What next?
Voltage, current and resistance settle each other. Power comes next.
How to use this calculator
- Fill in any two of voltage, current and resistance.
- Leave the value you want to find blank.
- Read the missing value plus power.
What your result means
Ohm’s law links the three: V = I × R. Enter any two and the third follows, along with power (P = V × I) in watts. It assumes a steady DC resistive circuit — real components change with temperature and frequency.
Why this one is different
Leave any one box empty and that is the one solved for, so the tool does not need a mode switch and cannot be set to the wrong one. Power is reported next to voltage, current and resistance every time, because the part that fails is usually the one dissipating more watts than it was chosen for.
The one equation that powers your whole house
Ohm’s law — voltage equals current times resistance — is the single most useful relationship in electronics. Georg Ohm was ridiculed for it in 1827; today it’s the first thing every engineer and electrician learns.
Know any two of voltage, current and resistance and the third follows instantly — along with power, the number that decides whether a component runs cool or burns out.
How it works
Ohm's law links the three basic quantities in an electrical circuit: voltage (the push), current (the flow) and resistance (the opposition). Know any two and the third follows. Power — how much energy the component uses per second — comes from combining them.
Formula
Example
12 V across a 4 Ω resistor:
P = 12 × 3 = 36 W
Frequently asked questions
Which two values do I enter?＋
Any two of voltage, current and resistance. The calculator solves for the missing one and also shows power.
Does this work for AC circuits?＋
It uses the simple DC form. For AC with reactance you'd use impedance instead of plain resistance.
How do I size a resistor for an LED?＋
Subtract the LED forward voltage from the supply, then divide by the current you want. An LED dropping 2 V at 20 mA on a 9 V supply needs (9 − 2) ÷ 0.02 = 350 Ω, so you would fit the nearest standard value of 330 or 390 Ω. Check the resistor’s power rating too: 7 V × 0.02 A is only 0.14 W, so a quarter-watt part is fine.
How do I calculate power from Ohm's law?＋
Power is voltage times current, which combines with Ohm's law to give P equals I squared R or V squared over R. Those two forms are how you size a resistor's wattage rating.
What happens in series and parallel circuits?＋
Series resistances add, and the same current flows through each. Parallel resistances combine as the reciprocal of the sum of reciprocals, always giving less than the smallest one, with the same voltage across each.
Which components do not obey Ohm's law?＋
Diodes, LEDs, transistors and filament lamps are all non-ohmic: their resistance changes with voltage, current or temperature, so a single R value does not describe them.
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Assumptions & limitations
Ohm's law is exact for ohmic components under steady conditions. Where real circuits depart from it:
- Applies to DC or to the resistive part of an AC circuit. With AC and reactive components you need impedance, not resistance.
- Assumes a linear (ohmic) component. Diodes, LEDs, transistors and lamps have resistance that changes with voltage, current or temperature.
- Resistance rises with temperature in most metals — a filament lamp measures very differently hot and cold.
- Ignores wire resistance, connector resistance and a supply's internal resistance, all of which matter at high current.