Ohms Law Calculator
Adjust the inputs below. Results update as you type.
How it works
Ohm's Law: V = IR. Watt's Law: P = VI = I²R = V²/R. Enter any two values to solve for the others. Series circuits: total R = sum of resistances; current is the same through all. Parallel circuits: 1/R_total = Σ(1/Rᵢ); voltage is the same across all. Safety note: household AC (120V/240V) can be lethal—always follow electrical codes and use qualified electricians for wiring.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
Ohm's law: V = I × R. Power: P = V × I = I²R = V²/R. Voltage, current, and resistance are linked, and any one follows from the other two.
Worked example
A 12-volt supply across a 4-ohm resistor drives I = 12 ÷ 4 = 3 amps and dissipates P = 12 × 3 = 36 watts.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Find productivity tools for everyday workflows
General recommendation, not professional advice.
Frequently asked questions
What is Ohm's law?+
It states that voltage equals current times resistance (V = IR). Rearranged, it finds any one quantity from the other two.
How do I calculate power?+
Power = voltage × current. You can also use P = I²R or P = V²/R depending on which values you know.
Does Ohm's law always hold?+
It holds for ohmic (resistive) components at constant temperature. Diodes, transistors, and some materials are non-ohmic.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.