Reference · updated 10 November 2026
Voltage drop calculator: wire size, length and current
Current through a cable loses a little voltage along the way: Vdrop = I × R, with R the resistance of the wire there and back. Enter the material, the conductor size, the one-way length, the current and the source voltage, and the calculator gives the drop in volts and in percent, and the voltage left at the load. This is an estimate from the resistance of the wire alone; it does not apply any wiring code.
iPhone and iPad (iOS 17 or later): free on the App Store. Android later.
Voltage drop calculator
This calculator needs JavaScript. Without it, use the formulas and the worked examples below.
Pick the conductor size in mm² or as an AWG number (the area of each AWG is shown next to it), the length in metres or feet, and the current in A, mA or µA. The length is one way, from the source to the load: the calculator counts the return conductor by itself. A decimal point or a decimal comma both work. The source voltage is optional; without it the calculator gives the drop in volts only. For three-phase, enter the line-to-line voltage.
The formula
The resistance of a conductor is R = ρ × L ÷ A, with ρ the resistivity in Ω·mm²/m, L the length in metres and A the cross-section in mm². The current flows out and back, so for DC and single-phase AC the length counts twice:
Vdrop = 2 × ρ × L × I ÷ A
For three-phase AC the line-to-line drop is √3 (1.732) times the one-wire drop:
Vdrop = √3 × ρ × L × I ÷ A
Then percent = Vdrop ÷ Vsource × 100 and Vload = Vsource − Vdrop. The power lost in the cable is Vdrop × I for two conductors and √3 × Vdrop × I for three-phase. One foot is taken as 0.3048 m.
Assumptions and limits
The calculator uses these resistivities at 20 °C: copper 0.017241 Ω·mm²/m (17.241 nΩ·m, annealed copper, 58.0 MS/m) and aluminium 0.0282 Ω·mm²/m (28.2 nΩ·m). The values are those of the Wikipedia table Electrical resistivity and conductivity (copper 1.72 × 10⁻⁸ Ω·m, aluminium 2.82 × 10⁻⁸ Ω·m at 20 °C, read 2026-11-10). The AWG areas are calculated from the AWG diameter formula, as in the AWG to mm² table.
- 20 °C, with no temperature rise. A warm cable has more resistance: copper rises by about 0.39 % per kelvin.
- Resistance only, no reactance. For long or large AC cables the real drop is higher.
- A solid, bare conductor. Stranded wire reads a little higher.
- Steady current. Starting currents and loads that change are not covered.
This is an estimate. It does not apply any wiring code or sizing rule: there are no NEC, IEC or other table look-ups, no allowed percentage, and no verdict on whether a cable is acceptable. For an installation, the wiring rules and the person responsible for the design decide.
Worked examples
A 12 V circuit. Copper, 2.5 mm², 20 m one way, 10 A, DC, 12 V source. Vdrop = 2 × 0.017241 × 20 × 10 ÷ 2.5 = 2.76 V, which is 23 % of 12 V. The load gets 9.24 V and the cable loses 27.6 W.
| Material | Size | Length | Current | Source | Drop | Percent | At the load |
|---|---|---|---|---|---|---|---|
| Copper | 2.5 mm² | 20 m | 10 A | 12 V DC | 2.76 V | 23 % | 9.24 V |
A 120 V circuit in AWG. Copper, AWG 12 (3.31 mm²), 50 ft one way, 15 A, single-phase, 120 V. The length is 15.24 m. Vdrop = 2 × 0.017241 × 15.24 × 15 ÷ 3.309 = 2.38 V, which is 1.99 %. The load gets 117.6 V.
| Material | Size | Length | Current | Source | Drop | Percent | At the load |
|---|---|---|---|---|---|---|---|
| Copper | AWG 12 | 50 ft | 15 A | 120 V AC | 2.38 V | 1.99 % | 117.6 V |
Three-phase in aluminium. Aluminium, 25 mm², 100 m, 40 A, three-phase, 400 V line to line. Vdrop = 1.732 × 0.0282 × 100 × 40 ÷ 25 = 7.82 V, which is 1.95 %. The load gets 392.2 V.
| Material | Size | Length | Current | Source | Drop | Percent | At the load |
|---|---|---|---|---|---|---|---|
| Aluminium | 25 mm² | 100 m | 40 A | 400 V 3-phase | 7.82 V | 1.95 % | 392.2 V |
Measure it live
The honest way to know the drop is to measure it. Connect a Bluetooth multimeter the app reads live, such as the OWON OW18B, and read the voltage at the source and the voltage at the load while the circuit carries its normal current. The difference is the drop, on both conductors together. Take both readings from the same reference, and keep the meter leads short.
The calculator in the app has preset formulas for Ohm's law and power, series and parallel, dividers, LED resistors, RC, dB and unit conversion, and a parallel operator for typing a sum directly. Cal Box fills it from your live reading and recent captures. For the wire's current and power at a known resistance, use the Ohm's law calculator.
What the app does here, and what it doesn't
| What it does | Plan |
|---|---|
| A calculator with SI prefix keys, engineering input such as 4k7 and a parallel operator. | Free |
| Presets for Ohm's law and power, series and parallel, dividers, LED resistors, RC, dB and unit conversion. | Free |
| Cal Box fills the calculator from your live reading and recent captures, and suggests Ohm's law. | Free |
Questions
How do I calculate voltage drop in a wire?
Multiply the resistance of the wire there and back by the current: Vdrop = 2 × ρ × L × I ÷ A for DC or single-phase AC, with L the one-way length. 2.5 mm² copper, 20 m, 10 A gives 2.76 V.
Does the calculator count the return wire?
Yes. Enter the one-way length, from the source to the load. For DC and single-phase AC the calculator counts two conductors; for three-phase it uses the factor √3.
Is a 3 % or 5 % drop acceptable?
The calculator does not say. Allowed values depend on the wiring rules that apply to the installation and on what the load needs, and it applies none of them. It only gives the resistive drop, to compare with what your rules and your load allow.
Why does my measured drop differ from the calculated one?
The calculation uses 20 °C and resistance only. A warm cable, a stranded conductor, a poor termination or reactance in an AC circuit all add drop. Measure the voltage at the source and at the load under load to see the real value.
How do I convert AWG to mm²?
The size list shows the area of each AWG next to its number. The full table is on the AWG to mm² page.
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