Ohm’s Law Calculator
Know two electrical quantities and find the third, plus the power dissipated. The tool converts the prefixes (milli, kilo, mega) for you.
Educational use: results are estimates made for study and reference. Checking the answer and deciding what to do with it is the sole responsibility of whoever uses the tool. Read the legal notice.
On this page
- How to use the calculator
- Ohm’s law and the power formula
- The simplest circuit in the world
- Safety first
- Worked examples
- Reference tables
- Common mistakes
- Questions from people just starting in electronics
- Georg Ohm and the cold reception
- Where this calculation is useful
- A few curiosities
- Basic electrical glossary
- More here
How to use the calculator
- Choose what you are looking for: voltage, current or resistance.
- Enter the other two quantities and pick the right prefix in the selector next to each one. For example: 20 mA, 220 Ω or 2.2 kΩ.
- Calculate. The result appears with the most suitable prefix (mA, kΩ...), along with the power dissipated and the step-by-step solution.
Why do prefixes matter so much? Because the most common mistake in this calculation is forgetting that 20 mA is 0.020 A. The calculator converts everything to the base units (volt, ampere and ohm), does the math and, at the end, gives you back a readable result.
Ohm’s law and the power formula
Solving for each quantity:
With Ohm’s law, electrical power can be written in three equivalent ways. The calculator uses the first, but the others are handy depending on the data you have:
How to think about it
A classic analogy is water in pipes. Voltage is the "pressure" pushing the water, current is the amount of water flowing per second, and resistance is the narrowing of the pipe. With the same pressure, a narrower pipe lets less water through. In the same way, current decreases when resistance increases.
In terms of proportion
- If you double the voltage (with the same resistance), the current doubles.
- If you double the resistance (with the same voltage), the current drops to half.
- If you double the current (with the same resistance), the power dissipated quadruples, because it grows with I².
The simplest circuit in the world
A source (cell or battery), a resistor and wires forming a closed path. The source voltage pushes the current, and the resistor decides how much current flows.
Safety first
This tool is purely educational. It is not a basis for residential, commercial or industrial electrical installations, nor for any work on power grids (outlets, electric showers, panels, voltages of 127 V, 220 V or higher). That work involves risk of shock, burns and fire and must be done by qualified professionals, following the applicable technical standards.
The results assume ideal components, exact values and constant temperature. In real components, tolerance (for example, ±5%), temperature and wear change the numbers. In electronics projects, confirm with measurements and with the components’ datasheets.
Bottom line: this tool is educational. Results do not replace your own checking, and how you use them is entirely your responsibility. More details in the legal notice.
Worked examples
1. The flashlight
A flashlight has two batteries adding up to 3 V, and the bulb has a resistance of 15 Ω. What is the current?
2. Resistor for an LED
An LED works at about 2 V and 20 mA, and the supply is 9 V. Which resistor should you put in series?
350 Ω does not exist in the common commercial series. You pick the nearest value just above it, such as 390 Ω (E12 series), which gives I = 7 ÷ 390 ≈ 17.9 mA: the LED is a little less bright, but protected.
3. How hot does a resistor get?
A 220 Ω resistor connected to 5 V.
An ordinary 1/4 W (0.25 W) resistor handles this power with room to spare.
4. Calculating the voltage
A current of 2 A flows through a resistance of 5 Ω.
5. Finding the resistance
A component measures 12 V when 20 mA flows through it.
Reference tables
Most common prefixes
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| mega | M | 1,000,000 | 1 MΩ = 1,000,000 Ω |
| kilo | k | 1,000 | 2.2 kΩ = 2,200 Ω |
| (unit) | – | 1 | 12 V |
| milli | m | 0.001 | 20 mA = 0.020 A |
| micro | µ | 0.000001 | 500 µA = 0.0005 A |
Resistor color code (the digits)
| Color | Digit | Color | Digit |
|---|---|---|---|
| Black | 0 | Green | 5 |
| Brown | 1 | Blue | 6 |
| Red | 2 | Violet | 7 |
| Orange | 3 | Gray | 8 |
| Yellow | 4 | White | 9 |
On four-band resistors, the first two bands give the digits, the third gives the multiplier (power of 10) and the fourth gives the tolerance. Brown-black-red, for example, is 1, 0 and × 100: 1,000 Ω, or 1 kΩ.
