Ohm's Law Calculator
Calculate voltage, current, resistance, or power using Ohm's Law (V = IR). Includes the power triangle, worked examples, and all four Ohm's Law formulas.
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. Enter any two values and the calculator solves for the third — plus all four power formulas.
Ohm's Law Formula Triangle
The three core variables:
- V = Voltage in volts (V)
- I = Current in amperes (A)
- R = Resistance in ohms (Ω)
V = I × R (Voltage = Current × Resistance)
I = V ÷ R (Current = Voltage ÷ Resistance)
R = V ÷ I (Resistance = Voltage ÷ Current)
Memory aid: cover the variable you want to solve for in the triangle — what remains is the formula.
Power Extensions — The Full Set
Ohm's Law extends to power (P, measured in watts) through four equivalent formulas:
P = V × I (Power = Voltage × Current)
P = I² × R (Power = Current squared × Resistance)
P = V² ÷ R (Power = Voltage squared ÷ Resistance)
P = V²I ÷ R (combined form)
| Know these | Calculate this |
|---|---|
| V and I | P = V × I |
| I and R | P = I² × R |
| V and R | P = V² ÷ R |
| P and V | I = P ÷ V |
| P and I | V = P ÷ I |
| P and R | I = √(P ÷ R) |
Worked Examples
Example 1 — LED circuit
A 5V LED circuit draws 20mA (0.020A) through a current-limiting resistor. What resistance is needed?
R = V ÷ I = 5 ÷ 0.020 = 250Ω
Use the nearest standard resistor value: 270Ω (E12 series). The LED will draw slightly less current — acceptable and safer for the LED.
Example 2 — Home appliance fuse selection
A 1,200W kettle runs on 240V. What current does it draw?
I = P ÷ V = 1,200 ÷ 240 = 5A
A 13A fuse is correct for UK plugs. Using a 3A fuse would blow; using a 13A fuse on a 5A device leaves headroom but is standard.
Example 3 — Car electrical system
A car speaker system is rated at 4Ω impedance and peaks at 100W. What peak current flows through the speaker wire?
I = √(P ÷ R) = √(100 ÷ 4) = √25 = 5A
Speaker wire should be rated for at least 5A with appropriate gauge (16 AWG minimum for short runs).
What Ohm's Law Applies To
Ohm's Law applies to linear, resistive components at a given temperature:
- Resistors (standard carbon film, metal film, wirewound)
- Heating elements (kettles, toasters, heaters)
- Incandescent bulbs (resistance changes with temperature, but Ohm's Law still holds at any given operating point)
- Copper wire and other conductors (treated as very low resistance)
Ohm's Law does NOT apply linearly to:
- Diodes and LEDs — current increases exponentially with voltage, not linearly
- Transistors — operate as amplifiers or switches, not resistors
- Capacitors — store and release charge over time; impedance is frequency-dependent
- Inductors — oppose changes in current; impedance is frequency-dependent
- Most semiconductors in general — they're non-linear by design
For AC circuits with capacitors or inductors, use impedance (Z) in place of resistance: V = I × Z, where Z is a complex number accounting for frequency.
Common Resistor Values and the E12/E24 Series
Resistors come in standard preferred values. The E12 series (12 values per decade) is the most common:
E12 series: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 (then ×10: 10, 12, 15...)
If your calculation gives 250Ω, choose 270Ω (next E12 value up) to slightly under-drive the circuit, which is the safer approach for LEDs and sensitive components.
Frequently Asked Questions
Who discovered Ohm's Law?
Georg Simon Ohm, a German physicist, published the relationship between voltage, current, and resistance in 1827. His work was initially ignored by the scientific establishment but is now foundational to all electrical engineering.
Does Ohm's Law work for AC circuits?
For purely resistive AC loads (heaters, incandescent bulbs), yes — the formula V = I × R applies using RMS voltage and current values. For circuits with capacitors or inductors, you need impedance (Z) which varies with frequency. At 50Hz or 60Hz mains frequency, impedance replaces resistance in the formula.
What is the unit of resistance and why is it called an ohm?
The ohm (Ω) is named after Georg Ohm. One ohm is defined as the resistance of a conductor through which a current of one ampere flows when a potential difference of one volt is applied. It's part of the SI system and formally defined in terms of the quantum Hall effect.
What happens if resistance is zero?
Zero resistance means a short circuit. With V = I × R: if R = 0, then V = 0 regardless of current, or equivalently, for any non-zero voltage, current would be infinite. In practice, wire resistance is very low (not zero) and circuit breakers or fuses blow before infinite current is reached.
How does temperature affect resistance?
For most metals, resistance increases with temperature. Copper's resistance increases approximately 0.4% per degree Celsius. This is why incandescent bulbs have lower resistance when cold than when operating at full temperature — the cold-start current surge is much higher than the steady-state current.
What is the power triangle?
The power triangle is a visual aid showing the relationship between real power (P, watts), reactive power (Q, VAR), and apparent power (S, VA) in AC circuits. It extends Ohm's Law into AC power analysis. For resistive-only circuits, there is no reactive power and P = S.
Related Calculators
- Amps to Watts Calculator — convert current and voltage to power
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- Resistor Calculator — decode resistor colour codes and calculate series/parallel resistance
- LED Resistor Calculator — find the correct current-limiting resistor for any LED