Ohm's Law Calculator
Fill in any two of voltage, current, resistance and power and get the other two — plus series and parallel networks, resistor colour bands, and the LED dropping resistor.
Instant answers
Works offline once loaded
Nothing you type is sent anywhere
Any two of V, I, R, P
Series and parallel
Colour-band decoder
Fill in any two — the other two are calculated
Clear a box to make it a result. The two most recently edited boxes are treated as the inputs.
Circuit
—
Power dissipated—
Energy in one hour—
Heat produced—
Suggested resistor rating—
Nearest E12 resistor value—
Nearest E24 resistor value—
Resistor colour bands
Resistor network
LED series resistor
Resistor needed—
Use this standard value—
Power in the resistor—
The twelve Ohm's law formulas
| To find | From V and I | From V and R | From I and R | With power |
|---|---|---|---|---|
| Voltage (V) | — | — | V = I × R | V = P / I · V = √(P × R) |
| Current (I) | — | I = V / R | — | I = P / V · I = √(P / R) |
| Resistance (R) | R = V / I | — | — | R = V² / P · R = P / I² |
| Power (P) | P = V × I | P = V² / R | P = I² × R | — |
Instructions
How to use this calculator
Step by step
- Type any two of voltage, current, resistance and power. The other two fill themselves in and are highlighted so you can always see which numbers you supplied and which the calculator worked out.
- To change which two are the inputs, just edit a different box — the two most recently edited fields become the inputs and the others become results.
- Read the summary line for all four quantities in engineering notation, which is how components are actually labelled: 4.7 kΩ rather than 4700 Ω.
- Check the suggested resistor rating. Standard practice is to fit a part rated for at least twice the power it will dissipate, because a resistor running at its rated power gets very hot.
- Use the E12 and E24 rows to find a resistor you can actually buy. Real resistors only come in fixed value series, so the exact number rarely exists.
- The colour band panel shows what the resistance looks like on a real part, with the tolerance range underneath — worth checking before you assume a measured value is wrong.
- For a network, add resistors and switch between series and parallel. For an LED, enter the supply voltage and pick the colour: the forward voltage differs enough between red and blue to matter.
Good to know
- Power goes as the square of current, so doubling the current quadruples the heat. This is why a marginal connection with a little extra resistance can get hot enough to start a fire.
- Always derate. A quarter-watt resistor dissipating a quarter of a watt will be too hot to touch and will drift in value. Fit the next size up.
- An LED without a series resistor will draw whatever current the supply can deliver and destroy itself, usually within seconds. The forward voltage is not a resistance.
- Two identical resistors in parallel give exactly half the value; in series exactly double. This is a genuinely useful way to hit a value you do not have in the drawer.
- Wire has resistance too. Over a long run at high current, the voltage drop in the cable can be a significant fraction of the supply — worth calculating before you wonder why the far end is dim.
- Ohm's law describes ohmic devices. Diodes, LEDs, transistors and lamps are not ohmic — their resistance changes with the operating point, so you cannot simply divide voltage by current and use the answer elsewhere.
The maths behind it
- Ohm's law V = I × R The definition of resistance for an ohmic conductor.
- Power P = V × I Watts, the rate at which energy is converted.
- Power from resistance P = I²R = V²/R Substituting Ohm's law into P = VI.
- Series resistance R = R₁ + R₂ + R₃ … Same current through each; voltages add.
- Parallel resistance 1/R = 1/R₁ + 1/R₂ + … Same voltage across each; currents add.
- LED series resistor R = (V_supply − V_forward) / I_forward The resistor takes whatever voltage the LED does not.
- Energy E = P × t Joules if t is in seconds; kilowatt-hours if you divide by 3.6 million.
Which two values should I enter?
Whichever two you actually know. If you have a power supply and a resistor, enter voltage and resistance. If you are measuring a running circuit, enter voltage and current. If you are choosing a resistor for a known load, enter power and voltage. Every pair works and gives the same physics.
Why does my resistor value not exist?
Because manufactured resistors come in fixed series — E12 has twelve values per decade, E24 twenty-four — chosen so that their tolerance bands just about cover the gaps. The calculator shows you the nearest available value in both series and how far off it is. In most circuits a few percent makes no difference.
What power rating do I need?
At minimum, more than the calculated dissipation; in practice, at least double it. A resistor at its rated power runs at well over 100 °C, drifts in value, and discolours the board. Doubling costs pennies and is what the suggested rating on this page gives you.
How do I read resistor colour bands?
Hold the resistor with the tolerance band — gold or silver, and usually separated by a wider gap — on the right. The first two bands (three on a five-band part) are digits, the next is the power of ten to multiply by. Brown-black-red-gold is 1, 0, ×100, ±5% — a 1 kΩ resistor.
Why does an LED need a resistor at all?
Because a diode is not ohmic. Above its forward voltage the current rises almost vertically with voltage, so a small excess causes a huge current. The resistor turns that into a well-behaved circuit where the current is set by a component that obeys Ohm's law. Without it, the LED sets its own current and the answer is "as much as it takes to destroy itself".
Does Ohm's law apply to AC?
In the form V = IR it applies to resistances at any frequency. For capacitors and inductors you need impedance instead, which has a phase as well as a magnitude, and power becomes the product of voltage, current and the power factor. This page handles the DC and purely resistive case.
What is a safe voltage to work with?
Below about 30 V DC, contact with dry skin is generally not dangerous. Above 50 V it can be. Mains voltage — 120 V or 230 V — is lethal, and the danger is the current through your chest rather than the voltage itself. If you are unsure, do not work on it live.