Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law. Enter any two values — get all four electrical quantities instantly.
A
Ω
Enter your values above to see the results.
Tips & Notes
- ✓Ohm's Law applies to resistive (ohmic) components at constant temperature. Non-ohmic devices like diodes and LEDs do not follow a linear V-I relationship — their resistance changes with operating conditions.
- ✓Power dissipates as heat in resistors: P = I²R. A 100 Ω resistor carrying 0.1 A dissipates P = 0.01 × 100 = 1 W. Always use resistors rated at least 2× the calculated power to avoid overheating.
- ✓In series circuits: R_total = R₁ + R₂ + ... and the same current flows through all components. In parallel: 1/R_total = 1/R₁ + 1/R₂ + ... and the same voltage appears across all branches.
- ✓Real voltage sources have internal resistance. A 12 V battery under heavy load may deliver only 11.2 V due to internal resistance voltage drop. This is why battery voltage sags when starting a car engine.
- ✓For AC circuits, use RMS values: V_RMS = V_peak / √2 ≈ 0.707 × V_peak. Household 120 V AC means 120 V RMS, with a peak of 170 V. Ohm's Law works normally with RMS values for resistive loads.
Common Mistakes
- ✗Mixing AC peak and RMS values — Ohm's Law gives correct results only when both voltage and current are expressed in the same form. Using peak voltage with RMS current (or vice versa) gives wrong power by a factor of √2.
- ✗Ignoring wire resistance in high-current circuits — even short wire runs have resistance. A 1 m AWG 18 copper wire (0.021 Ω/m) carrying 10 A drops 0.21 V and dissipates 2.1 W. This matters in automotive and power electronics.
- ✗Choosing resistor value without checking power rating — calculating R = V/I without also calculating P = I²R often leads to undersized resistors that overheat. Always verify power dissipation.
- ✗Applying Ohm's Law to non-linear components — diodes, LEDs, transistors, and Zener diodes have non-constant resistance. Use their characteristic curves or datasheet values, not simple V/I calculations.
- ✗Forgetting to account for component tolerances — a resistor marked 1 kΩ may actually be 950–1,050 Ω (5% tolerance). For precision circuits, calculate the effect of tolerance spread on voltage and current.
Ohm's Law Calculator Overview
Ohm's Law (V = IR) is the cornerstone of electrical engineering — the single equation that connects voltage, current, and resistance in every resistive circuit. Combined with the power formula P = VI, it provides complete characterization of DC electrical behavior.
Ohm's Law and power formulas:
V = I × R | I = V / R | R = V / I | P = V × I = I²R = V²/R
EX: 9 V battery, 470 Ω resistor → I = 9/470 = 19.1 mA → P = I²R = (0.0191)² × 470 = 172 mW. Use at minimum a 1/4 W rated resistor (250 mW). For 2× safety margin, use 1/2 W.Series vs. parallel circuits:
Series: R_total = R₁ + R₂ | Parallel: R_total = (R₁ × R₂)/(R₁ + R₂) | n equal resistors parallel: R/n
EX: 100 Ω + 220 Ω + 330 Ω in series → R = 650 Ω. Same three in parallel: 1/R = 1/100+1/220+1/330 = 0.01758 → R = 56.9 Ω. Parallel always lower than smallest individual value.Ohm's Law — formula matrix:
| Find | Known: V, I | Known: V, R | Known: I, R |
|---|---|---|---|
| Voltage (V) | — | — | V = I × R |
| Current (I) | — | I = V / R | — |
| Resistance (R) | R = V / I | — | — |
| Power (P) | P = V × I | P = V² / R | P = I² × R |
| Rating | Max Current (100 Ω) | Max Voltage (100 Ω) | Typical Package |
|---|---|---|---|
| 1/8 W | 35 mA | 3.5 V | SMD 0402/0603 |
| 1/4 W | 50 mA | 5 V | Axial, SMD 0805 |
| 1/2 W | 71 mA | 7.1 V | Axial, SMD 1206 |
| 1 W | 100 mA | 10 V | Axial or wirewound |
| 5 W | 224 mA | 22.4 V | Wirewound, heatsink |
Frequently Asked Questions
Ohm's Law: V = I × R. Rearranged: I = V / R, R = V / I. Enter any two known values to find the third. Example: 12 V supply, 330 Ω resistor → I = 12 / 330 = 36.4 mA. Power: P = V × I = 12 × 0.0364 = 0.436 W. Or P = V²/R = 144/330 = 0.436 W, or P = I²R = 0.0364² × 330 = 0.436 W. All three power formulas give the same result — choose the one using your known values.
Power can be calculated four ways: P = V × I (most direct, use when you know voltage and current); P = I² × R (use when you know current and resistance — common for calculating resistor heating); P = V² / R (use when you know voltage and resistance); P = V² × G (using conductance G = 1/R, useful for parallel analysis). Example: 5 V across a 68 Ω resistor: I = 5/68 = 73.5 mA; P = 5 × 0.0735 = 367 mW; verify P = 25/68 = 367 mW. Use a 1/2 W rated resistor.
Series: R_total = R₁ + R₂ + R₃. Same current flows through each resistor; voltages add. Parallel: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃. Same voltage across each; currents add. Two resistors in parallel: R_total = (R₁ × R₂) / (R₁ + R₂). Example: 100 Ω and 150 Ω in parallel → R_total = (100 × 150)/(100 + 150) = 15,000/250 = 60 Ω. With 12 V supply: I_total = 12/60 = 200 mA total (120 mA through 100 Ω + 80 mA through 150 Ω = 200 mA).
Current-limiting resistor for LED: LED needs 20 mA, supply 5 V, LED forward voltage 2.1 V → R = (5 − 2.1) / 0.020 = 145 Ω → use 150 Ω standard. Power dissipated = (5−2.1) × 0.020 = 58 mW. Transistor base resistor: 5 V logic, want 1 mA base current, V_BE = 0.7 V → R = (5 − 0.7) / 0.001 = 4,300 Ω → use 4.7 kΩ. Voltage divider: 10 V in, want 3.3 V out → choose R1 = 10 kΩ, R2 = (3.3 × 10,000) / (10 − 3.3) = 4,925 Ω → use 4.7 kΩ. Ohm's Law is used in every analog circuit design step.
For DC circuits, V = IR applies directly. For AC circuits with reactive components (capacitors, inductors), the relationship becomes V = I × Z where Z is impedance — a complex number combining resistance R and reactance X. Capacitive reactance: X_C = 1/(2πfC). Inductive reactance: X_L = 2πfL. Total impedance: Z = √(R² + X²). For a 100 Ω resistor in series with a 10 mH inductor at 1 kHz: X_L = 2π × 1000 × 0.01 = 62.8 Ω → Z = √(100² + 62.8²) = 118.2 Ω. The current is 18.2% lower than in a purely resistive circuit.
Voltage: set to DC V (or AC V for mains), connect red probe to higher potential point, black to lower/ground. Connect in parallel with the component. Never insert a voltage-measuring multimeter in series — it will short the circuit. Current: set to A or mA, break the circuit wire, and insert the multimeter in series so current flows through it. Resistance: power the circuit off completely, disconnect at least one leg of the component, set to Ω, probe both ends. Safety: never measure resistance on live circuits, never exceed the meter's input voltage rating, use properly rated probes for mains voltage work.