Ohm's Law Calculator

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. The formula is V = I × R, where V is voltage (volts), I is current (amperes), and R is resistance (ohms). This means: higher voltage pushes more current through the same resistance, and higher resistance restricts current flow for the same voltage. Ohm's Law applies to ohmic materials (resistors, wires, heaters) where resistance stays constant, but does NOT apply to non-ohmic devices like LEDs, diodes, or transistors where resistance changes with voltage or current.

Use standard SI base units for accurate calculations: Voltage in volts (V)—if you have millivolts (mV), divide by 1,000. Current in amperes (A)—if you have milliamps (mA), divide by 1,000 (e.g., 250 mA = 0.25 A); if microamps (µA), divide by 1,000,000. Resistance in ohms (Ω)—if you have kilohms (kΩ), multiply by 1,000 (e.g., 10 kΩ = 10,000 Ω); if megohms (MΩ), multiply by 1,000,000. Power in watts (W)—if milliwatts (mW), divide by 1,000. Unit conversion errors are the #1 cause of incorrect results, so always double-check that you've converted to base units before entering values into the calculator.

Yes, but you need to calculate equivalent resistance first, then use the calculator. For series resistors (connected end-to-end), add resistances directly: R_total = R1 + R2 + R3 + ... For parallel resistors (connected side-by-side), use reciprocals: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ... Once you have the equivalent resistance, enter it along with voltage to find total current, or enter total current to find voltage drop. For individual branch currents in parallel circuits, use the current divider formula or apply Ohm's Law to each branch separately. Some advanced calculators include dedicated series/parallel calculators—look for those features if you work with complex resistor networks frequently.

Exceeding a component's power rating causes overheating, damage, or fire. When a resistor dissipates more power than its rating (e.g., 1W through a 0.25W resistor), it heats up rapidly, potentially causing: (1) Component failure—the resistor burns out, goes open-circuit, or changes value. (2) PCB damage—excessive heat melts solder, lifts traces, or damages the board. (3) Fire hazard—in extreme cases, overheated components ignite. (4) Cascading failures—overheated component damages adjacent parts. Always use resistors rated at least 2× the calculated power for a safety margin. For example, if P = 0.3W, use a 0.5W or 1W resistor. Check datasheets for power derating curves—many components can't handle full rated power at high ambient temperatures.

Ohm's Law (V = IR) applies to ohmic materials—conductors where resistance remains constant regardless of voltage or current. This includes resistors, wires, heaters, and most metals at constant temperature. However, it does NOT apply to non-ohmic devices where resistance changes with voltage, current, or temperature: (1) LEDs and diodes—resistance drops dramatically when forward voltage is reached; V-I relationship follows exponential diode equation, not linear Ohm's Law. (2) Transistors—resistance varies with base/gate voltage; behavior is modeled by transistor equations, not Ohm's Law. (3) Thermistors—resistance changes significantly with temperature. (4) Varistors—resistance decreases sharply at high voltages. For these devices, use manufacturer datasheets, V-I characteristic curves, or specialized models instead of Ohm's Law.

This calculator is designed for DC (direct current) circuits where voltage and current are constant. For basic resistive AC circuits (purely resistive loads like heaters or incandescent bulbs), you can use RMS (root-mean-square) values of voltage and current, and Ohm's Law applies the same way: V_RMS = I_RMS × R. However, for AC circuits with reactive components (capacitors, inductors, transformers, motors), resistance is replaced by impedance (Z), which includes both resistance and reactance. The relationship becomes V = I × Z, where Z is complex (has magnitude and phase). You'll need an impedance calculator or AC circuit analysis tool that accounts for frequency, capacitance, inductance, and phase angles. This Ohm's Law calculator does not handle reactive components or AC-specific behaviors.

To find the current-limiting resistor for an LED: (1) Determine the supply voltage (e.g., 5V from USB or Arduino). (2) Find the LED's forward voltage from its datasheet (typically 1.8–2.2V for red, 2.8–3.6V for blue/white). (3) Find the LED's maximum current (typically 20 mA for indicator LEDs). (4) Calculate voltage across the resistor: V_resistor = V_supply - V_LED (e.g., 5V - 2V = 3V). (5) Use Ohm's Law: R = V_resistor / I_LED (e.g., R = 3V / 0.020A = 150 Ω). (6) Choose the next higher standard resistor value (180 Ω or 220 Ω) for safety. (7) Calculate power: P = I² × R to ensure the resistor can handle it (typically 0.25W is sufficient for single LEDs). Never connect an LED directly to a power source without a current-limiting resistor—it will burn out instantly.

