Ohm's Law Calculator

An Ohm's law calculator solves any two of voltage, current, resistance and power for the missing two using V = I × R and P = V × I. Enter 120 V across 240 Ω, for example, and it returns 0.5 A of current and 60 W of power instantly: the numbers behind an ordinary 60-watt incandescent bulb on a wall outlet.

This tool covers the electrical values inside a single load. If you already know the current and want to know how much voltage a long cable run loses on the way to that load, switch to the Voltage Drop Calculator, which factors in wire gauge, length and material. Use Ohm's law here to solve the circuit, then voltage drop to size the feed to it.

A worked example with real numbers

Suppose you know two things about a circuit: the supply is 120 V and the load is 240 Ω. Tick "Voltage" and "Resistance," enter those values, and the calculator works as follows:

• Current: I = V ÷ R = 120 ÷ 240 = 0.5 A
• Power: P = V² ÷ R = (120 × 120) ÷ 240 = 14,400 ÷ 240 = 60 W

So the full picture is 120 V, 0.5 A, 240 Ω and 60 W, exactly the 60-watt bulb from the intro. Start from a different pair and you get the same answer: if you instead knew the bulb is 60 W at 120 V, the calculator would give I = P ÷ V = 0.5 A and R = V² ÷ P = 240 Ω. Any two knowns lock in the whole circuit.

Solved Ohm's law problems (step by step)

These are the exact textbook-style questions people search for, worked out with the formula the calculator applies. Every result is computed; pick the row that matches your two known values.

The formulas

Four equations connect the four quantities. The calculator picks whichever ones fit the pair you entered:

Here V is voltage in volts, I is current in amps, R is resistance in ohms (Ω), and P is power in watts. Every other form (I = V ÷ R, R = V ÷ I, V = √(P × R) and so on) is just algebra on these four. That is why you only ever need two values to start.

Current and power for common voltages and resistances

Look up a resistance down the left and a supply voltage across the top; each cell shows the resulting current and power (P = V² ÷ R) for an ideal resistive load. The 5 V and 12 V columns cover electronics and automotive work; 120 V and 240 V are US household supplies.

Ohms to watts conversion

Ohms alone don't convert to watts: you also need a voltage (or current), because power depends on both. Assuming the whole voltage sits across the resistor, this table gives the current and power at 120 V and at 12 V using P = V² ÷ R.

The 120 V rows on the left make clear why a low-ohm load is dangerous on mains: a 1 Ω or 2 Ω short across a 120 V outlet tries to pass tens of amps and thousands of watts, which is exactly what a fuse or breaker is there to interrupt.

How to measure your two known values

If you are working on a real circuit rather than a textbook problem, here is how to get the two inputs you need with a multimeter:

1. Voltage: set the meter to DC (or AC) volts and place the probes across the component or supply, in parallel, without breaking the circuit.
2. Current: set the meter to amps, then break the circuit and put the meter in series so all the current flows through it. Mind the meter's current limit and fuse.
3. Resistance: power off and isolate the component, then set the meter to ohms and measure across it. Measuring resistance in a live circuit gives wrong readings.
4. Power: you rarely measure watts directly on a small project; instead measure two of the others and let the calculator derive it.

Pick whichever two are easiest and safest to obtain, enter them, and the tool fills in the rest.

Who this calculator is for

• Students and hobbyists checking homework or breadboard projects without rearranging the formula by hand.
• Makers and electronics tinkerers sizing current-limiting resistors for LEDs and picking the right wattage part.
• DIY homeowners sanity-checking whether a device's draw fits a circuit, an extension cord, or a power supply.
• Technicians and apprentices who want a quick, reliable second opinion in the field.
• Anyone curious who sees "120V," "15A," or "1500W" on a label and wants to know what the other numbers are.

Key terms explained

• Voltage (V, volts): the electrical "pressure" or potential difference that pushes current through a circuit.
• Current (I, amps): the rate of flow of electric charge. More current means more heat in the wires and components.
• Resistance (R, ohms / Ω): how strongly a component opposes current. A higher resistance means less current for the same voltage.
• Power (P, watts): the rate at which the circuit converts electrical energy to heat, light, or motion.
• Impedance (Z): the AC equivalent of resistance, which also accounts for capacitors and inductors. Swap R for Z in AC circuits.
• Power factor: in AC circuits, the fraction of apparent power that does real work. A purely resistive load has a power factor of 1.
• Energy (Wh / kWh): power multiplied by time. This is what your utility bills you for.

Three quick scenarios

1. Sizing an LED resistor. A 5 mm LED wants about 20 mA (0.02 A) and drops roughly 2 V; your supply is 5 V. The resistor must drop the remaining 3 V, so R = V ÷ I = 3 ÷ 0.02 = 150 Ω, dissipating P = I² × R = 0.02² × 150 = 0.06 W. A common ¼-watt resistor has plenty of headroom.

2. Checking a space heater on a circuit. A heater rated 1,500 W on 120 V draws I = P ÷ V = 1,500 ÷ 120 = 12.5 A. That is most of a 15 A branch circuit, so you should not run much else on it, and never on a thin extension cord.