Common commercial values (E12 series)
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82, multiplied by 1, 10, 100, 1,000 and so on. That is where values like 330 Ω, 470 Ω, 1 kΩ and 4.7 kΩ come from.
Common mistakes
Forgetting to convert milli and kilo
20 mA is 0.02 A, not 20 A. A calculation with the wrong prefix changes the result by a factor of a thousand.
Applying Ohm’s law to everything
It holds for "ohmic" components, such as ordinary resistors. LEDs, diodes and filament bulbs do not follow a linear relationship between voltage and current, which is why the LED example uses a typical voltage for the component.
Using the source voltage instead of the voltage across the resistor
In circuits with more than one component, the voltage is divided among them. For the LED resistor, what counts is 9 V minus 2 V, not the full 9 V.
Ignoring power
A resistor can have the right value and still burn out because it cannot handle the power. Calculate P and choose a component with a safety margin, generally double what is needed.
Mixing direct and alternating current
The form V = I × R applies directly to direct current and, for alternating current, to purely resistive loads using RMS values. With motors, capacitors and inductors, other concepts come into play.
Questions from people just starting in electronics
What does Ohm’s law say?
In an ohmic conductor, voltage equals current times resistance: V = I × R.
How do I calculate current from voltage and resistance?
Divide the voltage by the resistance: I = V ÷ R. With 12 V and 2.2 kΩ, the current is about 5.45 mA.
How do I calculate the resistor for an LED?
Subtract the LED voltage from the supply voltage and divide by the desired current: R = (Vsupply − Vled) ÷ I. Then choose the nearest commercial value, preferably the one just above.
How do I calculate the power of a resistor?
P = V × I, or I² × R, or V² ÷ R. Choose a resistor with a power rating of at least double the calculated value.
How many ohms is 1 kΩ?
1 kΩ is 1,000 Ω. And 1 MΩ is 1,000,000 Ω.
How many amperes is 1 mA?
1 mA is 0.001 A. So 20 mA is 0.020 A.
Does Ohm’s law apply to bulbs and LEDs?
Only approximately or at specific points. These components do not have a linear relationship between voltage and current.
Can I use this calculator to size home wiring and circuit breakers?
No. This tool is educational only. Electrical installations must be designed and carried out by a qualified professional, following the applicable standards.
What happens to the current if I double the resistance?
With constant voltage, the current drops to half.
Why does the calculator not accept zero resistance?
Because dividing by zero has no result. In practice, zero resistance causes a short circuit, with a very high and potentially dangerous current.
Georg Ohm and the cold reception
The German physicist Georg Simon Ohm published the relationship between voltage, current and resistance in 1827, after careful experiments with wires of different lengths and materials. At the time, the idea was poorly received by part of the scientific community, and Ohm went years without the recognition he deserved. Only decades later was his work widely accepted.
In his honor, the unit of electrical resistance is called the ohm and has the symbol Ω, the Greek letter omega. The unit of voltage honors Alessandro Volta, and the unit of current honors André-Marie Ampère.
Where this calculation is useful
- Bench electronics: choosing resistors for LEDs, voltage dividers and sensors.
- Arduino and microcontroller projects: protecting pins and sizing components.
- Physics classes: high school electric circuit problems.
- Automotive and low-voltage electronics: understanding the relationship between a 12 V battery, bulbs and fuses (for study purposes only).
- Understanding your electricity bill: the power of an appliance multiplied by the time it is used gives the energy consumed.
A few curiosities
- Superconductors are materials that, at very low temperatures, have practically zero resistance: current flows without losses.
- A 1 m copper wire with a 1 mm² cross-section has about 0.017 Ω, which is why long, thin cables heat up.
- The siemens is the unit of conductance, the inverse of resistance: G = 1 ÷ R.
- The human body does not have a fixed resistance: it depends on the skin, moisture and the path the current takes. That is why there is no safe rule of "harmless voltage" for running tests.
Basic electrical glossary
- Voltage (V)
- Electric potential difference that "pushes" charges through the circuit. Unit: volt.
- Current (I)
- Amount of charge passing a point in the circuit every second. Unit: ampere.
- Resistance (R)
- Opposition to the flow of current. Unit: ohm (Ω).
- Power (P)
- Rate at which electrical energy is converted into another form, such as heat or light. Unit: watt.
- Resistor
- A component made to provide a known resistance in a circuit.
- Series circuit
- Components connected one after another, so the same current flows through all of them.
- Tolerance
- Allowed variation of the actual value from the marked value of a component.