Why use this calculator
Ohm's law is the first relationship anyone learns in electronics and the one most often applied half way. People work out the current, fit the resistor and then discover it is scorching the board, because the number that decides the part is the power, not the current.
This calculator gives all four quantities at once and converts the current into milliamps, which is the unit component data sheets actually use. It also suggests a power rating at twice the calculated dissipation, the margin a bench technician applies by habit so a part is not run at its absolute limit in a warm enclosure.
Getting these four numbers right in one pass is the difference between a circuit that works for years and one that browns the board around a resistor.
How to use it
- Choose which quantity you want solved and the two inputs it needs will appear.
- Take the voltage from the supply rail or a meter reading across the component rather than from the battery label, because a loaded battery sags below its nominal figure.
- Enter the current in amps, not milliamps, so a 20 mA LED is typed as 0.02; this is the single most common data-entry mistake with this calculation.
- Enter the resistance in ohms, so a 4.7 kΩ part is 4700 and a 1.2 MΩ part is 1200000.
- Read the power result before you order anything, then compare it with the suggested rating to check the resistor you have in mind can shed the heat.
What each result means
- Voltage (V)
- The potential difference across the component, in volts, driving the current through it.
- Current (I)
- The current flowing through the component in amperes, which is what heats conductors and trips protection.
- Current in milliamps
- The same current in milliamps, the unit component data sheets and LED ratings normally use.
- Resistance (R)
- The opposition to current flow in ohms, assumed constant at the working temperature.
- Power (P)
- Heat dissipated in the component each second, in watts. This decides the physical size of the part.
- Energy if it runs all day
- Kilowatt-hours this load would consume over twenty-four hours of continuous operation, ready to price against a tariff.
- Resistor power rating to fit
- The next standard wattage above twice the dissipation, giving a safe thermal margin.
The formula
Ohm's law
V = I × R
Rearranged as
I = V / R
R = V / I
Voltage V is in volts (V), current I in amperes (A) and resistance R in ohms (Ω).
Power follows from
P = V × I
Which combines with Ohm's law to give the two forms worth memorising:
P = I² × R
P = V² / R, both in watts (W)
Energy is power multiplied by time, so W × hours / 1,000 gives kilowatt-hours (kWh), the unit an electricity bill is measured in. These relationships hold for ohmic conductors at a steady temperature and for DC or RMS values in AC resistive circuits.
How this number is used in practice
Choosing a series resistor for an LED is the textbook job. A 20 mA LED with a 2 V forward drop on a 12 V rail needs to lose 10 V, so the resistor is 10 / 0.02 = 500 Ω, and the nearest standard value is 510 Ω.
The part then dissipates 0.2 W, which immediately rules out a common 0.125 W resistor and points at a 0.5 W one. An electrician sizing a heating element works the identical arithmetic in reverse: a 2,400 W element on a 240 V supply draws 10 A and measures about 24 Ω cold, so a reading far from that figure means the element is failing.
On site the same relationship explains voltage drop. A long run of cable has resistance of its own, so the current through it drops volts before the load ever sees them, which is why installation standards cap the permitted drop at a few per cent of the nominal supply.
Derating matters just as much: a resistor rated at 0.25 W is rated in free air at room temperature, and inside a sealed enclosure at 60 °C it will not manage anything like that. Treat the calculated power as the minimum and fit a part rated well above it.
Above about 50 volts, and at any voltage where a fault can push serious current, the work belongs to a licensed electrician and must be isolated, locked off and proved dead before anything is touched.
Worked examples
A 12 V supply across a 100 Ω resistor
Current of 0.1200 A (120.00 mA) and 1.440 W of heat to get rid of.
An LED dropper: 5 V rail through 220 Ω
22.73 mA flowing and only 0.114 W dissipated, so a 0.25 W resistor is comfortable.
A 230 V appliance drawing 10 A
An effective resistance of 23.000 Ω and 2,300.000 W of power.
