How to use the Ohm's Law Calculator
Fill in two known quantities, select their Unit values, and click Calculate. The calculator fills in the other two and shows Voltage, Current, Resistance, and Power in base units. It also lists the formula, calculation steps, and assumptions used for the result.
- Enter values in exactly two fields: Voltage, Current, Resistance, or Power. Leave the other two blank.
- Select a Unit for each entered value. The available units are mV, V, kV, µA, mA, A, mΩ, Ω, kΩ, MΩ, mW, W, and kW.
- Click Calculate to solve for the two blank quantities and display the result.
- Click Copy result or Download result (.txt) to keep the displayed values and steps.
How the Ohm's law calculator calculates voltage and power
The calculator converts entered quantities to base units before applying V = I × R and P = V × I. For example, entering 120 V for Voltage and 2 A for Current gives:
Voltage: 120 V
Current: 2 A
Resistance: 60 Ω
Power: 240 WThe result always uses V, A, Ω, and W, even when an entered Unit is mA or kW. The arithmetic runs in the browser. The script has no fetch, XMLHttpRequest, sendBeacon, or form submission that sends the entered values elsewhere.
Input rules for the voltage, current, resistance, and power calculator
- Voltage, Current, and Power can be zero or greater. Resistance must be greater than zero.
- Enter exactly two values. Fewer or more filled fields produce an error.
- Use decimal notation such as
120or0.5. Commas and scientific notation such as1e3are rejected. - Each raw entry must be no greater than
1e12in absolute value. - Some zero combinations cannot produce a finite result. For example, Voltage and Power cannot be used together when Voltage is zero, because the calculation divides by Voltage.
What the Ohm's law calculator result includes
A successful result lists Voltage, Current, Resistance, and Power, followed by Formula & steps and Assumptions. Resistance is treated as constant. The calculation assumes a simple resistive DC circuit or an AC circuit at unity power factor, so it does not calculate impedance or phase angle for reactive loads.
Changing an input or Unit hides the previous result until you click Calculate again. Reset clears the four fields, restores V, A, Ω, and W as the selected units, hides the result, and clears a validation error. Copy result uses the displayed text. Download result (.txt) creates a plain-text file.
Where this calculation comes up
A circuit note gives 12 V and 6 Ω. Enter Voltage and Resistance to calculate Current and Power. The result is 2 A and 24 W.
A supply label gives Voltage and Current, such as 120 V and 2 A. Enter both values to calculate Resistance and Power.
A lab worksheet may give Resistance and Power instead. Enter those two quantities to calculate Voltage and Current under the calculator's resistive model.
When to use another science calculator
Use this page when the two known quantities are Voltage, Current, Resistance, or Power. Choose the Electrical Calculator for resistor combinations, voltage drop, wire-size estimates, battery runtime, or energy consumption. Choose the Battery Runtime Calculator for battery capacity, load, voltage, and efficiency. Choose the Power Calculator when power comes from work or energy and time. Browse Science Calculators for another physics, chemistry, optics, wave, fluid, gas, or radioactive-decay calculation.
Frequently asked questions
How do I calculate voltage, current, resistance, and power?
Enter exactly two of Voltage, Current, Resistance, and Power, then click Calculate. The calculator applies V = I × R and P = V × I to solve for the remaining two quantities.
How many values do I enter in the Ohm's law calculator?
Enter exactly two valid values and leave the other two fields blank. Voltage, Current, and Power cannot be negative. Resistance must be greater than zero.
Which units does this voltage and resistance calculator accept?
It accepts mV, V, and kV; µA, mA, and A; mΩ, Ω, kΩ, and MΩ; plus mW, W, and kW. Results are displayed in V, A, Ω, and W.
Can I use this Ohm's law formula calculator for AC circuits?
It assumes a simple resistive DC circuit or an AC circuit at unity power factor. It does not calculate impedance or phase angle for reactive loads.