Free electrical calculators

Small, honest calculators for students and technicians. Each page shows the formula, a worked example and the answer to common questions. They work in any browser, with or without JavaScript, and nothing you type is stored.

Ohm's law

Enter any two of V, I, R and P and get the other two.

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Cable size

Pick a standard mm² size by load current and voltage drop.

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Transformer efficiency

Efficiency from iron and copper loss, and the load for maximum efficiency.

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Motor torque

Shaft torque from kW or HP and rpm; slip and full-load current as extras.

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Solar panel size

Array size, panel count, battery bank and inverter from daily use.

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Electricity bill

Total bill from units and your own slab rates.

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Power factor correction

Capacitor kVAr needed to raise the power factor.

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The results are for learning and quick estimates. A real installation needs a proper design that follows the standards and the data sheets of the equipment you use.