Ohm's Law Calculator, V=IR
Calculate voltage, current, resistance, or power using Ohm's law.
PRACTICAL EXAMPLES
Enter any two values, leave the unknowns empty to solve for them automatically.
What Is the Ohm's Law Calculator, V=IR?
This Ohm's Law calculator solves for any two of the four electrical quantities, Voltage (V), Current (I), Resistance (R), and Power (P), given any other two. Enter any two values, select their units, and the calculator instantly computes the remaining two quantities in engineering notation (mV, kΩ, mW, µA, etc.).
- ›Full 4-way solve: enter any two of V, I, R, P and solve for the others, all six input combinations supported.
- ›Unit prefixes: voltage in V/mV/kV, current in A/mA/µA, resistance in Ω/kΩ/MΩ/mΩ, power in W/mW/kW/MW.
- ›Engineering notation output: results display in appropriate prefix form (e.g. 23 mW, 4.7 kΩ, 120 µA).
- ›Practical presets: LED resistor, phone charger, 60W bulb, load real examples with one click.
- ›Applied equations: shows all four equations solved numerically with your actual values.
Formula
| Find | From V & I | From V & R | From I & R | From P & (other) |
|---|---|---|---|---|
| V (Volts) | — | V = V | V = I × R | V = P/I, V = √(PR) |
| I (Amps) | I = I | I = V/R | — | I = P/V, I = √(P/R) |
| R (Ohms) | R = V/I | — | R = R | R = V²/P, R = P/I² |
| P (Watts) | P = V×I | P = V²/R | P = I²R | — |
How to Use
- 1Enter any two known values from: Voltage (V), Current (I), Resistance (R), Power (P).
- 2Select the appropriate unit for each value using the unit dropdown next to each field.
- 3Leave the two unknown fields empty, the calculator will solve for them.
- 4Click Calculate or press Enter to compute all four quantities.
- 5Results show in engineering notation for easy reading (23 mW, 4.7 kΩ, etc.).
- 6Click a practical preset (LED resistor, phone charger, 60W bulb) to load a real-world example.
- 7Click Clear to reset all fields.
Example Calculation
Example 1, LED current-limiting resistor
Example 2, 60 W bulb at 120 V
Example 3, Mains fuse sizing
Power triangle memory aid
Understanding Ohm's Law, V=IR
Ohm's Law Explained
Ohm's Law states that the voltage across a resistor is directly proportional to the current flowing through it: V = I × R. Discovered by Georg Simon Ohm in 1827, it is the fundamental law of electrical circuits. The constant of proportionality is the resistance R, measured in ohms (Ω). A resistor is said to be "ohmic" when it obeys this linear relationship over a range of voltages and currents.
Not all circuit elements are ohmic. Diodes, LEDs, and transistors have non-linear I-V characteristics. Incandescent light bulbs have resistance that increases with temperature. However, for resistors, capacitors (in AC), and inductors (in AC), Ohm's law (or its AC equivalent using impedance Z instead of R) applies.
Power and Joule's Law
Electrical power P = V × I was established by James Prescott Joule in 1841. Combined with Ohm's Law, it produces two derived forms: P = I²R (power as a function of current and resistance) and P = V²/R (power as a function of voltage and resistance). These three equivalent expressions allow power to be calculated from any two of {V, I, R}.
- ›P = V × I: directly, watts = volts × amps. Used when V and I are both known.
- ›P = I²R: "heating formula", resistive power dissipation. Cable heating is I²R loss.
- ›P = V²/R: useful when voltage is fixed and you vary the resistance (e.g. dimmer circuits).
All three forms are equivalent, the choice depends on which quantities are known. This calculator handles all six input combinations automatically.
