How it works
Ohm's law relates voltage V, current I and resistance R. Power P is V × I. If two of the four values are known, the other two follow from these equations.
V = I × RP = V × I = I² × R = V² / R
Worked example
A 4 Ω load on a 12 V supply draws 3 A and dissipates 36 W. A 10 W resistor carrying 1 A has 10 V across it and a resistance of 10 Ω.
Design tips
- If the calculated power is more than half the resistor rating, the margin is thin. Choose a part with a larger power rating.
- Leave a field blank to treat that value as unknown. At least two values must be entered for the rest to be solved.
- When the current comes from a supply and a load resistance, also check the wiring and connector contact resistance, which add to the total.
Limitations
- Only pure resistance is assumed. Inductors and capacitors in AC circuits cannot be handled with these equations alone.
- Resistance changes with temperature. The temperature coefficient (ppm/°C) of the resistor is not included.
Frequently asked questions
What happens if I enter three values?
The entered values are used as they are, and only the remaining values are calculated. If the three values do not agree with each other, the results may not match reality, so check your measurements first.
What if the resistor power rating is too low?
Choose a larger rated resistor so the calculated power stays well below the rating (usually 50 % or less), or split the power across several resistors in series or parallel.