When people use it
- Sizing a series resistor for an LED or a sensor bias network
- Checking the power rating a resistor needs before ordering the part
- Working back to an appliance's effective resistance from its rating plate
- Estimating the running cost of a resistive load left on continuously
- Diagnosing a faulty heating element by comparing measured and expected resistance
Tips
- Enter current in amps, not milliamps. A 20 mA LED is 0.02 A, and typing 20 gives an answer a thousand times too large.
- Power decides the component size, not current. Always check the watts before choosing a resistor package.
- Derate resistors to at least half their rated power in still air, and further inside a sealed or hot enclosure.
- P = V² / R and P = I² × R save a step when you already know two of the quantities and want the heat directly.
- For AC circuits use RMS values, and remember that reactive loads draw volt-amps that do not all turn into watts.
- Above roughly 50 V, and on any mains circuit, isolate, lock off and prove dead before measuring. Licensed work needs a licensed person.
What it assumes, and where it stops being accurate
- The conductor is ohmic, meaning resistance stays constant regardless of the applied voltage.
- Resistance is taken at the working temperature. Copper and tungsten rise substantially in resistance when hot, so a cold measurement understates it.
- DC or purely resistive AC is assumed. Inductance, capacitance and power factor are not modelled.
- Semiconductor junctions such as LEDs and diodes are not ohmic; subtract the forward voltage drop before applying this to a series resistor.
- Wiring, connector and source resistance are ignored, so a real circuit draws slightly less current than the ideal figure.
Terms used on this page
- Ohm (Ω)
- The SI unit of resistance: one volt across it drives exactly one ampere through it.
- Power factor
- The ratio of real power in watts to apparent power in volt-amps for an AC load.
- Derating
- Operating a component below its maximum rating to allow for heat, altitude or ageing.
- Voltage drop
- The loss of potential along a cable caused by the cable's own resistance.
Questions people ask
- What exactly does Ohm's law say?
- That the current through a conductor between two points is proportional to the voltage across it and inversely proportional to its resistance, written V = I × R. Double the voltage and the current doubles; double the resistance and the current halves. It applies to ohmic materials at a steady temperature, which covers ordinary resistors and metal conductors.
- How do I pick the resistor for an LED?
- Subtract the LED's forward voltage from the supply, then divide by the current you want. A 2 V LED at 20 mA on a 12 V rail needs (12 − 2) / 0.02 = 500 Ω, so fit the nearest standard 510 Ω. Then check the power: 10 V across 0.02 A is 0.2 W, which needs a 0.5 W part.
- Why does my resistor get so hot?
- Because it is dissipating more power than its package can shed. Every watt becomes heat, and a small surface-mount or quarter-watt part has very little surface area to lose it from. Work out P = V × I, then fit a resistor rated at roughly twice that figure, more if it sits in a sealed or already warm enclosure.
- Does Ohm's law work for AC?
- For purely resistive AC loads such as heaters and incandescent lamps, yes, provided you use RMS voltage and current. Once inductance or capacitance is involved, current and voltage fall out of step and you need impedance rather than resistance, with real power found from volt-amps multiplied by the power factor.
- What is the difference between watts and volt-amps?
- Watts are real power, the energy actually converted into heat or work. Volt-amps are apparent power, simply voltage multiplied by current. For a resistive load they are equal. For motors, transformers and switch-mode supplies the current lags or leads the voltage, so volt-amps exceed watts and the ratio is the power factor.
- Why is the voltage at my load lower than at the supply?
- Because the cable has resistance too, and the current flowing through it drops some voltage along the way. The longer and thinner the run, the larger the drop. Installation standards limit the permitted drop to a few per cent of nominal, which is usually met by increasing the conductor size rather than by anything at the load.
Use this with
Ohm's Law Calculator answers one part of the question. These pick up where it stops: Electrical Load Calculator, Power Bill Calculator, Energy Calculator and Solar Panel Savings Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- OSHA: electrical safety. Shock and arc-flash hazards, and safe working practice
- International Electrotechnical Commission. International standards for electrical installations and equipment
- UK Health and Safety Executive: electricity at work. Legal duties and safe systems of work around electricity
- NIST: International System of Units (SI). Base units, derived units and the SI definitions
Ohm's Law Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.