Ohm's Law Calculator

Plug a 60-watt light bulb into a standard 120-volt outlet and a single equation tells you everything else about the circuit: it draws 0.5 amps and behaves like a 240-ohm resistor. That equation is Ohm's law, and this calculator runs it both ways - enter any two of voltage, current, resistance or power, and it instantly returns the other two along with the exact formula it used. Whether you are picking a resistor for an LED, checking whether an extension cord can handle a heater, or just studying for an exam, this is the fastest way to get a reliable number.

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.

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.

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.

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.
• Always leave headroom: size resistors, wires, and supplies with margin. A common rule of thumb is to build in roughly 10% extra capacity (and 50-100% on resistor wattage) so nothing runs at its limit.
• Round to real parts: if the math says 147 Ω, you will buy the nearest standard value (150 Ω) - 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?" If you have a different job in mind, a sister tool fits better: use the Square Footage Calculator and Cubic Yard Calculator to measure spaces and materials, the Concrete, Gravel and Mulch Calculators to estimate bulk quantities, and the Paint Calculator to figure out how many gallons you need - and remember the same "order about 10% extra" advice applies whenever you buy materials.