Ohm's Law & Watt's Law Calculator
Free Ohm's law and Watt's law calculator. Enter any two of voltage, current, resistance, and power and instantly solve the other two — for HVAC techs, electricians, and students.
Enter any two values and leave the rest blank.
How Ohm’s law and Watt’s law work
Ohm’s law ties voltage, current, and resistance together:
V = I × R (so I = V ÷ R and R = V ÷ I)
Watt’s law brings in power:
P = V × I — and combined with Ohm’s law, P = I² × R and P = V² ÷ R.
Between them, any two known values let you solve the other two. The wheel of relationships:
| Find | From V & I | From V & R | From I & R | From P & … |
|---|---|---|---|---|
| V | — | — | I × R | √(P × R) |
| I | — | V ÷ R | — | P ÷ V |
| R | V ÷ I | — | — | V² ÷ P |
| P | V × I | V² ÷ R | I² × R | — |
Worked example
A condenser fan motor draws 2.5 A at 240 V:
- Power = 240 × 2.5 = 600 W
- Resistance (apparent) = 240 ÷ 2.5 = 96 Ω
A note on motors
These formulas assume a resistive, in-phase load. Motors and other inductive loads have a power factor, so true power is V × I × PF. Use these results as a baseline for resistive elements (heat strips, ignitors), and account for power factor on motor circuits.
Sizing a conductor run? Pair this with the Voltage Drop Calculator.
How FieldPad helps in the field
Solve the circuit here, then keep the readings with the work. FieldPad logs electrical measurements against the equipment and the job, documents the failed component, and turns the repair into a signed estimate or invoice — offline, on your iPhone, with full history on one record.
Stop Re-Running These Numbers by Hand
FieldPad is the all-in-one iPhone CRM for solo HVAC techs — clients, scheduling, estimates, invoices with signatures, inventory, and equipment history. Keep your readings on the job, the client, and the equipment, all in one offline-ready app. EPA Section 608 and Subpart C compliance included.
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