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How to use this calculator
Choose the formula (if there is more than one), choose what you want to find, then fill in the other values using the units shown. The calculator rearranges the formula, puts your numbers in, and shows every step.
How it's calculated
Ohm's law V = IR. Power P = VI = I²R = V²/R. Charge Q = It. Energy E = VIt. Resistors in series: R = R₁ + R₂. In parallel: 1/R = 1/R₁ + 1/R₂.
Worked example
A 12 V battery across a 4 Ω resistor: I = V / R = 12 ÷ 4 = 3 A.
Frequently asked questions
Why is the parallel total smaller?
Current has more paths to flow through, so the total resistance falls below the smallest resistor.
What is the unit of resistance?
The ohm, written Ω.
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Last updated October 2026. Results are for learning and guidance; always check important work. Calculations run in your browser and are not stored.