Ohm's Law Calculator
Ohms Law calculator: solve for voltage, current, resistance or power from any two known values. Includes formulas, reference table and worked examples.
Last updated: May 2026
Enter the required values
Choose what you want to solve, then fill in the matching fields.
Why this calculator is useful
Use this page to calculate the missing relationship between voltage, current, resistance and power when you are checking a basic circuit, supply setting or load estimate. The point is not to replace proper design review. The point is to catch obvious mismatches quickly and keep small electronics tasks moving. That is especially helpful in hobby work, repair work and prototype setups where you want a fast answer without opening a full simulator.
A calculator like this earns its place because it reduces the chance of a simple arithmetic mistake creating a bigger bench problem later. Calculate first, then measure on the real circuit.
Typical use cases
- bench supply setup, simple circuit design and troubleshooting
- Reading and sanity-checking datasheet examples
- Confirming values before soldering or ordering parts
The final circuit still depends on tolerances, load conditions and the actual parts in front of you, but a quick page like this gives you a reliable starting point.
Frequently Asked Questions
What does Ohm's Law actually tell you when you measure a circuit?
Ohm's Law describes the relationship between voltage (electrical pressure), current (flow), and resistance (opposition to flow). If you know two values, you can calculate the third. In practice, this means if you measure 12V across a 220Ω resistor, you immediately know the current is 0.0545A (54.5 mA)—helping you verify whether a circuit is drawing the expected current or if there's a hidden short/break.
When would I solve for current vs voltage vs resistance?
Solve for current when you know the supply voltage and component resistance (e.g., "Will this 5V LED resistor limit current to safe levels?"). Solve for voltage when you know the current and resistance (e.g., "How much voltage drops across this resistor?"). Solve for resistance when you need a specific current with a known voltage (e.g., "What resistor do I need for a 20mA LED on 12V?"). Solve for power to check if a resistor needs to be heatsunk or will overheat.
Why does power dissipation matter in circuit design?
A 1kΩ resistor on a 12V supply dissipates 0.144W—it'll run warm but stay safe. The same resistor on 230V (wall outlet voltage) dissipates 52.9W and will char instantly. Calculating power first prevents sourcing the wrong component size or creating a fire hazard. A 0.25W resistor rated for low power cannot safely handle high-power circuits.
How do I use this calculator to verify component ratings?
Calculate the expected current through a component (LED, resistor, thermistor), then check the datasheet maximum rating. If your circuit will push 150mA through a component rated for 100mA max, you've caught the problem before breadboarding. This protects both the component and the circuit it's connected to.
Should I verify the calculator result on a real circuit?
Yes. Ohm's Law assumes ideal components with fixed resistance. Real circuits have tolerances (resistors are ±5% at best), temperature effects (resistance changes with heat), and parasitic elements (capacitance, inductance). Use the calculator for planning, but measure the actual voltage and current on the bench to confirm.
Electronics workflow
Common bench sequence for identifying and verifying components.
- Resistor Color Code Calculator — Read the value off any 4-band or 5-band resistor from its color bands
- LED Resistor Calculator — Calculate the current-limiting resistor for any LED and supply voltage
- Voltage Divider Calculator — Design a two-resistor divider for signal scaling or reference voltages
- Watt / Volt / Amp Calculator — Verify power ratings and component dissipation
- Voltage Drop Calculator — Check wire and trace voltage drop over a given distance