Engineering Notation and SI Prefixes
This calculator outputs results in engineering notation, values scaled to powers of 1000 with appropriate SI prefixes. This matches how electrical engineers and technicians communicate quantities:
- ›Voltage: 0.012 V → 12 mV (millivolts); 5000 V → 5 kV (kilovolts)
- ›Current: 0.00025 A → 250 µA (microamps); 0.015 A → 15 mA (milliamps)
- ›Resistance: 4700 Ω → 4.7 kΩ; 2,200,000 Ω → 2.2 MΩ
- ›Power: 0.060 W → 60 mW; 1500 W → 1.5 kW
Practical Applications: LEDs, Fuses, and Power Supplies
Ohm's Law has immediate practical applications in electronics and electrical work:
- ›LED resistor calculation: subtract LED forward voltage from supply voltage, divide by desired current (typically 10–30 mA) to find the current-limiting resistor value.
- ›Fuse sizing: calculate maximum circuit current (I = P/V) and select the next standard fuse rating above that value.
- ›Wire gauge selection: calculate I²R heating losses for a given cable resistance to ensure safe operating temperature.
- ›Battery life estimation: I = P/V gives current draw; divide battery capacity (Ah) by current (A) to get runtime in hours.
Frequently Asked Questions
What is Ohm's Law and what does it say?
Ohm's Law states: V = I × R. The voltage (V, in volts) across a resistor equals the current through it (I, in amps) multiplied by its resistance (R, in ohms). Discovered by Georg Simon Ohm in 1827, it is the cornerstone of circuit analysis.
- ›Double the voltage → double the current (at fixed resistance)
- ›Double the resistance → halve the current (at fixed voltage)
- ›It holds for ohmic materials (metals, most resistors) at constant temperature
- ›Non-ohmic: diodes, LEDs, transistors, I-V relationship is non-linear
Combined with Kirchhoff's voltage and current laws, Ohm's Law allows the analysis of any linear electrical circuit.
What is the power formula and how is it derived from V = IR?
Power P = V × I (Joule's Law, 1841). By substituting Ohm's Law, two additional forms emerge:
All three are equivalent. Use P = V×I when voltage and current are known; P = I²R for wire heating loss; P = V²/R for loads at a fixed supply voltage.
How do I calculate a current-limiting resistor for an LED?
An LED requires a current-limiting resistor to prevent excess current from burning it out.
Standard resistor values: use the E24 or E12 series and choose the nearest standard value at or above your calculated R.
What is the difference between voltage, current, and resistance?
A useful analogy is a water pipe:
- ›Voltage (V, volts): electrical pressure, like water pressure. Higher voltage pushes more current through a given resistance.
- ›Current (I, amps): flow rate, the number of electrons passing per second. 1 amp = 1 coulomb per second ≈ 6.24 × 10¹⁸ electrons/second.
- ›Resistance (R, ohms): opposition to flow, like pipe narrowness. Higher resistance means less current for the same voltage.
- ›Power (P, watts): energy dissipated per second, rate of energy consumption. 1 W = 1 J/s.
What are common SI unit prefixes for electrical measurements?
Engineering notation groups by powers of 1000. This calculator outputs in the most appropriate prefix automatically. For example, 0.000047 A becomes 47 µA, and 47000 Ω becomes 47 kΩ.
Does Ohm's Law apply to AC circuits?
In AC circuits, Ohm's Law generalises to: V = I × Z, where Z is impedance (a complex number combining resistance R and reactance X).
- ›Pure resistor: Z = R (same as DC)
- ›Capacitor: Z = 1/(jωC), impedance decreases with frequency
- ›Inductor: Z = jωL, impedance increases with frequency
- ›Series RLC: Z = R + j(ωL − 1/ωC)
This calculator handles DC circuits (pure resistance). For AC impedance calculations, use an RLC impedance calculator. The magnitudes follow the same formulas: |V| = |I| × |Z|, P = |V||I|cos(φ) where φ is the phase angle.
How do I calculate electrical power consumption and energy cost?
Power (P, watts) tells you the rate of energy use. Energy = Power × Time:
First find the device's power consumption using this Ohm's Law calculator (P = V × I), then multiply by daily hours and electricity rate to estimate your monthly energy bill.