Ohm’s Law Calculator
Set any two of voltage, resistance and current and read the third — with the power, the heating in the resistor and the safety headroom shown at the same time, because those are what decide whether the part survives.
Ohm's Playground
Explore wider voltage and resistance ranges, pin multiple R curves to compare them on the V vs I graph, and log measurements in the table below.
No data yet — move the voltage slider.
One law, three rearrangements
Ohm’s Law says the current through a conductor is proportional to the voltage across it: V = I × R. Everything else is that one statement rearranged. Cover the quantity you want on the V-I-R triangle and what remains is the calculation — cover V and you are left with I × R, cover I and you are left with V / R.
Power is the part that burns things
The law tells you the current; it says nothing about whether the resistor survives. That is P = V × I, and because V and I are themselves linked, it can be written as I²R or V²/R — useful when you only know one of them. Power rises with the square of current, so doubling the current quadruples the heat. A resistor passing 100 mA at 100 Ω dissipates 1 W and needs a 2 W part; the same resistor at 200 mA needs 8 W.
Where the law stops applying
Ohm’s Law describes linear (ohmic) elements — ones whose V-I graph is a straight line through the origin. Resistors qualify. Diodes, transistors, lamps and thermistors do not: their resistance changes with operating point, so no single R describes them. On an AC circuit with capacitance or inductance, R is replaced by impedance Z and the same form holds as V = I × Z, but with phase to track as well.
Learn more → Ohm’s Law — Learn
Quick experiments
- Watch power outrun current. Hold the resistance at 6 Ω and take the voltage from 12 V to 24 V. The current doubles from 2 A to 4 A, but the power goes from 24 W to 96 W — four times, not two, because P = V × I and both terms grew.
- Find the short-circuit point. Drag the resistance down towards 0.1 Ω. Current shoots past 100 A and the safety panel turns red. This is what a dead short looks like numerically: the only thing limiting the current is the wiring itself.
- Approach an open circuit. Take the resistance up to 10 kΩ at 12 V. Current falls to about 1.2 mA and the resistor barely warms. Add a decade more and it is microamps — the reason a multimeter's 10 MΩ input barely disturbs the circuit it measures.
- Size a resistor for an LED. A red LED drops about 2 V and wants 20 mA from a 5 V supply, so the resistor must drop 3 V. Set 3 V and adjust R until the current reads 20 mA — you should land on 150 Ω, and the power readout confirms 60 mW, well inside a ⅛ W part.
- Prove the square law on heating. Fix the voltage and halve the resistance. Current doubles and power quadruples. This is why a slightly undersized resistor does not run slightly hot — it runs dramatically hot.
Formula reference
- Ohm's Law
The definition. Everything below is this rearranged.
- Solving for current or resistance
Cover the unknown on the V-I-R triangle.
- Power — three equivalent forms
Pick whichever two quantities you already know.
- Resistor power rating
Derating headroom; more in still air or at high ambient.
- Series resistor for a known drop
The LED resistor calculation.
- Conductance
In siemens. Convenient for parallel branches.
| Symbol | Meaning | Unit |
|---|---|---|
| Voltage across the element | V | |
| Current through the element | A | |
| Resistance | Ω | |
| Power dissipated as heat | W | |
| Conductance, the reciprocal of R | S |
Common mistakes
Applying Ohm's Law to a diode, LED or transistor.
These are non-linear — their V-I curve is not a straight line, so no single R describes them. An LED does not have a resistance; it has a forward voltage. Size the series resistor from the voltage left over after that drop, not from the LED itself.
Calculating current but never checking power.
The law tells you nothing about whether the part survives. A 100 Ω resistor at 30 V passes a harmless-looking 300 mA — and dissipates 9 W, which will destroy a quarter-watt part in seconds. Always compute P = I²R afterwards.
Mixing units — millivolts with amps, or kilohms with milliamps.
The formula is only valid in base units. 5 V across 2 kΩ is 2.5 mA, not 2.5 A. Convert everything to volts, amps and ohms first, or track the prefixes deliberately: V / kΩ gives mA directly.
Choosing a resistor rated for exactly the calculated power.
Ratings assume free air at 25 °C. A resistor dissipating its full rating runs extremely hot and drifts in value. Pick at least double, and more if it is enclosed or near other heat sources.
Assuming resistance is constant as things heat up.
It is not. A tungsten lamp filament measures a few ohms cold and ten times that when glowing, which is why lamps draw a large inrush current at switch-on. Metal-film resistors are stable enough to ignore this; hot components are not.
Frequently asked questions
How do I calculate current from voltage and resistance?
Divide the voltage by the resistance: I equals V over R. Twelve volts across six ohms gives two amps. Keep everything in base units first, because volts divided by kilohms gives milliamps, not amps.
What size resistor do I need for an LED?
Subtract the LED's forward voltage from the supply, then divide by the current you want. A red LED dropping about two volts, run at twenty milliamps from five volts, needs three volts across the resistor, so R equals 3 divided by 0.02, which is 150 ohms.
Does Ohm's Law work for AC circuits?
For a purely resistive load, yes, using RMS voltage and current. Once capacitors or inductors are involved, resistance is replaced by impedance and the relation becomes V equals I times Z, with a phase angle to track as well.
Why does Ohm's Law not apply to a diode or transistor?
They are non-linear. Their current does not rise in proportion to voltage, so their V-I curve is not a straight line and no single resistance describes them. Ohm's Law holds only for linear, or ohmic, elements such as resistors.
What power rating should I choose for a resistor?
Work out P equals I squared times R, then pick a part rated at least twice that. Ratings assume free air at room temperature, so a resistor run at its exact rating gets very hot and drifts in value.
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