How to Calculate Ohm's Law (Voltage, Current, Resistance)
Learn how to calculate voltage, current, and resistance using Ohm's Law (V = IR) with practical electronics examples. This fundamental law is the foundation of circuit analysis and electrical engineering.
What is Ohm's Law?
Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, with resistance (R) as the constant of proportionality: V = I × R. This relationship holds for ohmic materials (metals, resistors) over a wide range of conditions. Non-ohmic components like diodes and transistors do not follow this simple linear relationship.
Calculating Voltage
V = I × R. If 2 amperes (A) flow through a 10-ohm (Ω) resistor, the voltage drop across it is 2 × 10 = 20 volts (V). This is the voltage a battery or power supply must provide to push that current through that resistance. In circuit analysis, each component's voltage drop is calculated this way and all drops in a series loop must sum to the source voltage.
Calculating Current
Rearrange to I = V ÷ R. A 12 V car battery connected to a 6 Ω load draws 12 ÷ 6 = 2 A of current. Exceeding the rated current of a wire or component causes overheating — this is why fuses are sized to the maximum safe current for a circuit. Current is measured in amperes (A), or milliamperes (mA) for small electronics.
Calculating Resistance
Rearrange to R = V ÷ I. If a 9 V battery drives 0.045 A (45 mA) through an LED circuit, the total circuit resistance is 9 ÷ 0.045 = 200 Ω. Knowing this, you can select the correct resistor to protect the LED from excess current. Resistance is measured in ohms (Ω), kilohms (kΩ = 1,000 Ω), or megohms (MΩ).
Power in an Electrical Circuit
Combining Ohm's Law with the power formula P = V × I gives three useful forms: P = V × I, P = I² × R, and P = V² ÷ R. A 100 Ω resistor carrying 0.5 A dissipates P = 0.5² × 100 = 25 W of power as heat. Always choose components rated for the calculated power dissipation plus a safety margin (typically 50–100 % above the calculated value).
Series and Parallel Circuits
In a series circuit, resistances add directly: R_total = R₁ + R₂ + R₃. In a parallel circuit, the reciprocal formula applies: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃. For two resistors in parallel: R_total = (R₁ × R₂) ÷ (R₁ + R₂). Two 100 Ω resistors in parallel give R_total = (100 × 100) ÷ (100 + 100) = 50 Ω.
Practical Safety Considerations
Even modest voltages can be dangerous if enough current passes through the human body — currents above 10 mA can cause painful shocks and above 100 mA can be fatal. Always de-energize circuits before working on them, use fuses or circuit breakers sized to protect conductors, and verify your calculations before applying power to a new circuit design.
Try These Calculators
Put what you learned into practice with these free calculators.
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