Every electrical circuit, from the lighting in a room to the processor inside a phone, obeys a small set of physical rules. The most famous is Ohm's law, named for the German physicist Georg Simon Ohm, who described in 1827 how voltage, current and resistance relate to one another. The Ohm's Law Calculator applies that relationship and its power-law companions instantly: choose the quantity you want to find, enter the two values you know, and read off the result in the correct SI units. Whether you are wiring a lamp, sizing a resistor for an LED, or checking the rating of an appliance, the same few formulas do the work.
Ohm's Law: Voltage, Current and Resistance
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across those points, as long as the resistance of the conductor is unchanged. In symbols:
V = I × R
Here V is the voltage measured in volts, I is the current measured in amperes, and R is the resistance measured in ohms. Doubling the voltage across a fixed resistor doubles the current; halving it halves the current. If you keep the voltage fixed and double the resistance, the current drops to half. The three quantities form a triangle of relationships, and any two of them determine the third:
- Current I = V ÷ R — how much charge flows per second, given the pressure and the opposition.
- Voltage V = I × R — the electrical pressure needed to push a given current through a given resistance.
- Resistance R = V ÷ I — how strongly the circuit opposes the flow, found by dividing the measured voltage by the measured current.
The calculator's four mode tabs mirror these choices. On the Find Current tab you supply voltage and resistance; on the Find Voltage tab you supply current and resistance; on the Find Resistance tab you supply voltage and current; and the Find Power tab supplies voltage and current to solve for power.
Choosing the Mode That Matches Your Question
Decide first which unknown you need, then select the matching tab at the top of the calculator. Each tab shows exactly the two input fields that are relevant to that question, so there is no ambiguity about which numbers go where. With the default values on the Find Current tab — a 12 volt source driving a 6 ohm resistor — the current reads 2.0 amperes. On the Find Voltage tab, 2 amperes through 6 ohms returns 12 volts. On the Find Resistance tab, 12 volts divided by 2 amperes returns 6 ohms. And on the Find Power tab, 12 volts multiplied by 2 amperes returns 24 watts. Whichever tab is active, only its own inputs are used; the fields belonging to inactive tabs evaluate to zero so the result always belongs to the mode you selected.
How Electrical Power Fits In
Voltage, current and resistance tell you how a circuit behaves, but power tells you how much work it does, how hot a component gets, and how much a device costs to run. The power dissipated by a resistive element is the product of the voltage across it and the current through it:
P = V × I
Combining this with Ohm's law produces two more equivalent forms:
P = I² × R
P = V² ÷ R
The calculator computes power from whichever two quantities you supplied. Given voltage and resistance it uses P = V² ÷ R; given current and resistance it uses P = I² × R; and given voltage and current it uses P = V × I directly. The three paths always agree because they are the same law rearranged. Power is measured in watts, and a watt is one joule of energy transferred every second. That is why a 24 watt resistor in the example above converts 24 joules of electrical energy into heat each second — which is why component power ratings matter so much.
A Practical Example: Sizing a Resistor
Consider a classic beginner circuit: a 9 volt battery, a blue LED, and a series resistor. The LED needs about 3 volts and around 20 milliamperes, so the resistor must drop the difference of 6 volts while passing 20 milliamperes, or 0.02 amperes. Using the Find Resistance tab with V equal to 6 and I equal to 0.02 gives a resistance of 300 ohms. The power the resistor must handle is found with P = V × I, which is 6 times 0.02, or 0.12 watts, so an eighth-watt resistor at 0.125 watts sits right at the edge and a quarter-watt part is a safer choice. A resistor rated below the required dissipation would overheat, and the calculator's power output is exactly the number that keeps the component within its limits.
Voltage Drop Along a Cable
Another everyday use of Ohm's law is estimating voltage drop in wiring. A long cable has resistance proportional to its length, and the current drawn by the load multiplied by that resistance tells you how many volts are lost in the wire itself. A cable of 0.5 ohms carrying 5 amperes to a load drops 2.5 volts, so a nominal 12 volt supply arrives at the equipment with only 9.5 volts available. That single calculation decides whether a cable gauge is adequate, whether a charger reaches its device, and whether a power tool has enough voltage at the end of a long extension lead. The Find Voltage mode with current and resistance is precisely this calculation.
Understanding the Units
The calculator uses the international system units that make the formulas exact: volts, amperes, ohms and watts. Practical values often come with prefixes — millivolts, milliamperes, kilohms, megawatts — and those must be converted before they enter the equations. A current of 200 milliamperes is 0.2 amperes, a resistance of 4.7 kilohms is 4,700 ohms, and a supply of 5 volts at 0.2 amperes is exactly 1 watt. Keeping every input in the base unit keeps every result correct; mixing 5 volts with 200 milliamperes directly would silently give a five-times-too-large current.
Alternating Current and Real Components
Ohm's law as presented here applies exactly to purely resistive circuits, whether the supply is direct current or alternating current, as long as there are no capacitors or inductors in the path. A resistor is a resistor whether the waveform is steady or sinusoidal. Once a circuit includes capacitors or inductors, current and voltage can shift out of phase with each other, and the simple resistance is replaced by impedance, which depends on frequency. For everyday lighting, heaters, and the steady-state behaviour of many loads, the resistive model remains a faithful approximation and Ohm's law gives the right answers.
Reading the Four Outputs
- Current (I) — solved as V ÷ R on the Find Current tab, the amperes flowing through the circuit.
- Voltage (V) — solved as I × R on the Find Voltage tab, the volts needed to drive the given current through the resistance.
- Resistance (R) — solved as V ÷ I on the Find Resistance tab, the ohms opposing the flow.
- Power (P) — always derived from the two values you entered, the watts dissipated by the circuit, shown on every tab so you never lose sight of the heating effect.
Each output is labelled with its formula so you can see exactly which rearrangement produced it. Because all the quantities are linked, the results are internally consistent: the power shown on the Find Current tab, computed as V² ÷ R, agrees with what P = V × I would give from the very same pair of values.
Common Mistakes
- Mixing prefixed units, such as entering milliamperes where the formula expects amperes.
- Swapping the numerator and denominator, for example computing R = I ÷ V instead of R = V ÷ I.
- Forgetting that resistance must be greater than zero; a zero-ohm value makes the current undefined.
- Applying the simple formula to components like diodes whose resistance changes with operating point.
- Ignoring the power rating of a resistor and letting it overheat while passing a legal current.
Key Assumptions
- The circuit is purely resistive, with constant resistance over the operating range.
- All inputs are in SI base units of volts, amperes and ohms.
- Only the values on the active tab are used, and every result corresponds to that mode.
- Power is computed from the two known quantities and assumes the component dissipates all of it as heat.
- Line losses, source internal resistance and temperature effects are ignored.
From a single resistor to a house full of appliances, Ohm's law and its power forms answer most questions a practical circuit asks. Enter the two values you know, pick the tab that names the quantity you need, and the calculator returns the missing piece together with the power it implies. The same formula that sizes an LED resistor and checks a cable also stands behind the larger calculations of electricians and engineers.
Disclaimer
Results are provided as estimates for informational purposes only and may be inaccurate. Always verify outcomes with a qualified professional before making financial or personal decisions based on these calculations.