Ohm's Law is the foundation of all electrical engineering. Three basic quantities – voltage (U), current (I) and resistance (R) – are enough to analyse simple and complex circuits. Add power (P) and you have the complete "golden rectangle" of electronics. The calculator lets you enter any two known values and instantly computes the two unknown ones – no formula hunting, no manual arithmetic.
Step by Step: How to Use the Ohm's Law Calculator
- Enter the known values: Input the two quantities you know. For example, a 12 V battery and a 100-ohm resistor: enter U = 12 V and R = 100 Ω.
- Read the calculation: The calculator immediately computes current and power: I = 12 ÷ 100 = 0.12 A = 120 mA; P = 12 × 0.12 = 1.44 W.
- Calculate an LED series resistor: For a red LED (2 V forward voltage, 20 mA operating current) on a 5 V supply: R = (5 – 2) ÷ 0.02 = 150 Ω.
- Watch the units: Current in amperes (A) or milliamperes (mA), resistance in ohms (Ω), kilohms (kΩ) or megohms (MΩ), voltage in volts (V).
- Verify the result: Sanity check: P = U × I must always equal P = U² ÷ R – both calculation paths give the same answer.
Practical Examples
Example 1 – Calculating an LED series resistor: Blue LED on a 9 V battery: forward voltage approx. 3.2 V, rated current 20 mA. Required resistor: R = (9 – 3.2) ÷ 0.02 = 290 Ω. Nearest standard value: 300 Ω. Power dissipated in the resistor: P = (9 – 3.2) × 0.02 = 0.116 W → a standard 0.25 W resistor is sufficient.
Example 2 – Fan heater current draw: A 2,000-watt fan heater on 230 V. Current: I = P ÷ U = 2,000 ÷ 230 = 8.7 A. Resistance of the heating element: R = U ÷ I = 230 ÷ 8.7 = 26.4 Ω. The circuit breaker must be rated at least 10 A.
Example 3 – Car battery internal resistance: A 12 V car battery with 0.005 Ω internal resistance during starter cranking (150 A): voltage drop = I × R_i = 150 × 0.005 = 0.75 V. Terminal voltage falls to 11.25 V – this is normal and not a fault.
Ohm's Law: Calculate U, I, R and P
Core formulas: U = I × R (voltage); I = U / R (current); R = U / I (resistance); P = U × I (power). The golden rectangle of electrical engineering.
Frequently Asked Questions (FAQ)
- Does Ohm's Law apply to all electrical components?
- No – Ohm's Law applies strictly only to linear resistors, where U is proportional to I. Diodes, transistors, capacitors and inductors behave non-linearly and follow different laws. For AC circuits, the complex resistance (impedance) is used.
- What is the difference between real power, reactive power and apparent power?
- P = U × I applies to DC and purely resistive AC loads. With inductive or capacitive loads (motors, transformers), reactive power Q arises. Apparent power S = U × I (in VA) is then greater than real power P (in W). The ratio is called the power factor cos φ.
- How do I remember the Ohm's Law formulas?
- The classic memory aid is the URI triangle: write U at the top, I and R side by side at the bottom. To find U → I × R; to find I → U ÷ R; to find R → U ÷ I. For power there's a second triangle with P at the top, U and I at the bottom: P = U × I; U = P ÷ I; I = P ÷ U.
