Enter any two values, voltage (V), current (I), resistance (R) or power (P), and the calculator instantly works out the other two. It draws on the four formulas below, and the ones it uses for your numbers light up as you type, so you can see exactly how the answer was found.
How to Use the Ohm’s Law Calculator
- Pick any two known values, voltage (V), current (I), resistance (Ω), or power (P).
- Type them into the fields above. Use the unit dropdown to switch between V/mV/kV, A/mA/µA, Ω/kΩ/MΩ, and W/mW/kW.
- Results appear instantly. The remaining two values are calculated with step-by-step workings and the exact formulas used.
Example 1: Enter 12 V and 4 Ω to return 3 A and 36 W.
Example 2: Enter 500 mA and 10 kΩ to return 5,000 V and 2,500 W.
Example 3: Enter 24 V and 2 A to return 12 Ω and 48 W.
Voltage, Current, Resistance & Power Explained
The electrical pressure pushing current through a circuit. Measured in Volts.
V = I×R · V = P/I · V = √(P×R)The flow of electric charge through a conductor. Measured in Amps.
I = V/R · I = P/V · I = √(P/R)Opposition to current flow in a circuit. Measured in Ohms (Ω).
R = V/I · R = V²/P · R = P/I²Rate of energy consumed by a load. Measured in Watts (W).
P = V×I · P = V²/R · P = I²×RComplete Ohm’s Law Formula Table
Quick-reference for all voltage, current, resistance and power relationships.
| Find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage (V) | V = I × R | V = P / I | V = √(P × R) |
| Current (I) | I = V / R | I = P / V | I = √(P / R) |
| Resistance (R) | R = V / I | R = V² / P | R = P / I² |
| Power (P) | P = V × I | P = V² / R | P = I² × R |
Understanding Ohm’s Law
Ohm’s Law describes the relationship between voltage, current, and resistance in an electrical circuit. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, expressed as V = I × R.
The law was first published in 1827 by German physicist Georg Simon Ohm in his treatise The Galvanic Circuit Investigated Mathematically. Ohm derived the relationship through careful experimental measurements of current and voltage across metal wires of varying lengths and thicknesses. The SI unit of electrical resistance, the ohm (Ω), was named in his honour.
Each quantity has its own SI unit: voltage is measured in the volt (V), electric current in the ampere or amp (A), resistance in the ohm (Ω) and electrical power in the watt (W). One volt drives one ampere of current through a resistance of one ohm, and one watt is the power delivered when one volt pushes one ampere.
Ohm’s Law Triangle
Cover the value you want to find, the remaining two reveal the formula.
Cover V → I × R | Cover I → V ÷ R | Cover R → V ÷ I
Basic Circuit Diagram
How voltage (the push), current (the flow), and resistance (the opposition) interact in a simple DC circuit.
Complete Formula Wheel, 12 Formulas
All formulas for Voltage, Current, Resistance and Power in one chart.
What Determines Resistance?
Ohm’s Law links voltage, current and resistance, but the resistance of a component itself depends on four physical factors:
- Material (resistivity). Copper and silver conduct well (low resistance); nichrome and carbon resist more. Every material has a property called resistivity.
- Length. A longer conductor has more resistance. Roughly, double the length means double the resistance.
- Cross-sectional area. A thicker conductor has less resistance, with more room for charge to flow.
- Temperature. For most metals, resistance rises as temperature rises.
Ohmic vs non-ohmic: Ohm’s Law holds for “ohmic” components such as resistors, where resistance stays constant. Diodes, lamps and transistors are “non-ohmic”, so their resistance changes with voltage or temperature, and the calculator gives a snapshot for a single operating point rather than a fixed value.
To measure an unknown resistance precisely rather than calculate it, the classic method is a Wheatstone bridge calculator.
Ohm’s Law and Power (Watts’ Law)
Power is where Ohm’s Law meets Watts’ Law. Once the calculator knows any two of voltage, current and resistance, it also returns power (P) in watts using:
- P = V × I (power = voltage × current)
- P = I² × R (when you know current and resistance)
- P = V² / R (when you know voltage and resistance)
That is why entering just voltage and resistance still returns a wattage. It is handy for sizing resistors, LEDs, heating elements and power supplies so nothing overheats.
Ohm’s Law Calculator: Worked Examples
Sizing an LED resistor. A 9 V supply drives an LED that needs 2 V at 20 mA. The resistor drops the remaining 7 V: R = V ÷ I = 7 ÷ 0.02 = 350 Ω (use the nearest standard value, 360 Ω). Power in the resistor: P = I² × R = 0.02² × 350 = 0.14 W, so a 0.25 W resistor is fine.
What is 20 amps at 12 ohms? V = I × R = 20 × 12 = 240 V, and the power is P = I² × R = 20² × 12 = 4,800 W.
Series vs parallel. Ohm’s Law applies to any single component and to a whole circuit. For total resistance, combine resistors first: in series add them (R = R1 + R2…); in parallel use 1/R = 1/R1 + 1/R2…, then feed the total into the calculator.
Ohm’s Law Calculator FAQ
What is Ohm’s Law?
Ohm’s Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. The formula is V = I × R, where V is voltage in volts, I is current in amps and R is resistance in ohms.
How do you calculate Ohm’s Law?
Multiply current by resistance to get voltage: V = I × R. Rearrange for the others: I = V ÷ R and R = V ÷ I. Enter any two values into the calculator above and it solves the rest.
What are the 3 Ohm’s Law formulas?
V = I × R (voltage), I = V ÷ R (current) and R = V ÷ I (resistance). Add power and you get the full set of relationships shown in the formula table above.
What is 20 amps at 12 ohms?
240 volts. V = I × R = 20 × 12 = 240 V, and the power dissipated is 4,800 W (P = I² × R).
How do I use the Ohm’s Law calculator?
Enter any two known values (for example, voltage and resistance) and the calculator automatically works out the other two (current and power), with step-by-step workings. Change units using the dropdown next to each field.
What is Watts’ Law?
Watts’ Law links power to the other quantities: P = V × I, P = I² × R and P = V² / R. The calculator uses both laws together, so it returns power alongside voltage, current and resistance.
Can I use this calculator for AC circuits?
This calculator is designed for DC circuits. For purely resistive AC loads the same formulas apply using RMS values. For AC circuits with capacitors or inductors, impedance (Z) replaces simple resistance and V = I × Z.
Who discovered Ohm’s Law?
Georg Simon Ohm, a German physicist, published his discovery in 1827. The SI unit of resistance, the ohm (Ω), was named in his honour.
Related Calculators
Explore more free electronics tools from Kunkune:
- Browse all free electronics calculators, the full Kunkune toolkit for circuit design and analysis.
- Wheatstone bridge calculator, measure an unknown resistance precisely from three known resistors.
- MOSFET threshold voltage calculator, find the gate voltage needed to switch a transistor on.