Voltage drop percentage calculator

It reports the lost volts, expresses that loss as a percentage of source voltage, and checks the result against a maximum percentage you choose. Use it to evaluate an installed circuit run from real measurements without entering wire gauge, conductor material, current, or cable length. The result gives a direct, repeatable indication of whether the measured drop stays within your selected regulation target.

Enter comparable source and load measurements

Measure source voltage at the circuit origin and load voltage at the equipment end under the same operating condition. For a useful result, the circuit should be carrying the load you actually want to evaluate; an unloaded reading may conceal loss that appears only when current flows. Enter both readings in volts, using the same meter range and measurement convention. The source value must be greater than zero, and the load value cannot exceed it because this calculator is specifically designed for voltage drop rather than voltage rise. You may also enter the maximum acceptable drop percentage required by your design, specification, or inspection procedure. The default is 3 percent, but that is a convenient comparison value rather than a universal legal rule. Applicable limits can differ by circuit purpose, installation, jurisdiction, and project documentation. If you are checking compliance, take the threshold from the governing rule or approved design instead of assuming the default applies. Accurate readings and a correctly chosen threshold matter more than extra decimal places in the final percentage.

Understand the percentage and limit check

The calculator first subtracts load voltage from source voltage to find the absolute drop in volts. It then divides that drop by source voltage and multiplies by 100. For example, a source reading of 120 V and a load reading of 115.8 V produce a 4.2 V drop; 4.2 divided by 120 equals 0.035, so the voltage drop is 3.5 percent. The denominator is always source voltage, which makes results consistent across circuit voltage levels. The returned within-limit flag is true when the calculated percentage is less than or equal to the selected maximum, including an exact boundary match. The assessment field repeats that decision in a compact text value for reports and automation. Outputs are rounded deterministically, while the limit comparison uses the underlying calculation so display rounding does not incorrectly change a pass into a failure. A passing result means only that these readings satisfy the threshold supplied to the calculator. It does not independently certify conductor ampacity, protection, insulation, grounding, temperature correction, or any other electrical requirement.

Use the result to investigate a circuit run

A result above the chosen limit is a prompt to investigate the circuit, not a diagnosis by itself. Excessive drop may be associated with a long run, undersized conductors, high operating current, loose or damaged connections, contact resistance, or a source that changes as the load is applied. Repeat measurements safely and confirm that source and load readings represent the same operating moment when the supply is variable. Comparing results at different accessible points can help locate where a large part of the loss occurs, but electrical testing should be performed only by someone qualified for the equipment and environment. A result within the selected limit is useful for commissioning records, maintenance comparisons, and automated quality checks. Save the two raw readings, the threshold, and the calculated output together so another person can reproduce the decision later. For design-stage calculations based on wire gauge, run length, current, and conductor material, use a wire voltage-drop model instead; this capability intentionally evaluates measured or otherwise known endpoint voltages and does not estimate them from cable properties.