Free Ohm's law calculator
Enter any two of voltage, current, resistance, or power and get the other two from Ohm's law (V = I × R) and P = V × I — with the full power-wheel breakdown, updated live, as you type.
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Ohm's law applies to resistive loads at DC. Results may differ for reactive loads (inductors, capacitors).
Results are estimates. Consult a professional.
What Ohm's law is and what this calculator does
Ohm's law is the rule that ties together the three core quantities of an electrical circuit: voltage (V), current (I) and resistance (R). It states that the current through a resistor is proportional to the voltage across it — written V = I × R. Add the power equation P = V × I and you can describe almost any simple DC circuit with four numbers. This calculator solves for whichever values you do not know: enter any two of voltage, current, resistance or power, and it returns the other two instantly.
It is the first tool most people reach for when sizing a resistor, checking whether a wire can carry a load, or working out how much power a component will dissipate. Because the four quantities are locked together, knowing any two fixes the rest — there is no ambiguity once two are pinned down.
How the Ohm's law calculator works
You supply any two of the four quantities. The calculator picks the matching relationship, solves for the remaining two, and shows all four together so you can read off voltage, current, resistance and power at a glance. Everything is computed from two starting equations and their rearrangements.
Voltage, current, resistance and power — the inputs explained
Each input is one of the four quantities Ohm's law links. Enter any two and leave the others blank; the calculator fills them in. It helps to know what each one measures and the unit it is expressed in.
Voltage (V) — the push
Voltage is the electrical "pressure" or potential difference that drives current through the circuit, measured in volts (V). A higher voltage across the same resistance pushes more current.
Current (I) — the flow
Current is the rate of charge flow, measured in amperes, or amps (A). Small-signal electronics often works in milliamps (mA), where 1000 mA = 1 A — a frequent source of errors covered below.
Resistance (R) — the opposition
Resistance is how strongly a component opposes current, measured in ohms (Ω). For an ideal resistor it is a fixed number; larger resistance means less current for the same voltage.
Power (P) — the work rate
Power is the rate at which the circuit converts electrical energy to heat, light or motion, measured in watts (W). It is not an independent input — it follows from V and I — but you can enter it as one of your two knowns and let the calculator work backwards to the rest.
A worked example using the Ohm's law calculator
Sam has a 12 V supply and wants to drop it across a 100 Ω resistor. Before powering it up, Sam needs to know how much current will flow and how much heat the resistor must handle — so the right wattage part can be chosen.
Step 1 — Find the current with I = V ÷ R
I = 12 V ÷ 100 Ω = 0.12 A, which is the same as 120 mA. Watching the units here matters: 0.12 A and 120 mA are one and the same current.
Step 2 — Find the power with P = V × I
P = 12 V × 0.12 A = 1.44 W. The two power-wheel forms agree: P = I²R = 0.12² × 100 = 1.44 W, and P = V²/R = 144 ÷ 100 = 1.44 W.
The Ohm's law power wheel — every rearranged formula
Because V, I, R and P are interlocked, each one can be written from any two of the others. This is often drawn as the "Ohm's law power wheel". The table lists every rearrangement, so whatever pair you start with, the formula you need is here.
| Solve for | From V and I | From V and R | From I and R | From P and one other |
|---|---|---|---|---|
| Voltage V | — | V = √(P × R) | V = I × R | V = P / I |
| 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 | — |
All twelve forms come from V = I×R and P = V×I. The calculator selects the correct one automatically based on which two values you enter.
Ohmic vs non-ohmic devices: why Ohm's law fails on diodes and LEDs
Ohm's law is exact for ohmic devices — components whose resistance stays constant regardless of the voltage applied. Plain resistors, and metal wires at a steady temperature, are ohmic: double the voltage and the current doubles, so a plot of current against voltage is a straight line through the origin.
Many components are non-ohmic: their current-versus-voltage curve bends, so a single fixed R does not describe them. Diodes and LEDs are the classic example — below their forward voltage almost no current flows, and just above it the current rises steeply, so you cannot pick one resistance value for them. That is why you never connect an LED straight to a supply. Instead you add a series resistor and size it with Ohm's law applied to the resistor alone: subtract the LED's forward voltage from the supply, then divide by the target current. Our LED resistor calculator does exactly that.
- Ohmic (Ohm's law applies directly): fixed resistors, heating elements, and wire at constant temperature.
- Non-ohmic (apply Ohm's law only to the surrounding resistors): diodes, LEDs, transistors, lamps with a heating filament, and thermistors.
- AC with reactance: capacitors and inductors oppose alternating current through reactance, not plain resistance — use impedance Z, where V = I × Z.
Where a circuit is built from several resistors, combine them first with the series and parallel resistor calculator, then apply Ohm's law to the single equivalent resistance. To split a voltage with two resistors, the voltage divider calculator uses the same V = I×R relationship.
Common Ohm's law mistakes
- Mixing milliamps and amps. The single most common error. In the worked example above, 120 mA is 0.12 A — feeding 120 into a formula that expects amps makes the answer 1000× too large. Convert milliamps to amps (÷1000) and milliohms or kilohms to ohms before you calculate.
- Using Ohm's law on an AC reactive circuit. V = I×R holds for resistance only. With capacitors or inductors on AC, the opposition is impedance Z and varies with frequency, so use V = I×Z instead.
- Applying one R to a diode or LED. These are non-ohmic, as above — model the resistor in the circuit, not the diode itself.
- Ignoring the resistor's power rating. Ohm's law gives the current, but P = I²R tells you the heat. A 1/4 W resistor asked to dissipate 1.44 W will burn out — always check the wattage, not just the resistance.
- Forgetting temperature. Real resistance rises with temperature in most metals, so a circuit measured cold will draw slightly less current once it warms up.
How accurate is this Ohm's law calculator
The arithmetic is exact. For the two values you enter, V = I×R and P = V×I are solved to full floating-point precision, so the calculator itself introduces no error. The difference you see on the bench comes from the physical parts, not the math.
Real resistors carry a tolerance band — commonly ±5%, ±1%, or ±0.1% for precision parts — so a "100 Ω" resistor may actually be anywhere from 95 to 105 Ω, and the measured current shifts to match. Resistance also drifts with temperature, supply voltages sag under load, and meter leads add a fraction of an ohm. Treat the calculated figure as the design value, then confirm with a multimeter on the assembled circuit. The result is only as accurate as the resistance and voltage you put in.
Ohm's law definitions
Frequently asked questions about the free Ohm's law calculator
About this Ohm's law calculator
This Ohm's law calculator runs entirely in your browser — nothing you enter is sent anywhere. Enter any two of voltage, current, resistance, or power and it solves the remaining two from V = I × R and P = V × I, recalculating instantly as you type.
It pairs with the series and parallel resistor calculator, the LED resistor calculator, and the voltage divider calculator. Browse the full set of free calculators for more electronics tools.