wattstoamps
← Browse all tools

Resistive circuit arithmetic

Resistor power calculator

Know the voltage actually across a resistor and its resistance? Enter those two values to see current and power dissipation. The voltage is across this component, not automatically the source voltage.

Enter your values

Result

Results are based only on the inputs and assumptions shown here. Nothing you enter is sent or saved.

Formula: current = V ÷ R; resistor power = V² ÷ R = I²R

Worked example

With 12 V measured across a 24 Ω resistor, current is 0.5 A and ideal dissipation is 6 W. Those figures describe that component under the stated conditions.

Use the component voltage

OpenStax states the V²/R and I²R forms for resistors. In a larger circuit, the full supply voltage may be divided among components. Enter the voltage drop across this resistor; the main watts-to-amps calculator answers a different power-and-voltage question.

AC assumption

For AC, V must be RMS voltage across a purely resistive element. Reactive impedance, phase angle and power factor are not modeled here. For DC, use the steady voltage across the part.

No part selection is implied

Resistance changes with temperature and tolerances, and operating conditions can exceed a simple nominal reading. The result is calculated dissipation, not a recommended component rating, thermal design or wiring approval. All arithmetic runs locally.

Explore the related calculation

For a different starting point, try the power-factor calculator for AC loads or return to the main calculator. Browse the complete tool directory for this site.

Reference and corrections

The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.

Found a problem? Email contact@wattstoamps.app.