Ohm's Law Calculator
Solves for voltage, current, or resistance in a resistive circuit using V = I × R. Essential for electronics hobbyists, electricians, and students designing or troubleshooting DC and AC circuits.
Last updated: May 2026
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About this calculator
Ohm's Law is the foundational relationship between voltage, current, and resistance in an electrical circuit. The core formula is V = I × R, where V is voltage in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω). Rearranged, you get I = V / R (to find current) and R = V / I (to find resistance), allowing you to solve for any one variable when the other two are known. Variables: voltage (V), current (A); this calculator solves for resistance using R = voltage / current. The law was formulated by Georg Simon Ohm in 1827 and remains the starting point for almost all circuit analysis. It applies to linear, resistive components and is the foundation of resistor selection, fuse sizing, and wire-gauge math. Edge cases: Ohm's Law applies only to ohmic conductors where resistance stays constant regardless of voltage or current. Non-linear components — diodes, transistors, LEDs, electrolytic capacitors, lamps with cold-vs-hot filaments — do not follow it directly. Temperature affects resistance in metals (positive temperature coefficient) and semiconductors (negative), so Ohm's Law is an approximation at temperature extremes even for metallic resistors. In AC circuits with reactive components (capacitors, inductors), substitute impedance Z for resistance: V = I × Z. Power calculations combine Ohm's Law with P = V × I = I² × R = V² / R.
How to use
Example 1: 12 V battery driving 2 A through a circuit. Step 1: apply R = V / I = 12 / 2 = 6 Ω. Verify: at 6 Ω with 12 V applied, current = 12/6 = 2 A — matches input, and power dissipated = V × I = 12 × 2 = 24 W. Example 2: A 9 V battery powering a 470 Ω resistor — find the current. Step 1: I = V / R = 9 / 470 ≈ 0.01915 A = 19.15 mA. Verify: V = I × R = 0.01915 × 470 ≈ 9.0 V — confirms. Power = 9 × 0.01915 ≈ 0.172 W; the resistor must be rated for at least 1/4 W (250 mW) to handle this safely.
Frequently asked questions
What is Ohm's Law and why is it important in electronics?
Ohm's Law states that the voltage across a conductor equals the current through it multiplied by its resistance: V = I × R. It is important because it lets engineers and hobbyists predict how a circuit will behave before building it. By knowing any two of the three quantities, you can always calculate the third — making it indispensable for resistor selection, current-limiting design, fault diagnosis, and load calculations. It underpins almost every electrical and electronic system from a battery-powered LED to a power-grid substation. Together with Kirchhoff's voltage and current laws, Ohm's Law forms the basis of nearly all circuit analysis at any frequency.
How do I calculate resistance using Ohm's Law?
To calculate resistance, rearrange the formula to R = V / I. Measure or note the voltage across the component in volts and the current through it in amperes. Dividing voltage by current gives you resistance in ohms (Ω). For example, 9 V across a component carrying 0.5 A gives R = 9 / 0.5 = 18 Ω. This is particularly useful when selecting current-limiting resistors for LEDs (R = (Vsupply − VLED) / Iled) or when sizing pull-down resistors in digital circuits. Always also check the resistor's power rating: P = V × I — for the 18 Ω example, P = 9 × 0.5 = 4.5 W, requiring at least a 5 W resistor.
When does Ohm's Law not apply?
Ohm's Law applies only to ohmic (linear) conductors, meaning components whose resistance stays constant regardless of voltage or current. It does not accurately describe non-linear components such as diodes (exponential I-V curve), transistors (active region transconductance), LEDs (constant forward voltage), electrolytic capacitors (frequency-dependent impedance), or filament lamps (resistance increases sharply when hot). In those cases, the current-voltage relationship is governed by more complex device-physics models — Shockley diode equation, Ebers-Moll for BJTs, MOSFET square-law equations. Temperature also affects resistance in metals (positive temperature coefficient ~0.4%/°C for copper) and semiconductors (negative), so Ohm's Law is an approximation at extreme temperatures even for resistors. In AC analysis with reactive components you must use impedance instead of pure resistance.
What are common mistakes when applying Ohm's Law?
Mixing up units — entering current in mA but expecting amps — gives results off by a factor of 1,000. Confusing voltage drop across a component with the supply voltage when components are in series produces wrong resistance values. Forgetting that the formula uses RMS values for AC, not peak values (peak voltage is √2 × RMS). Applying Ohm's Law to a non-linear component like an LED gives nonsense — the LED's effective resistance changes with current. Ignoring the power dissipation P = V × I when sizing resistors causes them to overheat and fail. Treating impedance as pure resistance in AC circuits ignores the phase relationship between voltage and current. Finally, using cold-state resistance when calculating current through a heated component (incandescent bulb, motor) underestimates the steady-state value by 5–10×.
When should I NOT use a basic Ohm's Law calculator?
For AC circuits with significant capacitance or inductance, use complex impedance Z = R + jX in place of R; pure Ohm's Law ignores reactance and phase. For non-linear components (diodes, LEDs, transistors, varistors, thyristors), use the device-specific I-V curve or simulator (SPICE) instead of Ohm's Law. For high-power or high-current circuits, factor in wire resistance, contact resistance, and skin effect at high frequencies. For three-phase power systems, use three-phase formulas with √3 factors and phase angle. For transient/switching analysis, time-domain or Laplace-domain methods are required because R alone doesn't describe energy storage in L or C. Battery internal resistance changes with state of charge and load — pure Ohm's Law doesn't capture this. Finally, for safety-critical work (mains wiring, industrial controls), follow code requirements (NEC, IEC) which often demand engineered margins beyond what Ohm's Law alone suggests.