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Calculator Answers

Ohm's Law Calculator

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

An Ohm's law calculator is an academic physics utility that computes electrical relationships using the formula voltage equals current multiplied by resistance. Each field carries a unit selector covering the prefixes electronics work actually uses, so 12 mA can be entered as 12 with mA selected rather than converted by hand. Results are always reported in base units: volts, amperes, ohms and watts. By entering any two known variables, the calculator applies algebraic Ohm's law and Joule's law equations to solve for the two unknown parameters. Physics students, electronics hobbyists, and technicians use this tool to verify circuit parameters and prevent component damage.

Pick the two quantities you know and clear the others. The calculator returns voltage, current, resistance, and power, and shows which formulas were used to derive the unknowns.

Quick Answer

Calculate voltage, current, resistance, and power using Ohm's law. Enter exactly two known values, pick the unit under each field (mV, mA, kilohms and so on), and the other two are computed in base units.

Enter exactly two values

The other two are computed using Ohm's law and the power relationships. Pick the unit under each field; results are always shown in base units.

e.g. 12

e.g. 2

e.g. 6

e.g. 24

Formulas

  • V = I × R
  • I = V / R
  • R = V / I
  • P = V × I
  • P = I² × R
  • P = V² / R
Ohm's law

Computed quantities

12 V, 2 A, 6 Ω, 24 W

R = V / I · P = V × I

Voltage12 V
Current2 A
Resistance6 Ω
Power24 W

Ohm's law is V = I × R for ideal resistors. The power relationships follow by substitution: P = V × I, P = I² × R, and P = V² / R. Real circuits have temperature dependence, reactance, and non-ideal behavior; this calculator assumes a simple resistive DC or RMS AC circuit.

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Examples

12 V, 2 A

R = 6 Ω · P = 24 W

120 V, 60 W

I = 0.5 A · R = 240 Ω

5 A, 4 Ω

V = 20 V · P = 100 W

100 W, 50 Ω

V ≈ 70.71 V · I ≈ 1.41 A

How it works

Ohm's law is the single equation relating voltage, current, and resistance. Combined with P = V × I, it produces the six standard formulas used to find any unknown from any two knowns.

Ohm's law · V = I × R

Power · P = V × I = I² × R = V² / R

Finding wattage from any two values

Wattage is simply power measured in watts, and this calculator solves for it. Give it any two of voltage, current, resistance and power, and it returns the other two, so entering a voltage and a current gives the wattage directly from P = V x I. It works out an electrical quantity; it does not size a wire or a circuit breaker for you.

What units to type

Each field has a unit selector beneath it. Type the number you have, then pick its unit: mV, V or kV for voltage, uA, mA or A for current, ohms, kilohms or megohms for resistance, and mW, W or kW for power. Base units are selected by default, so a value you already have in volts or amperes needs no change.

This exists because the alternative fails quietly. Entering a prefixed number as if it were a base one does not produce an error, it produces a confident answer wrong by a factor of a thousand. Twelve milliamps through a one kilohm resistor is 12 V and 0.144 W. The same two numbers read as 12 amps and 1 ohm give the same 12 V, which is what makes the mistake so easy to miss, and 144 W, which is a thousand times too much.

  • Results stay in base units. A small current is shown as a decimal rather than switched to milliamps on your behalf, so what you read stays tied to the formulas above.
  • Clearing resets the units. Leaving a stale kilohms selected beside an empty field is how the next entry comes out wrong, so Clear all returns every selector to base.
  • The list is deliberately short. Only the prefixes each quantity is genuinely used with are offered. Megavolts are a real unit; they are not a realistic entry on a page about DC circuits.

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Frequently asked questions

Ohm's law is V = I × R: the voltage across a resistor equals the current through it times its resistance. The law applies to ideal resistors in DC circuits and to RMS values in AC circuits with purely resistive loads.

Start with P = V × I (the basic definition of electrical power) and substitute Ohm's law. Replace V with I × R to get P = I² × R. Replace I with V / R to get P = V² / R. All three forms describe the same power dissipation.

Because two independent values determine the other two through Ohm's law and the power relationships. Entering three creates an overdetermined system; if the third does not match what Ohm's law predicts, there is no consistent answer. Clear extra fields before computing.

An ohm is the SI unit of electrical resistance: 1 ohm is the resistance that produces 1 ampere of current when 1 volt is applied. Symbol is the Greek letter omega (Ω). Larger units include kΩ (1,000 Ω) and MΩ (1 million Ω).

For purely resistive AC loads, yes; substitute RMS voltage and RMS current. For circuits with capacitance or inductance, the relationships use complex impedance instead of pure resistance, and power has real (W), reactive (VAR), and apparent (VA) components. This calculator handles the basic resistive case only.

No. The calculator is a math tool for the basic electrical relationships. Real electrical work involves code compliance, conductor sizing, overcurrent protection, grounding, and safety procedures that this tool does not address. Consult a licensed electrician for permitted electrical work.