⚡ Ohm's Law Calculator
Solve for Voltage (V), Current (I), or Resistance (R). Power is calculated automatically.
Ohm's Law (V = I × R) was formulated by Georg Simon Ohm in 1827. It describes the relationship in resistive circuits. Real-world components may deviate (diodes, transistors). For AC circuits, impedance (Z) replaces resistance. Power triangle: P = V×I = I²×R = V²/R.
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Ohm's Law Explained: Voltage, Current, and Resistance
Ohm's Law is the fundamental relationship governing electrical circuits: V = I × R (Voltage = Current × Resistance). Knowing any two of these three values lets you calculate the third. This law was discovered by German physicist Georg Ohm in 1827 and remains the foundation of all circuit analysis.
Understanding the Three Variables
Voltage (V) is electrical pressure — the force that pushes electrons through a circuit, measured in volts (V). A US household outlet is 120V; European outlets are 230V. Car batteries are 12V; a 9V battery is common in electronics.
Current (I) is the flow rate of electrons, measured in amperes (A or amps). A typical LED uses 0.02A; a hairdryer uses 12–15A; a large EV charger uses 80A or more.
Resistance (R) is the opposition to current flow, measured in ohms (Ω). Conductors like copper have very low resistance; insulators like rubber have extremely high resistance. Resistors in circuits deliberately add resistance to control current flow.
Power and Ohm's Law
A related formula is Watt's Law: P = V × I (Power in watts = Voltage × Current). Combined with Ohm's Law: P = I²R = V²/R. These relationships let you calculate power consumption, size fuses correctly, and design safe circuits.
Ohm's Law: V, I, and R Relationships
Ohm's Law defines the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = I × R. From this, I = V ÷ R and R = V ÷ I. Voltage is measured in volts (V), current in amperes or amps (A), and resistance in ohms (Ω). A simple example: a 12V car battery connected to a 6Ω resistor produces a current of 12 ÷ 6 = 2 amps. If you double the resistance (to 12Ω), current halves to 1 amp — voltage stays the same. Ohm's Law only applies to ohmic (linear) components where resistance is constant — not to diodes, transistors, or other non-linear components where the V-I relationship is more complex.
Power Calculations: Adding Watts to the Equation
Electrical power (P, measured in watts) combines with Ohm's Law to give several useful formulas: P = V × I, P = I² × R, and P = V² ÷ R. A 60W light bulb on a 120V circuit draws 60 ÷ 120 = 0.5 amps of current. The same 60W bulb at 230V (European) draws only 0.26 amps — but would burn out instantly if plugged into 120V because the resistance remains constant (R = V² ÷ P = 14,400 ÷ 60 = 240Ω for 120V design) and 230V would deliver far more power than the filament can handle. Understanding these relationships is essential for sizing electrical wiring (too small a wire for the current causes overheating) and fuses (which protect circuits by breaking at a specified current).
Series vs Parallel Circuits and Resistance
Resistors can be combined in series or parallel. Series: total resistance = R₁ + R₂ + R₃ (resistances add directly). Current is the same through all components; voltage divides proportionally. Parallel: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ (total resistance is less than any individual resistor). For two equal resistors in parallel, total resistance is half of one resistor. Voltage is the same across all parallel branches; current divides. Most household electrical circuits are wired in parallel — each outlet and fixture operates at 120V independently, and adding more devices doesn't reduce the voltage to existing ones (but does increase total current drawn, eventually tripping the circuit breaker at 15 or 20 amps).
