Solve for voltage, current, resistance, or power using Ohm's Law.
This tool provides estimates for informational purposes only and is not a substitute for professional advice. Individual results vary based on personal circumstances and assumptions.
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Ohm's Law is the foundational principle of electronics and electrical engineering. Whether you are a student studying basic circuit theory, a hobbyist building your first Arduino project, a DIY electronics enthusiast calculating resistor values, or a professional electrician sizing wire for a circuit, Ohm's Law is the equation you will use more than any other. This calculator lets you solve for any one of the four key electrical quantities β voltage, current, resistance, or power β when you know any two of the others. This guide explains what Ohm's Law is, how to use the formulas, and how they apply to real-world electrical work.
Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. It was formulated by German physicist Georg Simon Ohm in 1827 and remains the cornerstone of circuit analysis.
The fundamental formula is: V = I Γ R
This formula can be rearranged to solve for any unknown: I = V Γ· R, or R = V Γ· I. The calculator above handles all three arrangements automatically β just enter any two known values and select what you want to solve for.
Electric power is the rate at which energy is consumed or produced. The power formula, often used alongside Ohm's Law, is: P = V Γ I
By combining Ohm's Law with the power formula, you get additional forms: P = IΒ² Γ R, and P = VΒ² Γ· R. This gives you four variables (V, I, R, P) with six possible formula combinations β the calculator handles all of them.
The classic memory aid is the Ohm's Law triangle: write V at the top, I and R at the bottom corners. Cover the unknown to see the formula:
A similar triangle works for power: P at the top, V and I at the bottom. Cover P β P = V Γ I. Cover V β V = P Γ· I. Cover I β I = P Γ· V.
You have a 9V battery connected to a 470Ξ© resistor. What current flows?
I = V Γ· R = 9 Γ· 470 = 0.0191 A (19.1 mA)
You want to connect a red LED (forward voltage 2V, maximum current 20 mA) to a 5V power supply. What resistor do you need?
Voltage across resistor = 5V β 2V = 3V. Current = 20 mA = 0.02 A.
R = V Γ· I = 3 Γ· 0.02 = 150Ξ©. Use the nearest standard value: 150Ξ© or 180Ξ©.
A 100Ξ© resistor has 12V across it. How much power does it dissipate?
P = VΒ² Γ· R = 144 Γ· 100 = 1.44 watts. Use a resistor rated for at least 2W to allow a safety margin.
A 240V circuit (UK mains voltage) has a 2,400W electric heater. What current does it draw?
I = P Γ· V = 2400 Γ· 240 = 10 amps. This requires a 13A socket or larger circuit.
For a 120V US circuit with the same 2,400W heater: I = 2400 Γ· 120 = 20 amps β requiring a dedicated 20A circuit.
Understanding the difference between US and UK mains electricity is important for appliance calculations:
This voltage difference explains why US appliances running at 120V draw twice the current compared to equivalent UK appliances at 240V. A 1,200W appliance needs 10A at 120V but only 5A at 240V. This is why plugging an unprotected US appliance into a UK socket can damage it β and vice versa.
When resistors are connected in series (end-to-end), their total resistance is simply the sum: R_total = R1 + R2 + R3... The same current flows through all. Example: 100Ξ© + 220Ξ© + 470Ξ© = 790Ξ© total.
When resistors are connected in parallel, the total resistance is lower than any individual resistor. Formula: 1/R_total = 1/R1 + 1/R2 + 1/R3... For two resistors in parallel: R_total = (R1 Γ R2) Γ· (R1 + R2). Example: 100Ξ© and 100Ξ© in parallel = (100 Γ 100) Γ· (100 + 100) = 50Ξ©.
Use our Resistor Calculator for series, parallel, and color code calculations.
Ohm's Law is not just a theoretical tool β it has direct safety implications. Human body resistance varies from about 1,000Ξ© (wet skin) to 100,000Ξ© or more (dry skin). At 120V mains voltage (US), a wet contact could allow 120 Γ· 1000 = 0.12A of current β well above the 0.01A (10 mA) that causes painful shock and the 0.1A (100 mA) that can cause ventricular fibrillation.
Never work on live circuits without proper training and equipment. Even low voltages can be dangerous in certain conditions. Always disconnect power, verify the circuit is de-energized with a multimeter, and follow local electrical safety codes (NEC in the US; BS 7671 in the UK).
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit: V = I Γ R. If you increase the voltage while keeping resistance constant, more current flows. If you increase resistance while keeping voltage constant, less current flows.
Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ξ©). Power is measured in watts (W).
V = I Γ R. If current is 0.5 amps and resistance is 200 ohms: V = 0.5 Γ 200 = 100 volts.
Ohm's Law applies to DC (direct current) circuits directly. For AC (alternating current) circuits, the concept of impedance (Z) replaces simple resistance, incorporating capacitive and inductive reactances. V = I Γ Z still holds, but Z is a complex number.
Disclaimer: This calculator is for educational and design estimation purposes. For any electrical installation or safety-critical work, consult a qualified electrician and follow local electrical codes.