Ohm's Law Calculator
Calculate voltage, current, resistance, and power using Ohm's Law.
Ohm's Law tells us that the voltage across a resistor equals how much current flows through it multiplied by its resistance — like water pressure depending on how hard water pushes (current) and how narrow the pipe is (resistance).
Calculate resistance given voltage and current.
📥 Input Parameters
📤 Results
📐 Calculation Steps
Identify known variables
Determine which two of the three quantities (V, I, R) are known; the third is solved for.
Select appropriate form of Ohm’s Law
Rearrange V = I × R to solve for the unknown variable.
Substitute values and compute
Perform arithmetic using consistent units (volts, amperes, ohms).
⚡ Power Dissipation
Electrical power converted to heat (or work) in the resistive element.
🧮 Formula
Ohm's Law
V = I \times R
Fundamental linear relationship among voltage, current, and resistance in ohmic conductors.
| Symbol | Variable | Unit | Description |
|---|---|---|---|
| V | Voltage | V (volts) | Electric potential difference across the component. |
| I | Current | A (amperes) | Rate of flow of electric charge through the component. |
| R | Resistance | Ω (ohms) | Opposition to current flow due to material properties and geometry. |
📋 Worked Example: Resistor Sizing for LED Circuit
A standard 330 Ω, 0.25 W resistor is suitable — provides safe margin below power rating and within ±5% tolerance range.
🔍 Result Interpretation
Indicates near-short-circuit condition or measurement error.
Verify connections, meter calibration, and absence of parallel paths.
Typical for control circuits, sensors, and signal conditioning.
Validate with thermal derating if continuous operation exceeds 75% of rated power.
Common in insulation testing, pull-up/down networks, or high-impedance inputs.
Account for leakage currents and PCB contamination effects.
Suggests open circuit or dielectric integrity (e.g., >1 MΩ for Class I equipment).
Use megohmmeter; confirm environmental conditions (humidity, cleanliness).
🔥 Guidance
Resistors exceeding 70°C may degrade nearby electrolytic capacitors or solder joints.
Localized overheating can separate copper traces from FR-4 substrate, especially with poor thermal vias.
Concentrated dissipation without thermal relief increases failure rate of nearby semiconductors.
💡 Engineering Recommendations
Calculated resistance < 1 Ω with >1 A current
Use four-wire (Kelvin) measurement to eliminate lead resistance error.
Contact resistance and thermal EMF become dominant error sources below 1 Ω.
Power dissipation > 50% of resistor’s rated power
Derate by 50% for ambient temperatures > 40°C or enclosure mounting.
Thin-film resistors exhibit lower thermal resistance than carbon composition; verify datasheet derating curves.
AC application above 1 kHz
Include impedance (Z) analysis — consider skin effect, parasitic capacitance, and inductance.
Pure R becomes inadequate; use complex notation: Z = R + jX, where X includes reactance components.
⚡ Quick Facts
🔬 Engineering Insight
"Ohm’s Law is deceptively simple — but its real-world application demands rigorous attention to assumptions: constant temperature, uniform cross-section, negligible electromagnetic field coupling, and purely resistive behavior. Always validate with measurement under actual operating conditions; simulation tools often ignore contact resistance, trace inductance, and thermal coefficient drift — leading to 10–20% errors in precision analog designs."
📜 Standards & References
❓ Frequently Asked Questions
Yes — for purely resistive loads at any frequency. For reactive loads (capacitors/inductors), use impedance (Z) instead of R in the generalized form V = I × Z.
Never measure resistance on energized circuits — it risks damaging the meter and gives false readings due to parallel current paths and internal bias voltages.
Not directly — diodes are non-ohmic devices with exponential I-V characteristics. Ohm’s Law only approximates behavior over small linearized regions (e.g., dynamic resistance r_d = dV/dI).
Temperature rise (increasing R in metals), material nonlinearity (e.g., varistors), frequency-dependent effects (skin depth), and quantum-scale phenomena (nanoscale conduction).
For engineering design, ±1% tolerance on R and ±0.5% on V/I is typical. Critical applications (e.g., shunt calibration) require metrological traceability to NIST or national standards labs.