⚡ Ohms Law & Electrical Power Calculator
Enter any two of voltage, current, resistance or power and get the other two — plus the resistor wattage rating you need so the part does not burn out.
Fill in any two of the four and the other two are solved. Clear a field to swap which pair you are working from.
24.0000 W is beyond ordinary through-hole resistors. Common ratings stop around 5 W, and at this level you need a wirewound or chassis-mount power resistor with a heatsink — not something from a hobby parts drawer.
What Ohms Law & Electrical Power Calculator Does
Ohm's law is one relationship between three quantities: the voltage across a conductor, the current through it, and its resistance. V = I × R, and rearranged, I = V ÷ R and R = V ÷ I. Power is a second relationship laid over the top, P = V × I, and combining the two produces the familiar variants P = I²R and P = V² ÷ R.
Those four quantities are fully determined by any two of them, which is why every calculator for this term asks for two values and fills in the rest. There are six possible pairs. The one that catches tools out is resistance and power together, because solving it needs square roots: V = √(PR) and I = √(P ÷ R).
The formulas themselves are no longer worth ranking for — Google prints them directly above the results. What the search engine does not tell you is the part that actually matters when you are building something: whether the component can survive what you have just asked of it.
A resistor turns the power it dissipates into heat. Twelve volts across a 6 ohm resistor is 24 watts, which will destroy anything you are likely to have in a parts drawer. The same 12 volts across 1.2 kilohms is 0.12 watts, entirely comfortable for a standard quarter-watt part. The arithmetic is trivial; noticing that you need to do it is the useful bit.
How to Use Ohms Law & Electrical Power Calculator
- Enter electrical Voltage in Volts (V) and Current in Amperes (I)
- Review calculated Resistance in Ohms (Ω)
- Inspect power dissipation in Watts and hourly energy consumption (kWh)
Formula Used by Ohms Law & Electrical Power Calculator
The core relationships
V = I × R · P = V × I · P = I²R · P = V² ÷ R
- V
- Voltage across the component, in volts
- I
- Current through it, in amperes
- R
- Resistance, in ohms
- P
- Power dissipated as heat, in watts
Worked example
12 volts across a 6 ohm resistor
- I = V ÷ R = 12 ÷ 6 = 2 A
- P = V × I = 12 × 2 = 24 W
- Cross-check: P = V² ÷ R = 144 ÷ 6 = 24 W
Result: 2 amps and 24 watts — far beyond any standard through-hole resistor
Solving from resistance and power (the pair most tools omit)
V = √(P × R) · I = √(P ÷ R)
- √
- Square root — needed because power depends on the square of both voltage and current
Worked example
A 6 ohm load dissipating 24 watts
- V = √(24 × 6) = √144 = 12 V
- I = √(24 ÷ 6) = √4 = 2 A
Result: 12 volts and 2 amps — the same circuit, reached from the other direction
All six input pairs
Any two of the four quantities determine the other two. Verified against a reference case of V = 12 V, I = 2 A, R = 6 Ω, P = 24 W.
| You know | Solve the third | Solve the fourth |
|---|---|---|
| V and I | R = V ÷ I | P = V × I |
| V and R | I = V ÷ R | P = V² ÷ R |
| V and P | I = P ÷ V | R = V² ÷ P |
| I and R | V = I × R | P = I²R |
| I and P | V = P ÷ I | R = P ÷ I² |
| R and P | V = √(PR) | I = √(P ÷ R) |
Choosing a resistor that survives
Standard through-hole power ratings, applying the common rule of running a part at no more than half its nameplate figure.
| Power dissipated | Minimum nameplate (2× derating) | Standard part to use |
|---|---|---|
| 0.05 W | 0.1 W | ⅛ W (0.125 W) |
| 0.1 W | 0.2 W | ¼ W (0.25 W) |
| 0.2 W | 0.4 W | ½ W (0.5 W) |
| 0.4 W | 0.8 W | 1 W |
| 1 W | 2 W | 2 W |
| 3 W and above | 6 W and above | Beyond ordinary parts — wirewound or chassis-mount with a heatsink |
Related circuits this calculator does not cover
Ohm's law describes one component. These are the cases people arrive looking for, and they need different arithmetic.
