Utility July 13, 2026 · 8 Min Read

Voltage Calculator – Guide & Formulas

Calculate electrical voltage from current and resistance, or from power and current, including voltage drop over long wire runs.

Calculate electrical potential (Voltage) or estimate voltage drop over distance. Solve using fundamental Ohm's Law variables, or input wire parameters to compute voltage drop across long cables.

Key Takeaway

Use the free Voltage Calculator to calculate electrical voltage from current and resistance, or from power and current, including voltage drop over long wire runs. Get instant results with step-by-step explanations.

How to Use the Voltage Calculator

  1. Choose between "Ohm’s Law Solver" (standard equations) or "Voltage Drop Solver" (line wire losses).
  2. For Ohm’s Law: Enter power, current, or resistance to compute Volts.
  3. For Voltage Drop: Enter source voltage, wire material (Copper/Aluminum), wire gauge (AWG), run distance in feet, and load current in Amps.
  4. Review your computed terminal voltage, percentage drop, and power dissipated in the cables.

The Formula

Voltage (Volts) = Current (Amps) * Resistance (Ohms) | Voltage Drop = 2 * Distance * Wire Resistance per 1000ft * Current / 1000

Variable Definitions

  • V (Volts): Electrical electromotive pressure measured in Volts
  • Voltage Drop: The amount of voltage lost due to the internal electrical resistance of the wire over long distances
  • AWG: American Wire Gauge standard describing wire thickness; smaller numbers are thicker

Sizing Voltage Drop for an Outbuilding

Estimate the voltage drop running a 120V circuit to a shed 150 feet away carrying a 15-Amp load using a standard 12 AWG copper wire (which has a resistance of 1.93Ω per 1,000 feet).

  1. Step 1: Find total wire length factor (2-way trip): 2 × 150 ft = 300 feet of single-conductor equivalent.
  2. Step 2: Get total wire resistance: 300 ft × (1.93Ω / 1000 ft) = 0.579 Ohms total resistance.
  3. Step 3: Apply Ohm’s Law for voltage loss: V_drop = 15 Amps × 0.579Ω = 8.68 Volts.
  4. Step 4: Compute final terminal voltage: 120V - 8.68V = 111.32 Volts. This represents a 7.23% voltage drop (exceeding the recommended 3.0% maximum for clean power).

Frequently Asked Questions

What is the acceptable limit for voltage drop?

The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits, and 5% total combined across the feeder and branch circuits, to ensure electronics run reliably without overheating.

Why is voltage drop worse at lower voltages?

For the same power delivery, lower voltage systems require higher currents (Amps). Because voltage drop is directly proportional to current, low-voltage lines (like 12V RV or solar wiring) suffer severe relative losses.

How does wire material impact voltage drop?

Copper has higher conductivity and lower resistance than aluminum. For the same current, an aluminum wire needs to be roughly two wire sizes thicker than a copper wire to keep the same voltage drop.