Resistor Calculations

Calculate resistor voltage drop, current and power dissipation. Check an existing wattage rating or find a suggested rating with a selectable power margin. Includes B+ dropping resistors and parallel resistor combinations.

Calculation mode Voltage + Resistance Current + Resistance Voltage + Current B+ voltage drop Two resistors in parallel

Voltage

Resistance

Power margin 1.5 times 2 times (default) 3 times 5 times

Existing resistor rating (optional)

W

Leave blank for a suggested rating only.

Calculated results

Important: Wattage recommendations are based on steady-state power dissipation and the selected design margin. Maximum working voltage, temperature derating, pulse or surge conditions, and mounting requirements must still be checked against the resistor manufacturer's specifications. For an AC waveform, use its true RMS voltage across the resistor. Valve amplifier supplies can retain dangerous voltages after switch-off.

Formulas and examples

Ohm's law: I = V / R, V = I x R, R = V / I.

Power: P = V x I = V squared / R = I squared x R.

Parallel: 1 / Rtotal = 1 / R1 + 1 / R2. Each parallel resistor has the same voltage across it, but each dissipates power according to its individual resistance.

B+ dropping resistor: The voltage drop is the measured voltage before the resistor minus the voltage after it. Use measurements taken with the amplifier operating under load.

Example: 28.2 V across 200 ohms gives 141 mA and 3.98 W of heat. At a two-times design margin, the next listed resistor rating is 10 W.