12 Volt Voltage Drop Calculator: Formula and Examples

This 12-volt voltage drop calculator estimates the voltage lost along a two-conductor DC cable run from current, one-way distance, and conductor resistance. Enter the load current, the one-way cable length, and the conductor’s resistance per unit length; the calculator returns the round-trip voltage drop and the percentage of the supply voltage that drop represents.

Formula

For a two-wire DC circuit, voltage drop is V_drop = 2 × I × L × R, when length and resistance use compatible units. In this calculator, the formula is V_drop = 2 × I × L × R_km ÷ 1,000. Here, I is current in amperes (A), L is one-way distance from source to load in metres (m), and R_km is the resistance of one conductor in ohms per kilometre (Ω/km). The factor of 2 accounts for current traveling out on one conductor and returning on the other.

The percentage is 100 × V_drop ÷ 12, using a nominal 12 V supply. Estimated load voltage is 12 − V_drop. These are arithmetic estimates: actual supply voltage and conductor resistance can vary with conditions. Enter the resistance for the actual conductor material and size, rather than treating the default as universal.

Worked Example

Using the calculator’s defaults: current I = 10 A, one-way distance L = 5 m, and conductor resistance R_km = 3.28 Ω/km.

First convert the resistance for the 5 m run: 3.28 Ω/km × 5 m ÷ 1,000 = 0.0164 Ω per conductor. The two conductors together have 0.0328 Ω. The voltage drop is 10 A × 0.0328 Ω = 0.33 V when rounded to two decimal places. The drop percentage is 100 × 0.328 V ÷ 12 V = 2.73%. The estimated load voltage is 12 − 0.328 = 11.67 V. The calculations use the unrounded drop before displaying the rounded result.

What the Calculator Is For

A voltage-drop estimate helps explain why a load may receive less voltage than the source provides. It is useful when checking a 12 V DC setup such as a vehicle accessory, lighting circuit, or battery-powered device, provided you have a reliable resistance value for the cable. Longer runs and higher current generally produce greater drop; a lower-resistance conductor produces less.

Distance means the one-way route from the source to the load, not the total length of both conductors. The calculator doubles that distance internally. If you already enter the total out-and-back conductor length, do not use this calculator’s one-way input without adjusting the method, or the result will be doubled incorrectly.

How to Get a Useful Resistance Value

Resistance depends on conductor material, cross-sectional area, and temperature. Use a published resistance figure for the specific cable, with the units converted to Ω/km if necessary. For example, a value stated in Ω/1,000 ft is numerically close to Ω/km only after conversion: 1,000 ft is 304.8 m, so multiply Ω/1,000 ft by approximately 3.281 to express it in Ω/km.

For a rough theoretical estimate, copper’s resistance at about 20°C is approximately 0.0172 Ω·mm² per metre of conductor length. A conductor’s resistance can be estimated as resistivity multiplied by length and divided by cross-sectional area. Real cable specifications are preferable because the alloy, stranding, manufacturing tolerances, and operating temperature affect resistance. Do not use a nominal wire gauge alone as though it were a resistance measurement.

How to Read the Results

The first result is the estimated voltage lost across the cable pair. The percentage expresses that loss relative to 12 V, and the final output subtracts the estimated loss from a 12 V source. For instance, a displayed 0.33 V drop corresponds to an estimated 11.67 V at the load under the example’s assumptions.

A real battery or power supply may not sit at exactly 12 V, and its output can change with charge, load, and operating state. To estimate load voltage from a measured source voltage, subtract the calculated drop from that measured voltage; the displayed load-voltage result specifically assumes 12 V. Small displayed differences can also result from rounding, so use unrounded values for intermediate calculations when checking the result.

Common Mistakes and Limits

Mixing units is a frequent source of error. This calculator requires distance in metres and resistance in Ω/km; entering Ω/m as though it were Ω/km makes the result 1,000 times too large. Likewise, keep current in amperes, not milliamperes: divide milliamperes by 1,000 before entering them.

The calculation assumes steady DC current, a two-conductor circuit, and equal conductor length and resistance on the outgoing and return paths. It does not account for connectors, switches, fuses, contact resistance, changing current, or voltage variation at the source. At very high currents, resistance and temperature effects can be significant. It is a voltage-drop estimate, not a recommendation for cable, fuse, or installation sizing; confirm component selection and installation requirements with a qualified professional.

Frequently Asked Questions

Why does the formula multiply the distance by two?
The current travels from the source to the load and returns to the source through a second conductor. If the entered distance is one-way, doubling it accounts for both paths.

Can I use this for AC wiring?
This calculator uses a simple DC resistance model. AC voltage-drop calculations can depend on frequency, power factor, and conductor reactance, so this result should not be treated as a general AC calculation.

What if my cable resistance is given in ohms per 1,000 feet?
Convert it to ohms per kilometre before entering it. Multiply the Ω/1,000 ft figure by about 3.281; also convert the one-way distance to metres if needed.

Why might measured voltage differ from the result?
The actual source voltage, cable temperature, connections, and real current may differ from the inputs or assumptions. Measure voltage at the source and load while the circuit is operating to compare actual conditions.

Does the calculator tell me what cable size to install?
No. It calculates estimated voltage drop from a resistance value you provide. Cable and protective-device selection must account for applicable requirements and operating conditions, so consult a qualified professional.

Final Thoughts

Use the calculator to estimate the round-trip voltage loss in a 12 V DC cable run, then verify the resistance, units, and actual operating conditions. Its results are most useful as a transparent calculation—not a substitute for checking the complete installation.

Formula used

in i | Current | A | 10 | 0 | 10000
in d | One-way cable length | m | 5 | 0 | 100000
in r | Conductor resistance | ohm/km | 3.28 | 0 | 100000
out v | Voltage drop | V | 2 * i * d * r / 1000 | 2
out p | Drop as percentage of 12 V | % | 100 * v / 12 | 2
out vr | Voltage at load from 12 V source | V | 12 - v | 2

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.

More cards in this drawer

The whole Numbers and Powers drawer →