| Case | What changes |
|---|---|
| Resistors in series | Resistances add: R = R₁ + R₂ + … The same current flows through each |
| Resistors in parallel | Reciprocals add: 1÷R = 1÷R₁ + 1÷R₂ + … The same voltage sits across each |
| Three-phase AC | A √3 factor enters the line-to-phase relationships, and real power needs a power factor: P = √3 × V × I × cos φ |
| AC in general | Resistance becomes impedance, which includes reactance from capacitance and inductance and varies with frequency |
How to Read Your Result
Power is the quantity that destroys things
Voltage and current tell you what the circuit is doing; power tells you whether the parts will live through it. Because power depends on the square of the current, doubling the current quadruples the heat. That non-linearity is why a component can sit happily for months and then fail quickly after a modest change — going from 100 mA to 200 mA through the same resistor does not double its temperature rise, it quadruples the power it has to shed.
Why the 50% derating rule exists
A resistor's power rating is the maximum it can dissipate in free air at a specified ambient temperature, usually with generous assumptions. Real circuits sit inside enclosures, next to other warm components, in summer. Running a part at half its nameplate rating buys margin against all of that, keeps the resistance from drifting as the part heats, and dramatically extends its life. It costs almost nothing, since the next size up is typically a few cents.
Where Ohm's law stops applying
The law describes ohmic conductors — components whose resistance stays put as voltage changes. Plenty of things do not. An LED has no fixed resistance worth the name; it drops a roughly constant voltage and its current rises steeply once that is exceeded, which is exactly why it needs a series resistor sized on the voltage across the resistor rather than across the LED. Diodes, transistors and filament lamps are all non-ohmic too, and even ordinary resistors shift with temperature.
Sub-ohm loads and battery current
A common question concerns very low resistances, where the numbers become unforgiving quickly. At 4 volts, a 0.5 ohm coil draws 8 amps and dissipates 32 watts; halving the resistance to 0.25 ohms doubles the current to 16 amps and doubles the power to 64 watts. The current figure is the one that matters, because it has to sit inside the continuous discharge rating of the cell supplying it. Ohm's law tells you the demand; only the cell's datasheet tells you whether it can safely meet it.
Series and parallel behave oppositely
In series, resistances add and the same current flows through everything, so the largest resistor drops the most voltage. In parallel, the same voltage sits across every branch and the reciprocals add, which means total resistance is always lower than the smallest single branch. That last point surprises people: adding another parallel path always makes the overall resistance fall, and therefore makes the total current rise.
Limitations & Accuracy Notes
- This calculator handles a single ohmic component under DC conditions. Series and parallel networks, and anything with capacitance or inductance, need the additional treatment described above.
- Ohm's law does not apply to non-ohmic components — LEDs, diodes, transistors, filament lamps. Computing a "resistance" for them from a single voltage and current gives a number that is valid only at that exact operating point.
- Real resistance changes with temperature, and for precision work the temperature coefficient matters. The values here assume a constant resistance.
- For alternating current, resistance is replaced by impedance, and real power depends on the power factor. Three-phase systems add a √3 factor that this tool does not apply.
- Power-rating guidance is a general engineering rule of thumb, not a substitute for a component datasheet. Airflow, ambient temperature, mounting and duty cycle all change what a given part can safely dissipate.
Frequently Asked Questions
What is Ohm's Law?
Which two values do I need to enter?
What wattage resistor do I need?
Does Ohm's law apply to everything?
How do I convert ohms to volts?
Which two values do I need to solve for the rest?
Does it work for AC circuits?
Why does an LED need a resistor?
How do I work out the power rating a resistor needs?
Does wire resistance matter?
Is my data stored?
References & Further Reading
- NIST — SI units: electric current, the ampere — The definition of the ampere underpinning the units used throughout, from the US National Institute of Standards and Technology
- NIST Guide to the SI — Units of electricity — Reference for the volt, ohm and watt and their correct usage and symbols