Ohm's Law Calculator
Solve for voltage (V), current (I), resistance (R), or power (P) by entering any two known values. Supports all Ohm's Law and power formula combinations. No signup, runs entirely in your browser.
⏱ 6 min read · Complete guide below
How to Use the Ohm's Law Calculator
- 1Pick the variable you want to solve for — voltage, current, resistance, or power.
- 2Enter any two known values in base SI units (volts, amps, ohms, watts). Convert prefixes first: 10 mA = 0.01 A, 4.7 kΩ = 4700 Ω.
- 3The calculator picks the right formula automatically — V = IR, P = VI, P = I²R, or P = V²/R — and shows the result instantly.
- 4Check the remaining derived value too: with any two inputs, all four quantities are determined.
Worked Example: Sizing a Resistor for an LED
You want to run an LED from a 9 V battery. The LED drops 2 V across itself and is rated for 15 mA of current. The resistor must absorb the remaining voltage: 9 V − 2 V = 7 V. Using Ohm's Law, R = V / I = 7 / 0.015 = 466.7 Ω. Resistors come in standard values, so you round up to the nearest common size: 470 Ω. Re-checking with the real part: I = 7 / 470 ≈ 14.9 mA — safely under the LED's rating.
Now verify the resistor's power rating. Solve for power with P = I²R = (0.0149)² × 470 ≈ 0.10 W. A standard quarter-watt (0.25 W) resistor handles this with more than double the margin — following the rule of thumb of choosing a resistor rated for at least twice the dissipated power. This two-step pattern — solve for R from the voltage budget, then solve for P to pick the physical part — is the most common real-world use of Ohm's Law.
The Ohm's Law Triangle and the Power Wheel
A simple memory aid captures the three core quantities: picture V at the top of a triangle, with I and R side by side beneath it. Cover the value you want and the triangle shows the formula — cover V and you see I × R; cover I and you see V over R; cover R and you see V over I. Power extends this into what electronics texts call the “power wheel,” because power connects to all three quantities: P = V × I, and by substitution P = I²R and P = V²/R. That is why entering any two of the four values here is enough to determine the other two — the equations interlock so completely that two knowns always fix the whole circuit.
Watch Your Units and Prefixes
The most frequent mistake with Ohm's Law is not the formula but the units. The equations only work in base SI units — volts, amps, ohms, and watts — yet real components are labelled with prefixes: milliamps, kilohms, microfarads, milliwatts. Before entering values, convert: 10 mA is 0.01 A, a 4.7 kΩ resistor is 4,700 Ω, and 5 mW is 0.005 W. Forgetting a prefix throws the answer off by a factor of a thousand or more, which is why a result that looks wildly too large or too small is almost always a units error rather than a formula error. Convert first, then calculate.
Series and Parallel Circuits
Ohm's Law applies to a single resistance, so for a real circuit you first reduce the network to one equivalent resistance. In a series circuit, resistances simply add: R_total = R₁ + R₂ + R₃, and the same current flows through each. In a parallel circuit, the reciprocals add: 1/R_total = 1/R₁ + 1/R₂, giving a total resistance smaller than any individual branch, and each branch shares the same voltage. Work out the equivalent resistance by hand, then feed it into this calculator with your source voltage to find the total current and power draw — the standard workflow for analysing any resistive circuit.
Tips for Circuit Calculations
Series resistors
In a series circuit, total resistance is R₁ + R₂ + R₃. Use the total resistance in this calculator to find current and voltage for the whole circuit.
Parallel resistors
In parallel: 1/R_total = 1/R₁ + 1/R₂. Calculate the equivalent resistance first, then use it here to find total current and power draw.
Power dissipation
When sizing a resistor, calculate the power it will dissipate using P = I²R. Choose a resistor rated for at least twice that power to prevent overheating.
Milli and kilo prefixes
Convert before entering values: 10 mA = 0.01 A, 4.7 kΩ = 4700 Ω, 5 mW = 0.005 W. All inputs must be in base SI units.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it: V = I × R. This holds for ohmic materials (resistors) under constant temperature conditions.
What equations does the calculator use?
The calculator uses: V = IR, I = V/R, R = V/I, and the power equations P = VI, P = I²R, and P = V²/R. It automatically selects the right formula based on which two values you provide.
What are the units?
Voltage (V) is in Volts, Current (I) in Amps (A), Resistance (R) in Ohms (Ω), and Power (P) in Watts (W). Use consistent SI units for accurate results.
Does Ohm's Law apply to all components?
No. Ohm's Law applies only to ohmic (linear) components like resistors. Diodes, capacitors, inductors, and transistors are non-ohmic — their V-I relationship is non-linear.
How do I solve for power if I only know V and R?
Select "P" as the solve-for variable and enter V and R. The calculator uses P = V²/R. Similarly, if you know I and R, it uses P = I²R.
Is my data stored?
No. All calculations run locally in your browser. No data is sent to any server.
How do I remember the Ohm's Law formulas?
Use the Ohm's Law triangle: put V at the top and I and R below it. Cover the quantity you want to find and the remaining two show the formula — cover V and you get I × R, cover I and you get V ÷ R, cover R and you get V ÷ I. For power, remember P = V × I, and the two derived forms P = I²R and P = V²/R let you find power from any pair of the other quantities.
Why is my calculated value a thousand times too big or too small?
Almost always a units problem. Ohm's Law works only in base SI units — volts, amps, ohms, and watts — but components are labelled with prefixes like milliamps and kilohms. If you enter 10 for milliamps instead of 0.01 amps, or 4.7 for a 4.7 kΩ resistor instead of 4700 ohms, your answer will be off by a factor of a thousand. Convert every value to base units before calculating.
Does Ohm's Law work for AC circuits?
For purely resistive AC circuits, yes — V = IR holds using RMS (root-mean-square) values of voltage and current. However, AC circuits that contain capacitors or inductors also have reactance, so the full relationship uses impedance (Z) rather than plain resistance, and voltage and current can be out of phase. This calculator handles the resistive case; for reactive AC circuits you need impedance and phase, which are beyond simple Ohm's Law.
How do I combine resistors in series or parallel first?
Reduce the network to a single equivalent resistance before using the calculator. In series, add the resistances: R_total = R₁ + R₂ + R₃. In parallel, add the reciprocals: 1/R_total = 1/R₁ + 1/R₂ + …, which gives a value smaller than the smallest branch. Once you have the single equivalent resistance, enter it here with your source voltage to find the total current and power.
What power rating should I choose for a resistor?
Calculate the power the resistor will dissipate using P = I²R (or P = V²/R), then choose a resistor rated for at least twice that value to provide a safe margin and avoid overheating. For example, a resistor dissipating 0.1 W is comfortably handled by a standard quarter-watt (0.25 W) part. Under-rating a resistor is a common cause of components running hot or failing, so always check the power as well as the resistance.