Ohm's Law Calculator
The one relationship every circuit obeys. Pick the two values you know — any pair of voltage, current, resistance, and power — and get all four at once, rearranged for you.
Use base units: volts (V), amps (A), ohms (Ω), watts (W). 20 mA = 0.02 A.
Formula & how it works
The core relations are V = I × R and P = V × I. From any two known quantities the rest follow — for instance R = V ÷ I, P = V² ÷ R, V = √(P × R), and I = √(P ÷ R).
Worked example
Five volts across a component drawing 0.02 A (20 mA) gives R = 5 ÷ 0.02 = 250 Ω and P = 5 × 0.02 = 0.1 W. Knowing a 100 Ω resistor on 12 V instead? Then I = 0.12 A and P = 1.44 W.
Working with Ohm's law
Any two unlock the rest
Voltage, current, resistance, and power form a tight web of relationships. Measure or choose any two and the other two are fixed. That is why a multimeter reading of voltage and current is enough to know both the resistance a load presents and the power it draws.
Mind the power
It is easy to get resistance right and forget power. A resistor that is the correct value but rated for too little wattage will overheat. The power figure here is what the part actually dissipates; choosing a resistor rated at roughly double that leaves comfortable margin.
Units and prefixes
Small hobby circuits mix milliamps, kilohms, and milliwatts, which is where sign errors creep in. Convert everything to base units first — amps, ohms, volts, watts — do the math, then read the result back in whatever prefix is convenient. Twenty milliamps is 0.02 A; ten kilohms is 10,000 Ω.