Ohm's Law CalculatorFree online science calculator — instant results, no sign-up
Calculate voltage, current or resistance — enter any two.
About the Ohm's Law Calculator
The Ohm's Law Calculator finds voltage, current, or resistance when you enter the other two quantities and leave the unknown as 0. It also calculates electrical power from the completed values. With the default voltage of 12 V and current of 2 A, it gives a resistance of 6 Ω and power of 24 W.
- Voltage (leave 0 to solve) (V)
- The potential difference across the component in volts (V), defaulting to 12; leave it at 0 to solve for voltage.
- Current (leave 0 to solve) (A)
- The current flowing through the component in amperes (A), defaulting to 2; leave it at 0 to solve for current.
- Resistance (leave 0 to solve) (Ω)
- The component's resistance in ohms (Ω), defaulting to 0 to indicate that resistance should be calculated.
Formula and method
V = I × R; I = V / R; R = V / I; P = V × I. With the default inputs: R = 12 V / 2 A = 6 Ω; P = 12 V × 2 A = 24 W.
- VVoltage across the component, measured in volts (V).
- ICurrent through the component, measured in amperes (A).
- RElectrical resistance, measured in ohms (Ω).
- PElectrical power, measured in watts (W).
The field set to 0 identifies the unknown. The calculator rearranges Ohm's law to solve that quantity using the two supplied values, then multiplies voltage by current to calculate power. For the defaults, resistance is the unknown, so it divides 12 V by 2 A to obtain 6 Ω, then multiplies 12 V by 2 A to obtain 24 W.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Voltage (leave 0 to solve)12 V
- Current (leave 0 to solve)2 A
- Resistance (leave 0 to solve)0 Ω
Results
- Resistance6 Ω
- Power24 W
How to use the Ohm's Law Calculator
- 1Choose whether you want to find voltage, current, or resistance.
- 2Enter the two known quantities in volts, amperes, or ohms, converting any prefixed units to these base units first.
- 3Leave exactly one field at 0 to mark the quantity to solve; for the default example, keep voltage at 12, current at 2, and resistance at 0.
- 4Run the calculation to obtain the missing quantity and electrical power.
- 5Read the results with their units: the default inputs produce Resistance = 6 Ω and Power = 24 W.
Reading your result
The solved-quantity row reports the unknown: voltage in V, current in A, or resistance in Ω. In the default example, Resistance = 6 Ω means that a resistive component carrying 2 A across 12 V has a resistance of 6 Ω. The Power row reports the rate of electrical energy transfer in watts; Power = 24 W means the component dissipates 24 joules per second under these conditions.
Limitations
This calculation assumes an ohmic component with constant resistance under the stated operating conditions. It is directly applicable to steady DC resistive circuits; for purely resistive AC circuits, consistent RMS voltage and current values may be used. Reactive AC circuits require impedance and power-factor analysis, and nonlinear devices such as diodes do not generally follow a constant-resistance model. Because 0 marks an unknown, this input scheme cannot distinguish a genuinely zero known value from a quantity to solve. Provide exactly two known nonzero values; the tool does not determine component tolerances, temperature effects, or safe power ratings.
Common questions
Why does leaving resistance at 0 produce 6 Ω rather than a short circuit?+
Here, 0 is an instruction to solve for resistance, not a physical resistance value. With voltage at 12 V and current at 2 A, the calculator uses R = V / I to return 6 Ω.
How do I solve for voltage or current instead of resistance?+
To solve for voltage, leave voltage at 0 and enter current and resistance; the calculator uses V = I × R. To solve for current, leave current at 0 and enter voltage and resistance; it uses I = V / R.
Why is power shown when I am solving for resistance?+
Once voltage and current are known, power follows from P = V × I. The default inputs therefore give both resistance of 6 Ω and power of 24 W.
Does the 24 W output tell me which resistor power rating to buy?+
It gives the calculated dissipation, not a recommended component rating. A resistor must be rated to handle that dissipation with suitable margin and derating for ambient temperature, cooling, and the manufacturer's operating limits.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.