Ohm's Law Calculator
Quickly calculate voltage, current, resistance, or power in any electrical circuit. Just enter two known values and get instant results.
Formula
V = I × R
Ohm's Law Triangle
Cover the value you want to find. V over I×R means V = I × R
How the Ohm's Law Calculator Works
Select what you want to calculate from the dropdown: voltage, current, resistance, or power using any of three formulas. Enter the two required values in their respective fields, then click Calculate.
The calculator applies Ohm's Law (V = I × R) or the power formulas (P = V × I, P = I² × R, P = V² / R) depending on your selection. Results display with proper units: volts (V), amperes (A), ohms (Ω), or watts (W).
When you enter all three values (voltage, current, and resistance), the circuit summary shows all four parameters including calculated power. The Ohm's Law triangle diagram helps you remember the relationships between variables.
When You'd Actually Use This
Sizing resistors for LEDs
You have a 5V supply and an LED that needs 20mA at 2V forward voltage. Calculate the required resistance: R = (5V - 2V) / 0.02A = 150Ω.
Checking circuit power consumption
Your device draws 0.5A at 12V. Use P = V × I to find it consumes 6 watts. This tells you what power supply rating you need.
Troubleshooting electrical faults
A circuit should draw 2A but only draws 0.5A. Use R = V / I to check if resistance increased, indicating a loose connection or damaged component.
Electronics homework problems
Your assignment gives voltage and resistance, asks for current. Plug in the values and verify your manual calculation before submitting.
Selecting wire gauge
You know the current and need to check voltage drop across a wire. Calculate expected voltage loss to ensure your components get adequate power.
Understanding heater specifications
A heater is rated 1500W at 120V. Calculate current draw: I = P / V = 1500W / 120V = 12.5A. This tells you what circuit breaker you need.
What to Know Before Using
Ohm's Law applies to resistive circuits.The formulas work for DC circuits and AC circuits with pure resistance. Circuits with capacitors or inductors require impedance calculations.
Power has three equivalent formulas.P = V × I works with any two values. P = I² × R is useful when you know current and resistance. P = V² / R works with voltage and resistance.
Units must be consistent.Use volts, amperes, and ohms for correct results. Milliamps need conversion (100mA = 0.1A). Kilohms need conversion (4.7kΩ = 4700Ω).
The triangle is a memory aid.Cover the value you want to find. V over I means V = I × R. I under V means I = V / R. The visual helps remember which operation to use.
Pro tip: Power dissipation in resistors generates heat. A resistor dissipating 0.5W needs at least a 1W rating for safety margin. Double the calculated power for safe resistor selection.
Common Questions
What if I only have one value?
You need at least two values to calculate the others. Ohm's Law has three variables. With only one known, there are infinite possible solutions.
Can I use this for AC circuits?
For pure resistive AC loads (heaters, incandescent bulbs), yes. For motors, transformers, or electronics, you need to account for power factor and impedance.
Why are there three power formulas?
They're mathematically equivalent, derived by substituting V = I × R into P = V × I. Each form is convenient when you have different pairs of known values.
What does the circuit summary show?
When you enter voltage, current, and resistance, it calculates power and displays all four values together. This gives a complete picture of the circuit.
How accurate are the results?
The math is precise to many decimal places. Real-world measurements have tolerance. Resistor values vary by 1-10%, and multimeters have their own accuracy limits.
Can current be negative?
In this calculator, no. Negative current indicates direction opposite to your reference. The magnitude is what matters for power and resistance calculations.
What if resistance is zero?
Zero resistance means a short circuit. Current would be infinite (I = V / 0). In reality, wires have some resistance and power supplies have current limits.
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