Voltage Regulation Calculator — Transformer & Power Supply
Enter the no-load and full-load terminal voltages of a transformer, generator or power supply to find the voltage regulation percentage — the key measure of how stable the output is under varying load.
V
V
Lower % = better regulation; negative means output rises under load
- 1
Voltage change V_NL − V_FL
12.5 − 12 = 0.5 - 2
Voltage regulation %
(0.5 ÷ 12) × 100 = 4.17Positive means the output sags under load; negative means it rises.
How does this calculator work?
VR % = (V_NL − V_FL) / V_FL × 100. Enter the terminal voltages with no load and with full load. Less than 1 % is excellent (regulated supply), 2–5 % is good (quality transformer), above 10 % is poor. Negative VR means voltage rises under load (capacitive or over-excited reactive load).
Formula
How this is calculated
Voltage regulation (VR %) quantifies how much the output voltage of a power source changes between no load (nothing connected to the output) and full load (rated maximum current drawn). A perfect, ideal source would have VR = 0 % — its terminal voltage never changes regardless of load. Real sources have internal impedance, so the output drops as load current increases, making VR positive. In some reactive circuits or over-excited alternators the voltage can actually rise under load, giving a negative VR.
The formula VR % = (V_NL − V_FL) / V_FL × 100 is standard for transformers (IEEE C57), generators and linear power supplies. Note that the denominator is the full-load voltage (the rated output), not the no-load voltage — this convention makes VR a measure of how far the output strays from its intended value.
Typical values: an ideal linear regulator or regulated switching supply: <0.1 %; a well-designed power transformer: 2–5 %; a utility distribution transformer: 2–10 %; a small generator under heavy resistive load: 3–10 %. Higher VR means the output sags more under load, which degrades performance of sensitive equipment and can cause voltage-sensitive devices to malfunction. Power factor of the load also affects measured VR — purely resistive loads (PF = 1) typically show the largest sag.
Frequently asked questions
For regulated power supplies, VR < 1 % is considered excellent. Good power transformers and generators aim for 2–5 %. Values above 10 % generally indicate poor regulation that may affect load performance. Utility distribution transformers are typically 4–8 %, with tight regulation being more important close to sensitive loads.
Yes. Negative VR means the terminal voltage actually rises under load, which can happen in capacitive loads, over-excited synchronous generators, or circuits with leading power factor. A negative result from this calculator indicates V_FL > V_NL, which is unusual but not impossible in reactive power systems.
Voltage drop (in circuits or cables) refers to the absolute voltage lost across a resistance due to current flow — it is measured in volts. Voltage regulation is a dimensionless percentage that describes how stable a source's terminal voltage is between no load and full load conditions. Both concepts relate to internal impedance, but drop is used for passive conductors while regulation characterises active sources like transformers and power supplies.
Also known as
TG we-Calculate Editorial Team. (2026). Voltage Regulation Calculator — Transformer & Power Supply [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/voltage-regulation-calculator
TG we-Calculate Editorial Team. "Voltage Regulation Calculator — Transformer & Power Supply." TG we-Calculate. 2026. https://we-calculate.com/calculator/voltage-regulation-calculator.
TG we-Calculate Editorial Team, "Voltage Regulation Calculator — Transformer & Power Supply," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/voltage-regulation-calculator
@misc{wecalculate_voltage_regulation_calculator, title = {Voltage Regulation Calculator — Transformer & Power Supply}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/voltage-regulation-calculator}}, year = {2026}, note = {TG we-Calculate} }
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