How it works
Voltage pushes current through resistance (V = I × R), and the circuit turns energy into heat or work (P = V × I). Know any two of the four and the other two follow. Pick units such as mV, kΩ or mA directly; results are shown in a sensible unit, with the nearest standard resistor value.
12 V across 4 Ω → 3 A · 36 W
Which pair to enter
- V + R: the classic case — what current flows and how much power is lost as heat.
- P + V: sizing a fuse or a supply from a device's rating plate.
- I + R: voltage drop along a long cable run.
Tips
- Resistance and power can't be negative; zero resistance means a short circuit the tool refuses to compute.
- Mind the units: 1 kΩ is 1,000 Ω and 1 mA is 0.001 A — convert before entering.
- Everything runs in your browser; nothing is uploaded.
Standards and sources
- DC circuit relations V = IR and P = VI (Ohm's law, Joule heating)
Last reviewed October 12, 2026
Frequently asked questions
Which two values should I enter?
Whatever you know: V+R for current and heat, P+V from a rating plate for fuse sizing, I+R for cable voltage drop. Every pair works.
Why was zero resistance refused?
Zero ohms is a short circuit: current would be infinite, which no real answer can describe. Enter the actual small resistance instead.
Can resistance or power be negative?
No, for ordinary passive components. Negative values are refused so wiring mistakes surface as errors, not nonsense.
Do I need to convert milliamps?
Yes. Enter 0.001 A for 1 mA and 1000 Ω for 1 kΩ — the tool computes in base units.