Ohm’s law calculator
This Ohm’s law calculator takes any two of voltage, current, resistance and power and works out the other two. It uses V = I × R and P = V × I and handles SI prefixes, so you can work in milliamps, kilohms or microwatts without converting by hand. It’s built for sizing resistors, checking power dissipation and sanity-checking bench measurements on DC circuits.
How to use it
- Pick the two quantities you already know. Any pair works: voltage and current, voltage and resistance, current and power, and so on.
- Enter each value with its SI prefix. For example, 20 mA rather than 0.02 A, or 4.7 kΩ rather than 4700 Ω.
- Read the two missing quantities from the results.
- Check the power figure against the rating of the part you plan to use, especially for resistors.
Enter the voltage across the specific component you care about, not the whole supply voltage. That distinction causes most wrong answers.
How Ohm’s law and the power formula fit together
Georg Simon Ohm published the relationship in 1827. He found that the current through a conductor is proportional to the voltage across it. The constant of proportionality is resistance:
V = I × R
Voltage is in volts (V), current in amperes (A) and resistance in ohms (Ω). One ohm is the resistance that lets one ampere flow when one volt is applied.
Electrical power is the rate at which energy is delivered, measured in watts (W):
P = V × I
Substituting one equation into the other gives every combination the calculator needs. With any two values known, the other two follow:
| You know | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| I and R | V = I × R | P = I² × R | ||
| V and R | I = V / R | P = V² / R | ||
| V and I | R = V / I | P = V × I | ||
| P and I | V = P / I | R = P / I² | ||
| P and V | I = P / V | R = V² / P | ||
| P and R | V = √(P × R) | I = √(P / R) |
SI prefixes scale each unit by powers of ten. The ones you’ll meet most often in electronics are:
| Prefix | Symbol | Factor |
|---|---|---|
| pico | p | 10⁻¹² |
| nano | n | 10⁻⁹ |
| micro | µ | 10⁻⁶ |
| milli | m | 10⁻³ |
| kilo | k | 10³ |
| mega | M | 10⁶ |
A worked example. Say you want to run an LED from a 5 V supply. The LED drops about 2 V and you want 20 mA through it. The resistor sees 5 − 2 = 3 V. Enter 3 V and 20 mA, and the calculator gives R = 150 Ω and P = 60 mW. A standard 1/4 W resistor handles that comfortably. The calculator does not subtract the LED’s forward voltage for you, so do that step first.
What the tool does not do. It treats every value as a plain DC quantity in a single component. It does not calculate:
- AC impedance, reactance, phase angle, power factor or RMS conversions.
- Series or parallel resistor networks.
- Resistor colour codes or tolerance ranges.
- Nonlinear parts such as diodes, LEDs, transistors or incandescent filaments, whose resistance changes with voltage, current or temperature.
FAQ
Which two values do I need to enter?
Any two of the four. There are six possible pairs, and each one determines the other two quantities exactly. Entering power and resistance is just as valid as entering voltage and current.
Does this calculator work for AC circuits?
It works for purely resistive AC loads if you enter RMS voltage and RMS current. It does not handle capacitors or inductors, because they add reactance and a phase shift between voltage and current. For those circuits you need impedance and power factor, which this tool does not calculate.
Why doesn’t Ohm’s law match my LED or light bulb?
Ohm’s law describes ohmic components, where resistance stays constant. An LED barely conducts below its forward voltage and then conducts heavily above it. A tungsten filament’s resistance rises sharply as it heats up. The calculator is still useful for the resistor in series with these parts, but not for the parts themselves.
How do I choose a resistor’s wattage from the result?
Take the power figure and pick a resistor rated comfortably above it. Many makers choose a part rated at least double the calculated dissipation, because resistors run hot near their limit. If the result is 60 mW, a 1/4 W (250 mW) resistor leaves plenty of margin.