3. Finding an unknown resistance. You measure 9 V across a component and 0.03 A through it. R = V ÷ I = 9 ÷ 0.03 = 300 Ω, using P = V × I = 0.27 W of power.

Series and parallel resistors

Real circuits rarely have a single resistor, so before you reach for Ohm's law you often need to combine several into one equivalent resistance, then enter that value here. The rules are short:

• In series (resistors end-to-end, one current path), resistances simply add: R = R₁ + R₂ + R₃ … The same current flows through every part, and the voltage divides between them in proportion to each resistance.
• In parallel (resistors side by side, multiple current paths), the reciprocals add: 1 ÷ R = 1 ÷ R₁ + 1 ÷ R₂ + … The total is always smaller than the smallest individual resistor, and each branch sees the full supply voltage.

For example, two 100 Ω resistors in series make 200 Ω, but the same two in parallel make just 50 Ω. A handy shortcut for exactly two resistors in parallel is R = (R₁ × R₂) ÷ (R₁ + R₂). Once you have the single equivalent resistance, type it into the calculator with your supply voltage and it returns the total current and power the network draws. Then work back to each branch: in parallel, find each branch current with I = V ÷ R using that branch's resistance; in series, find each resistor's voltage with V = I × R using the shared current.

From power to energy and electricity cost

Ohm's law and the power equation give you watts, the instantaneous rate of energy use, but your electricity bill is in kilowatt-hours (kWh), which is power multiplied by time. To turn a calculated wattage into a running cost, use two more steps:

Suppose the calculator tells you a heater draws 1,500 W. Run it 5 hours a day and that is 1,500 ÷ 1000 × 5 = 7.5 kWh per day. At the U.S. average residential price of 18.31 cents per kWh (EIA, July 2026), that is about $1.37 a day, or close to $41 a month. The same chain works for any device: solve its power here from any two known values, multiply by the hours you actually use it, then multiply by your local rate (printed on your utility bill). Energy, not power alone, drives the bill: a 10 W LED left on for 5 hours uses 0.05 kWh, while a 1,500 W hair dryer used for 10 minutes uses 0.25 kWh, five times as much.

What changes the result the most

• Resistance: for a fixed voltage, current and power scale inversely with resistance: halving R doubles both.
• Voltage: power rises with the square of voltage (P = V² ÷ R), so even a small voltage increase sharply raises heat.
• Unit prefixes: entering 2.2 kΩ as "2.2" instead of "2200," or 20 mA as "20" instead of "0.02," throws the answer off by orders of magnitude.
• AC vs. DC: reactive loads (motors, transformers) need impedance and power factor; treating them as purely resistive overstates real power.

Practical tips

• Convert prefixes first: work in plain volts, amps, ohms and watts. 500 mA = 0.5 A; 4.7 kΩ = 4,700 Ω; 2 MΩ = 2,000,000 Ω.
• Leave margin: pick resistors rated well above their calculated dissipation (about twice is a common design practice), and size wires and breakers by the NEC, which uses 125% of a continuous load for branch circuits.
• Round to real parts: if the math says 147 Ω, you will buy the nearest standard value (150 Ω), so recheck the current and power at that value.
• Mind the heat: watts become heat. A part dissipating near its rating gets hot; give it air or pick a bigger one.

Limitations and assumptions

This calculator models an ideal DC circuit with a single, constant, purely resistive load. Keep these caveats in mind:

• It ignores temperature effects: real resistance changes as components warm up.
• It ignores wire, contact and source resistance, which subtract a little voltage in real circuits.
• For AC with reactance (motors, capacitors, inductors), substitute impedance for resistance and apply the power factor.
• It does not size wires, breakers or fuses; that follows electrical code and ampacity tables, not Ohm's law alone.
• Component tolerances (often ±5%) mean the real value drifts slightly from the calculated one.

Related calculators

Ohm's law answers "what are the other electrical values?" Once you know the current, the next question is usually how much voltage you lose over a long cable run, which the Voltage Drop Calculator handles with wire gauge, length and material against the 3% NEC recommendation. The Wire Size Calculator picks a conductor for a load and run length, and the Watts to Amps Calculator converts appliance ratings for AC and DC circuits. For networks of resistors, combine them first with the Parallel Resistor Calculator, split a voltage with the Voltage Divider Calculator, and read a part's value from its bands with the Resistor Color Code Calculator. To price the energy a load uses, finish with the Electricity Cost Calculator.

Sources

• National Institute of Standards and Technology: Special Publication 811, Guide for the Use of the International System of Units (the volt, ampere, ohm and watt, and the prefixes kilo, mega and milli).
• National Fire Protection Association: NFPA 70, National Electrical Code, 2023 edition (adopted in most states; local adoption differs, so check which edition your jurisdiction enforces): 210.19(A) and 210.20(A), conductors and overcurrent protection at 125% of continuous loads.
• U.S. Energy Information Administration: Electric Power Monthly, Table 5.6.A (average residential price 18.31 cents per kWh, July 2026).

Ohm's law and the power equation themselves are exact definitions for an ideal resistive load, so every number above can be reproduced by hand.