Compute percent voltage regulation from no-load and full-load voltage, solve the inverse in both directions, rate regulation quality, and compare several transformers side by side. Everything runs in your browser — no data leaves your device.
What this tool does (and does not do)
This tool = voltage REGULATION: %VR = (V_no-load − V_full-load) / V_full-load × 100 — the transformer / power-supply concept of how much the output sags from no-load to full-load.
Quality bands (general guide, not a certification standard): <3% excellent, 3–5% good, >5% poor. The band edges are compared with exact rational arithmetic (100·(Vnl−Vfl) vs 3·Vfl / 5·Vfl), so a result of exactly 3.00% rates “good”, not “excellent” due to floating-point noise. If V_no-load < V_full-load the tool reports a voltage rise (negative %VR) with a warning instead of a quality band.
Frequently asked questions
What is voltage regulation (%VR)?
Voltage regulation measures how much a transformer's or power supply's output voltage sags between no-load and full-load conditions. It is defined as %VR = (V_no-load − V_full-load) / V_full-load × 100. For example, a transformer that outputs 240 V with no load and 230 V at full load has %VR = (240 − 230) / 230 × 100 = 4.3478%.
How do I calculate voltage regulation from no-load and full-load voltage?
Subtract the full-load voltage from the no-load voltage, divide by the full-load voltage, and multiply by 100. Example: V_no-load = 240 V, V_full-load = 230 V → (240 − 230) / 230 × 100 = 4.3478%. Use Mode 1 of this calculator and it does the arithmetic for you, including mV/V/kV unit conversion.
How do I find the full-load voltage if I know %VR and the no-load voltage?
Rearrange the formula: V_full-load = V_no-load / (1 + %VR/100). Example: %VR = 4.347826% and V_no-load = 240 V → V_full-load = 240 / 1.04347826 = 230.00 V. Use Mode 2 (inverse) of this calculator.
What is a good voltage regulation percentage?
As a general guide (not a certification standard): below 3% is excellent, 3–5% is good, and above 5% is poor. A 240 V → 230 V transformer at 4.3478% falls in the 'good' band. Real requirements depend on the application — sensitive electronics may need tighter regulation than lighting or heating loads.
Can voltage regulation be negative?
Yes. Negative %VR means the output voltage rises under load (V_no-load < V_full-load), which can happen with certain transformer designs, leading power-factor loads, or measurement issues. This calculator reports it as a 'rise' with a warning instead of forcing it into the excellent/good/poor bands.
What is the difference between voltage regulation and voltage drop?
Voltage regulation (%VR) describes a source — how much a transformer or supply's own output sags from no-load to full-load. Voltage drop describes the wiring — how much voltage is lost along cables between source and load. This tool only computes %VR; for divider and wire-drop calculations use our voltage converter and voltage calculator tools instead.
Is voltage regulation the same as line regulation or load regulation?
They are related but different. %VR (this tool) compares no-load vs full-load output of a transformer or supply. Line regulation measures output change vs input-voltage change; load regulation measures output change across a load range for a regulated supply. For an unregulated transformer, %VR is the standard figure quoted on datasheets.
Why does my transformer have poor voltage regulation?
Poor regulation (above ~5%) usually comes from high winding resistance or high leakage reactance — typical of small, cheap, or heavily loaded transformers. Fixes include a larger (lower-impedance) transformer, shorter/thicker secondary wiring, reducing the load, or adding an active voltage regulator downstream.
How do I compare voltage regulation of several transformers?
Enter each candidate's no-load and full-load voltages in Mode 5 (comparison table). The calculator computes %VR for every row, labels each as excellent/good/poor, and ranks them — rank 1 is the tightest (lowest %VR) regulation. Example: 415/400 V → 3.75% (rank 1), 240/230 V → 4.3478% (rank 2), 120/114 V → 5.2632% (rank 3).
Does this calculator work for generators and alternators too?
Yes — %VR = (V_no-load − V_full-load) / V_full-load × 100 applies to any AC or DC source where you can measure terminal voltage unloaded and at rated load, including generators, alternators, bench supplies, and battery packs under load. Just enter the two voltages in the same unit.
We use cookies and third-party services (including Google AdSense) to personalize content, analyze traffic, and serve relevant ads. By clicking "Accept", you consent to our use of cookies as described in our Privacy Policy.
Report an issue
2 taps, no typing needed — diagnostics are sent automatically.