Ohms Law & Circuit
Ohms Law Calculator
Enter any two of voltage, current, resistance or power and this calculator solves the other two using the matching equation from the Ohms Law Wheel.
Select a quadrant to highlight the three formulas that solve for that quantity.
V-I-R-P Solver
Fill in any two boxes and leave the other two blank.
The Formulas Behind This Calculator
Ohms Law states that the current through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance between them. Written the three usual ways, that is V = I × R, I = V ÷ R and R = V ÷ I.
On its own that covers three of the four quantities. Power enters through Joule's Law, P = V × I, which measures how quickly electrical energy is converted into heat or another form of work. Substituting the Ohms Law forms into the power equation produces the remaining relationships, and the full set is what the Ohms Law Wheel arranges into four groups of three.
The calculator selects the pair of equations that matches the two values you supply, so you never have to rearrange anything by hand. Which pair you enter also determines which formula appears in the working panel underneath the results.
How to Use the Ohms Law Calculator
Fill in exactly two of the four boxes and leave the others empty. A counter beneath the fields tells you whether you still need a value or whether you have entered one too many. Each field has its own unit selector, so a 4.7 kΩ resistor and a 20 mA current can be typed as they appear on your notes rather than converted into ohms and amperes first.
Select Calculate to solve. Results appear as four tiles, with the two you supplied marked as entered and the two the calculator derived marked as solved. Below them, the working panel shows the formula chosen and the same formula with your numbers substituted in, so the arithmetic can be checked line by line.
The wheel graphic on the homepage responds to the same logic: selecting a quadrant highlights the three formulas that produce that quantity. Reset clears every field and returns the panel to its starting state, and Copy result puts the whole summary on your clipboard for a report or a lab notebook.
Worked Example: Voltage Across a Known Resistor
Suppose you measure 12 V across a resistor and 2 A flowing through it, and you want the resistance and the power it is dissipating.
Twenty-four watts is a serious amount of heat for a small component. A wirewound resistor rated at 50 W or more, mounted with airflow, would be the sensible choice here, and a quarter-watt part would fail almost instantly.
Now reverse the problem. A heating element is marked 1500 W on a 230 V supply. Entering power and voltage gives I = P ÷ V = 6.52 A and R = V² ÷ P = 35.3 Ω. The current figure decides the cable and protective device; the resistance figure is what you would expect to measure with a meter once the element has cooled, allowing for the fact that resistance rises with temperature.
Reading Your V-I-R-P Results
Voltage is the potential difference across the component, measured in volts. Current is the rate of charge flow through it, measured in amperes. Resistance describes how strongly the component opposes that flow, measured in ohms. Power is the rate of energy conversion, measured in watts, and it is the figure that determines heat.
Each result is shown twice: once in the unit you selected, and once in engineering notation with an SI prefix chosen automatically. A result of 0.000002 A appears as 2 µA, and 4700 Ω appears as 4.7 kΩ. Full floating-point precision is kept internally throughout the calculation, and rounding is applied only when the number is displayed.
If a combination cannot be solved, the calculator explains why instead of showing an error value. Asking for resistance while entering a current of zero, for example, would require dividing by zero, so the tool says so rather than returning infinity.
Where Ohms Law Is Applied
Bench work is the most common use. When you have probed a circuit with a multimeter and know two values, the wheel fills in the rest without a second measurement, which is useful when the third value would require breaking the circuit to insert an ammeter.
Component selection is the second. Working out the power in a resistor before ordering it prevents the classic mistake of specifying the right resistance with the wrong wattage. A common guideline is to choose a part rated for at least twice the calculated dissipation, giving margin for enclosure temperature and tolerance.
Teaching and study make up the third. Because the substituted equation is shown alongside every answer, the tool can be used to check homework rather than simply produce it, and the pattern of which formula applies to which pair of inputs becomes familiar quickly.
Limits of the Ohms Law Model
Ohms Law describes linear, resistive behaviour at a constant temperature. Several common situations fall outside that description, and it is worth recognising them.
- Semiconductors such as diodes, LEDs and transistors are non-linear. Their effective resistance changes with the applied voltage, so a single ratio of V to I does not characterise them.
- AC circuits containing inductance or capacitance need impedance rather than resistance, and real power then depends on the power factor as well as on volts and amps.
- Conductor resistance rises with temperature. A tungsten lamp filament can measure ten times lower cold than it does when incandescent, which is why switch-on surge current is so high.
- Results assume ideal sources and perfect connections. Internal source resistance and contact resistance both reduce the voltage actually delivered to the load.
Use the figures as engineering estimates for study, prototyping and planning. Installation work must follow local codes and be carried out by a qualified person on de-energised circuits.
Questions
Ohms Law Calculator FAQs
Can I enter three values instead of two?
No. Two values fully determine a resistive circuit, so a third would either be redundant or contradict the others. The calculator detects three entries and asks you to clear one before solving.
Why does the calculator refuse a current of zero?
Because several wheel formulas divide by current. R = V ÷ I and R = P ÷ I² both become undefined when I is zero, so the tool explains the problem rather than returning infinity.
Does it work with milliamps and kilohms?
Yes. Every field has a unit selector, and each entry is converted to volts, amperes, ohms and watts before any arithmetic runs. The result is then converted back into the unit you chose.
Is this suitable for AC circuits?
It applies to purely resistive AC loads if you use RMS values. Motors, transformers, fluorescent ballasts and switch-mode supplies also need impedance and power factor, which this calculator does not model.
Explore the Full Ohms Law Wheel
Open the full Ohms Law Wheel calculator to solve voltage, current, resistance and power from any two known values.