Ohm's Law Calculator
Calculate voltage (V=IR), current (I=V/R), resistance (R=V/I), and power (P=VI) with Ohm's Law.
Enter any two electrical values to find the remaining two.
Ohm’s Law is the most fundamental relationship in electronics and electrical engineering. It was formulated by Georg Simon Ohm in 1827 in Germany.
The core formula:
V = I × R
Where:
- V = Voltage (in Volts, V) — the electrical pressure or potential difference
- I = Current (in Amperes, A) — the flow of electric charge
- R = Resistance (in Ohms, Ω) — the opposition to current flow
Rearranged forms:
- Current:
I = V / R - Resistance:
R = V / I
Power formula:
P = V × I
Which can also be expressed as:
P = I² × R(when you know current and resistance)P = V² / R(when you know voltage and resistance)
Where P = Power in Watts (W).
The Ohm’s Law Triangle: A helpful memory aid is to draw a triangle with V on top and I and R on the bottom. Cover the variable you want to solve for: cover V to see I × R, cover I to see V/R, cover R to see V/I. The same works for the power triangle with P on top and V × I on the bottom.
Practical examples:
- A 60W light bulb on a 120V circuit draws 0.5A of current and has 240Ω resistance.
- A 5V USB port delivering 2.4A to charge a phone provides 12W of power.
- A 12V car battery powering a 100W headlight draws about 8.33A through 1.44Ω.
How to use this calculator: Enter any two of the four values (voltage, current, resistance, or power) and leave the other two blank. The calculator will compute the missing values. You only need two known values — the other two are derived mathematically.
Tips:
- Always double-check units: milliamps (mA) = amps ÷ 1000, kilohms (kΩ) = ohms × 1000.
- For safety, always calculate current before working on a circuit to ensure wires and components are rated correctly.
- Real-world circuits may differ slightly from calculated values due to temperature effects on resistance.