Ohm's Law Calculator

What Ohm's Law Says

Ohm's Law is the most useful relationship in electronics, and it says something simple: the current flowing through a conductor is proportional to the voltage across it, and inversely proportional to its resistance. Push harder and more flows; add resistance and less flows.

Georg Ohm published the relationship in 1827 after painstaking experiments with wires of different lengths and thicknesses — work that was dismissed at the time and only recognised years later. The unit of resistance carries his name.

The Formulas

Those six rearrange into twelve equations covering every pair of known values — which is why the calculator above accepts any two inputs. The three power forms are algebraically identical; you pick whichever uses the values you already have.

The Ohm's Law Triangle

The classic memory aid: write V on top, with I and R side by side underneath. Cover the quantity you want and the remaining arrangement is the formula.

A second triangle with P on top and V beside I does the same job for power. Useful for exams; less useful once you have internalised that there is really only one relationship being rearranged.

How to Use This Calculator

1. Enter any two of the four values. Leave the other two blank.
2. Click Calculate. The remaining two are filled in, along with a full breakdown.
3. To change something, edit any field and calculate again. The calculator uses the two fields you most recently typed into, so you do not need to clear the others first.
4. Watch the units. Enter amps, not milliamps — 20 mA is 0.02 A. Getting this wrong by a factor of 1,000 is the most common error.
5. Check the power figure against your components. A resistor that has to dissipate more than its rating will overheat.

Worked Example 1: Finding Voltage

A 12 Ω load draws 2 A. What voltage is across it?

Typical of a 24 V DC system — solar, marine, or light industrial control.

Worked Example 2: Finding Current

A 470 Ω resistor across a 12 V supply. How much current flows?

Worked Example 3: Finding Resistance

A 2,400 W kettle on Australian 230 V mains. What is its element resistance?

Worked Example 4: Finding Power

A circuit at 230 V drawing 10 A.

Run that for one hour and you consume 2.3 kWh. At an indicative 30c per kWh that is about 69 cents — the calculator shows a running-cost estimate on the same basis.

The LED and Resistor Problem

This is the single most common real use of Ohm's Law, and it is worth walking through properly because the intuitive approach gets it wrong.

An LED is not a resistor. It has a roughly fixed forward voltage and will draw destructive current if connected directly across a supply. The resistor's job is to drop the difference.

Subtract the forward voltage before dividing. Using 12 V instead of 9.9 V gives 600 Ω and an under-driven, dim LED. For colour-coded resistor values, our resistor calculator decodes the bands.

Typical Voltages You Will Meet

Useful both as a sanity check on your inputs and as a guide to which side of the Australian licensing line you are working on.

Everything above those last two rows is where hobby electronics lives, and where this calculator does most of its work.

AC vs DC: Where Ohm's Law Needs Care

Ohm's Law holds exactly for DC circuits and for AC circuits whose load is purely resistive — heaters, incandescent lamps, kettle elements. Introduce anything with inductance or capacitance and resistance alone stops describing the opposition to current.

What You Can Legally Do in Australia

This is where an Australian page has to differ from the American ones, because the law does. In the United States much residential electrical work is legal for homeowners. In Australia it is not.

If you are sizing resistors for a 5 V microcontroller project or working out current draw in a 12 V camper setup, this calculator is exactly the right tool and you are well inside the law. If you are calculating figures for anything connected to a switchboard, use it to understand the numbers and then engage a licensed electrician to do the work.

Everyday Applications

• Appliance current draw. Divide the appliance's wattage by 230 to get amps. A 2,000 W heater draws 8.7 A — most of a 10 A outlet on its own.
• Will this circuit trip? Add up the current of everything on one circuit. Australian lighting circuits are commonly 10 A and power circuits 20 A, but only an electrician can tell you what your board actually has.
• Sizing a 12 V system. Camper, caravan and marine work is all extra-low voltage. Current draw at 12 V is roughly twenty times what the same wattage draws at 230 V, which is why 12 V wiring is so much thicker.
• Battery runtime. A 100 Ah battery supplying 5 A lasts roughly 20 hours in theory — less in practice, since usable capacity depends on chemistry and depth of discharge.
• Voltage drop over a long run. Cable has resistance, so current through it loses voltage. Over long 12 V runs this matters quickly — see our voltage drop calculator.
• Running costs. Multiply kilowatts by hours for kWh, then by your tariff. The electricity cost calculator does it across a whole household.

Where Ohm’s Law Gets Used

Every one of these reduces to the same six equations — what changes is which two values you start with.

Electrical Safety

General guidance only — and none of it makes unlicensed mains work legal or advisable.

• Current is what harms you, not voltage. Currents in the tens of milliamps across the chest can be fatal. Because body resistance varies enormously with skin moisture and contact area, there is no safe voltage to be casual about.
• 230 V mains is lethal. Australian mains is more than twice the US voltage, and for the same body resistance that means more than twice the current.
• Capacitors hold charge after power is removed. Equipment can remain dangerous long after being unplugged — microwaves and CRT displays notoriously so.
• Treat every circuit as live until proven otherwise, with a tester you have verified works.
• RCDs protect people; fuses and breakers protect wiring. They are different jobs, and having one does not substitute for the other.
• Respect component ratings. Overheating resistors, overloaded wiring and undersized components cause fires in low-voltage systems too — a 12 V circuit can absolutely start one.
• If in doubt, stop. A licensed electrician's callout fee is trivial against the alternatives.

Common Mistakes

1. Milliamps entered as amps. 20 mA is 0.02 A. This factor-of-1,000 slip is by far the most frequent error, and it produces answers that look plausible.
2. Ignoring the power rating. Getting the resistance right is half the job. If the component cannot dissipate the wattage, it overheats regardless.
3. Forgetting an LED's forward voltage. Divide the supply voltage by target current instead of the difference, and your LED runs dim.
4. Applying plain Ohm's Law to motors and transformers. Reactive AC loads need impedance and power factor. V × I gives apparent power, not real power.
5. Assuming resistance is constant. It is not. Incandescent filaments and heating elements rise substantially in resistance as they heat, which is why they draw a large inrush current when first switched on.
6. Mixing up series and parallel. Resistances add in series. In parallel the reciprocals add, so the total is always less than the smallest individual resistor.
7. Treating a calculation as authorisation. Working out the figures for a mains circuit is legal. Doing the work without a licence is not.

One Relationship, Endless Uses

Almost everything practical in electrical work comes back to V = IR and its power companions. Get comfortable with those six equations and you can size a resistor, sanity-check an appliance's current draw, or work out why something is running hot.

From here, decode resistor bands with the resistor calculator, check losses over long cable runs with the voltage drop calculator, and price consumption with the electricity cost calculator.