Voltage is measured across (in parallel with) the component: place one probe on each side of the component while it remains in the circuit. Multimeters have very high input impedance (typically 10 MΩ), so they don't affect the circuit. Current is measured through (in series with) the circuit: you must break the circuit, insert the meter in the path of current flow, and let current pass through the meter. Multimeters in current mode have very low resistance (typically <1 Ω) to minimize voltage drop. NEVER measure current in parallel—connecting an ammeter across a voltage source creates a short circuit, blowing the meter's fuse or damaging it. Common mistake: leaving the meter in current mode and trying to measure voltage—this shorts the circuit. Always verify meter mode, probe placement, and range setting before measuring.

Discrepancies between calculated and measured values arise from real-world factors not included in ideal Ohm's Law: (1) Component tolerance—resistors are rated ±1%, ±5%, or ±10%; a 100 Ω ±5% resistor can be 95–105 Ω. (2) Wire resistance—wires, traces, and connections add resistance, especially for long runs or high currents. (3) Battery/supply voltage sag—batteries drop voltage under load due to internal resistance; nominal 9V battery may measure 8.5V when delivering current. (4) Temperature effects—resistance increases with temperature for most conductors (~0.4%/°C for copper). (5) Measurement error—multimeter accuracy (±1–2%), incorrect range setting, or poor probe contact. (6) Non-ideal components—LEDs, diodes, and semiconductors don't follow Ohm's Law. To improve accuracy, measure actual component values with an ohmmeter, use measured (not nominal) voltages, and account for wire resistance in critical applications.

Calculate power dissipation using P = I² × R, P = V² / R, or P = V × I (all equivalent), then choose a resistor rated at least 2× the calculated power for safety and reliability. Standard power ratings are 0.125W (1/8 watt), 0.25W (1/4 watt), 0.5W (1/2 watt), 1W, 2W, 5W, 10W, and higher. Example: if P = 0.3W, use a 0.5W or 1W resistor. Higher wattage resistors run cooler, last longer, and handle transient overloads better. Note that power ratings assume 25°C ambient temperature—resistors derate (can handle less power) at higher temperatures. For example, a 1W resistor might only handle 0.5W at 70°C. Check manufacturer derating curves if operating in hot environments (automotive, industrial, enclosed boxes). Physically larger resistors dissipate heat better and are rated for higher power.

Yes—paralleling resistors distributes power dissipation across multiple components, increasing total power capacity. Two 100 Ω, 0.5W resistors in parallel give 50 Ω equivalent with 1W total capacity (each resistor handles half the current, so power splits evenly). Formula: for N identical resistors in parallel, R_eq = R / N and P_total = N × P_single. Example: you need 10 Ω at 5W but only have 0.5W resistors. Use ten 100 Ω, 0.5W resistors in parallel: R_eq = 100 / 10 = 10 Ω ✓, P_total = 10 × 0.5 = 5W ✓. Verify each resistor's power: if total current is I, current per resistor is I / N, so power per resistor is (I/N)² × R = I² × R / N²—well within 0.5W rating. This technique is common in power electronics, dummy loads, and when high-wattage resistors are unavailable or expensive. Ensure good thermal contact and airflow for reliability.

A voltage divider uses two resistors in series to create a lower output voltage from a higher input voltage. The output is taken between the resistors: V_out = V_in × (R2 / (R1 + R2)), where R2 is the resistor connected to ground. Example: create 3.3V from 5V. Choose R2 = 10 kΩ, solve for R1: 3.3 = 5 × (10 / (R1 + 10)) → R1 ≈ 5.15 kΩ (use 5.1 kΩ standard value). Verify: V_out = 5 × (10 / 15.1) = 3.31V ✓. Important limitation: voltage dividers have high output impedance and are sensitive to loading. Connecting a low-impedance load (e.g., motor, LED, low-value resistor) in parallel with R2 changes the effective R2, dropping output voltage significantly. Use voltage dividers for high-impedance inputs (op-amps, ADCs, sensors) or add a buffer (op-amp follower) for low-impedance loads. For sensors, voltage dividers interface high-voltage signals to low-voltage ADCs safely.

Wire resistance depends on material, length, and gauge (thickness). Use AWG (American Wire Gauge) tables: smaller AWG numbers = thicker wire = lower resistance. Common values: 18 AWG copper ≈ 0.0065 Ω/ft (0.021 Ω/m), 14 AWG ≈ 0.0025 Ω/ft (0.008 Ω/m), 10 AWG ≈ 0.001 Ω/ft (0.003 Ω/m). For a cable run, resistance = resistance per unit length × 2 × distance (factor of 2 for round trip). Example: 50 ft of 18 AWG wire = 0.0065 × 2 × 50 = 0.65 Ω total. At 10A, voltage drop V = I × R = 10 × 0.65 = 6.5V (13% of 50V, too high). Power loss: P = I² R = 100 × 0.65 = 65W (significant heat). Use thicker wire: 14 AWG gives 0.25 Ω (2.5V drop, 25W loss—better). For DC power systems, target <3% voltage drop; for AC, <5%. Always calculate wire resistance for long runs, high currents, or low-voltage systems (12V, 5V) where voltage drop is a larger percentage.