Ohm's Law Calculator
Ohm's Law relates voltage, current, and resistance in electrical circuits: V = I × R. Enter any two values to calculate the third. This calculator also computes power dissipation (P = IV) and energy consumed over time.
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Formula
V = I × R
Voltage (V) in Volts equals current (I) in Amps times resistance (R) in Ohms. Rearranged: I = V/R and R = V/I. Power dissipated as heat: P = V×I = I²R = V²/R, in Watts. Enter any two of V, I, R and the calculator solves for the third.
How to use the Ohm's Law Calculator
- 1
Enter your voltage (v) — 0 to calculate
Value should be in V.
- 2
Enter your current (i) — 0 to calculate
Value should be in A.
- 3
Enter your resistance (r) — 0 to calculate
Value should be in Ω.
- 4
Read your results instantly
Results update in real time as you type.
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Understanding Ohm's Law
Ohm's Law is the foundation of circuit analysis. Voltage is electrical pressure — the force driving current through a circuit. Current is the flow of charge (electrons) through a conductor. Resistance opposes that flow. The law states that for many materials, the relationship between these three quantities is linear: double the voltage and you double the current; double the resistance and you halve the current.
Not all materials obey Ohm's Law (they are 'non-ohmic'). Diodes, transistors, and lightbulb filaments all have non-linear V-I relationships. But resistors, wires, and most passive components follow it closely, making Ohm's Law one of the most useful tools in electronics.
Voltage, current, and resistance in everyday life
A standard US wall outlet provides 120V AC. The current drawn depends on the appliance's resistance. A 1,200 W hair dryer draws 10 A (R = V/I = 120/10 = 12 Ω). A 60 W light bulb draws 0.5 A (R = 120/0.5 = 240 Ω). Circuit breakers trip when current exceeds a safe level — typically 15 or 20 A for household circuits.
In electronics, resistors are used to control current and voltage levels. A 5V microcontroller powering an LED (which needs ~20 mA at ~2V) needs a series resistor of R = (5-2)/0.02 = 150 Ω to limit the current and protect the LED.
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Power and safety
Power dissipated in a resistor P = I²R generates heat. This is why resistors have power ratings (¼W, ½W, 1W). Exceeding the rating causes overheating and failure. For wiring, the same principle applies: undersized wire has higher resistance per unit length, dissipating more heat at high currents — a fire hazard.
Electrical safety is about both voltage and current. Voltage pushes current through your body; it's the current that is dangerous. As little as 10 mA through the heart can cause ventricular fibrillation. Body resistance varies (1,000–100,000 Ω depending on skin moisture), but wet skin can drop to ~1,000 Ω. At 120V, that means up to 120 mA — potentially lethal.
Tips & Insights
Leave the unknown at 0
Enter 0 for the value you want to calculate. The calculator detects which value is missing and solves for it.
Series vs. parallel resistance
Resistors in series add: R_total = R₁ + R₂. Resistors in parallel: 1/R_total = 1/R₁ + 1/R₂. Use the total resistance with Ohm's Law for the whole circuit.
P = I²R for heat calculations
The I²R formula is most useful for calculating heat generated in a wire or resistor when you know the current and resistance but not the voltage.
Worked Examples
LED resistor calculation
R = 3V ÷ 0.02A = 150 Ω. Use a 150 Ω resistor to limit LED current to 20 mA at 3V.
Household appliance
I = 120V ÷ 20Ω = 6A. Power = 120 × 6 = 720 W.
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Frequently Asked Questions
What is Ohm's Law?
V = I × R: Voltage equals current times resistance. Rearranged: I = V/R (current = voltage ÷ resistance) and R = V/I.
What is voltage?
Voltage is electrical potential difference — the 'pressure' driving current through a circuit. Measured in Volts (V).
What is resistance measured in?
Ohms (Ω). 1 Ohm means 1 Volt drives 1 Amp of current. Higher resistance means less current for the same voltage.
Does Ohm's Law apply to AC circuits?
For pure resistors, yes. For circuits with capacitors and inductors, impedance (Z) replaces resistance, and V = IZ, but the relationship is more complex with phase shifts.
What is the relationship between watts and amps?
P = V × I. Watts = Volts × Amps. At 120V, 1 amp = 120 watts. At 240V, 1 amp = 240 watts.
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