD |
|---|---|
| 6-pulse VFD, no choke | 80–120% |
| 6-pulse VFD with 3–5% line reactor | 35–45% |
| 12-pulse drive | 10–15% |
| Active front-end drive | under 5% |
| LED lighting, SMPS | 20–150% |
Problems caused by harmonics
- Overheating of transformers, cables and neutral conductors (triplen harmonics add in the neutral).
- Nuisance tripping of breakers and failure of capacitor banks through resonance.
- Errors in meters that are not true-RMS.
Frequently asked questions
What is THD?
Total harmonic distortion — the RMS of all harmonic components divided by the fundamental, expressed as a percentage.
What are the IEEE 519 voltage THD limits?
8% THD for systems up to 1 kV, 5% for 1–69 kV, 2.5% for 69–161 kV and 1.5% above 161 kV, at the point of common coupling.
Which harmonics do drives produce?
Six-pulse VFDs and rectifiers mainly produce the 5th, 7th, 11th and 13th harmonics. Single-phase loads such as PCs and LED drivers add the 3rd.
Can I enter percentages instead of volts or amps?
Yes — enter 100 for the fundamental and each harmonic as a percentage of it.