Voltage Regulation Calculator — Transformer, Load and Line Regulation

Background.

Put a meter across a transformer's secondary with nothing connected and it reads 9.990 V. Connect the load it was built to drive and the reading falls to 9.348 V. The missing 0.642 V was lost inside the transformer, across the resistance of its windings and their leakage reactance, before it ever reached the terminals. Voltage regulation turns that loss into a percentage, and for this transformer it comes to 6.8678 % — more than twice the 3 % a good power transformer is expected to hold on a resistive load.

Quoted figures go astray on the division. Kuphaldt's transformer chapter and Fiore's equations for load and line regulation all divide by the lower, loaded voltage; divide the same 0.642 V by the no-load reading and the transformer seems to improve to 6.4264 % without anything about it having changed. A quoted figure often does not say which base it used, so both are shown here, with the denominator named.

Three different tests share the word. Transformer or generator regulation compares no load with full load while the primary voltage is held steady. Load regulation is the power-supply version: output at minimum load current against output at maximum, input fixed. Line regulation holds the load and moves the input instead, from the highest line voltage to the lowest, and regulator datasheets divide it by the input swing to quote %/V. The fraction is the same in all three. What you change on the bench between the readings is not, and a regulator can be excellent at one test and mediocre at another because different parts of its circuit set each figure.

What is voltage regulation calculator?

Voltage regulation is the change in a source's output voltage between two defined operating conditions, expressed as a percentage of the output at the more demanding one. For a transformer or generator the conditions are no load and rated load, so the figure measures internal impedance: winding resistance and leakage reactance sit in series with whatever is connected, and every amp drawn through them drops voltage the load never sees. For an electronic regulator it measures how well the feedback loop corrects for a change in load current (load regulation) or in input voltage (line regulation). In all three cases the ideal is 0 %.

How to use this calculator.

1. 01Choose the definition that matches the test you ran. The two voltage fields relabel themselves: no-load and full-load for a transformer, minimum- and maximum-load current for load regulation, highest and lowest line for line regulation.
2. 02Enter the light-condition reading. For a transformer, disconnect the load entirely; for a regulator, draw the smallest current it is specified for, since some need a minimum load to regulate at all.
3. 03Enter the heavy-condition reading, which becomes the denominator. Take it with the same meter, at the output terminals rather than the far end of the load leads, so that cable drop and meter error are not counted as regulation.
4. 04For line regulation only, enter how far you moved the input: a sweep from 3 V to 40 V of headroom is a span of 37 V. The field is ignored in the other two modes.
5. 05Read the regulation, then check the denominator and the other-base figure before comparing it with a number someone else has quoted.

The formula.

Regulation % = (V_light − V_heavy) ÷ V_heavy × 100. The subtraction is the voltage lost between the two conditions; dividing by V_heavy states that loss relative to what the load receives when it is working hardest, which is how Kuphaldt (§10.6) defines transformer regulation and how Fiore writes load and line regulation in his Equations 8.1 and 8.2.

The other-base figure divides the same spread by V_light. The two are linked exactly: alternative = r ÷ (1 + r/100), so the gap between them is r² ÷ (100 + r) percentage points. At 1 % regulation that gap is under a hundredth of a point; at 6.8678 % it is 0.44 of a point; on a supply that sags 20 % it reaches 3.33 points. The looser the source, the more the convention changes the number.

In line mode the percentage is also divided by the input span to give %/V, matching the LM317 datasheet's definition: percentage change in output per 1 V change at the input. In transformer mode the result is judged against Kuphaldt's benchmark of under 3 % on a resistive load; load and line regulation get no pass mark beyond the ideal of zero, since what counts as good depends on the part.

What drives a transformer's figure is its series impedance and the load's power factor. The standard approximation puts regulation near the resistive drop times cos φ plus the reactive drop times sin φ, at rated current. A lagging, inductive load adds the reactive term on top of the resistive one, which is why a transformer measured on a resistor bank regulates worse driving a motor. A leading, capacitive load makes sin φ negative and can lift the loaded voltage above the no-load voltage.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Tony R. Kuphaldt, 'Electric Circuits II — Alternating Current' (Lessons in Electric Circuits, Volume II), Workforce LibreTexts, §10.6 'Voltage Regulation'. Primary source for the transformer definition and the quality benchmark. States: 'The measure of how well a power transformer maintains constant secondary voltage over a range of load currents is called the transformer's voltage regulation'; that its worked secondary 'spans a range of 9.990 volts at (virtually) no load and 9.348 volts at the point we decided to call full load'; and that 'a good power transformer should exhibit a regulation percentage of less than 3%' on a simple resistive load, with inductive loads producing worse regulation. NOTE: the section's own arithmetic is rendered as an image which could not be read reliably, so this page does not quote the source's printed percentage — it applies the stated formula to the source's stated voltages and obtains 6.8677792041 %. Retrieved 2026-07-29.
2. [2]James M. Fiore, 'Operational Amplifiers and Linear Integrated Circuits: Theory and Application', Engineering LibreTexts §8.2 'The Need for Regulation'. Primary source for the two power-supply definitions. Equation 8.1: 'Line Regulation = (V_max − V_min)/V_min × 100', where 'V_max is the load voltage produced at the maximum AC line potential and V_min is the load voltage for the lowest AC line potential', and 'the perfect line regulation figure would be 0%'. Equation 8.2: 'Load Regulation = (V_max − V_min)/V_min × 100', where 'V_max is the largest load voltage produced, and V_min is the minimum load voltage produced. These points usually occur at the minimum and maximum load currents, respectively', and it 'would ideally be 0%'. Independent of the Kuphaldt source and agreeing with it on the denominator. Retrieved 2026-07-29.
3. [3]Texas Instruments, 'LM317 3-Pin Adjustable Regulator' datasheet, SLVS044Z, September 1997, revised April 2025. Source of the %/V form of line regulation and of the benchmark figures quoted on this page. Features list: 'Line regulation: 0.01%/V (typ)' and 'Load regulation: 0.1% (typ)'. Electrical Characteristics: line regulation 0.01 typical, 0.04 maximum, unit %/V, at TJ = 25 °C over 3 V ≤ (VIN − VOUT) ≤ 40 V; load regulation for VO ≥ 5 V, 0.1 typical and 0.5 maximum, unit %VO, over IO = 10 mA to 1500 mA. Footnote (2) defines the measure: 'line regulation is expressed as the percentage change in output voltage per 1V change at the input'. PDF datasheet; text extracted locally because the fetch tool could not read it. Retrieved 2026-07-29.
4. [4]James M. Fiore, 'Operational Amplifiers and Linear Integrated Circuits: Theory and Application', Engineering LibreTexts, Chapter 8 'Voltage Regulation' (chapter landing page listing §8.1–8.3). Consulted for the surrounding treatment of linear regulators that gives the two equations their context — in particular that the two figures characterise different parts of a supply and are measured with different quantities held constant. Retrieved 2026-07-29.