No mode switching
You never pick which formula to use. Type into any two boxes and the calculator tracks which two you touched last, treats those as knowns, and releases the others. Change your mind mid-calculation and it follows you.
Ohm’s Law Calculator
Enter any two of voltage, current, resistance and power, and the other two are solved instantly. Full unit prefixes from microamps to megohms, the exact formulas used shown every time, a live Ohm’s law wheel, and a suggested resistor power rating so you do not cook the part you just picked.
Any two values in
µ, m, k, M prefixes
Shows the working
Runs in your browser
Voltage
KnownCurrent
CalculatedResistance
KnownPower
CalculatedTeal segments are your inputs. Violet segments are solved from them.
Formulas used
Energy in 1 hour–at a constant load
Energy in 24 hours–running continuously
Conductance–1 / R, in siemens
Suggested part rating–2× headroom
A resistor dissipating 1.44 W should be rated at least double that in still air. Derate further inside a sealed enclosure or next to other warm components.
Everything is calculated locally in your browser; nothing is uploaded or stored. Ohm’s law applies to ohmic loads at a steady DC operating point. Diodes, LEDs, motors under load and AC circuits with reactance need the extra terms described below.
The full wheel
All twelve Ohm’s law formulas
Ohm’s law proper is V = I × R. Combined with the power law P = V × I it produces twelve rearrangements, one for every pair of known quantities. This is the complete set the calculator draws on.
| You know | Formula | Solves for | Typical use |
|---|---|---|---|
| I, R | V = I × R | Voltage | Voltage drop across a series resistor |
| P, I | V = P ÷ I | Voltage | Supply voltage for a rated load |
| P, R | V = √(P × R) | Voltage | Drive voltage for a heating element |
| V, R | I = V ÷ R | Current | Current through a known resistor |
| P, V | I = P ÷ V | Current | Sizing wire and fuses for an appliance |
| P, R | I = √(P ÷ R) | Current | Current in a resistive heater |
| V, I | R = V ÷ I | Resistance | Working out an unknown load |
| V, P | R = V² ÷ P | Resistance | Ballast resistor for a target dissipation |
| P, I | R = P ÷ I² | Resistance | Shunt and sense resistor design |
| V, I | P = V × I | Power | Load on a supply or battery |
| V, R | P = V² ÷ R | Power | Heat in a pull-up or bleeder resistor |
| I, R | P = I² × R | Power | Losses in cable and connectors |
Built differently
Why this Ohm’s law calculator is better
Most versions give you four boxes and a number. This one behaves the way you actually work at a bench.
Real unit prefixes
Microamps, milliamps, kilohms and megohms are selectable per field, so you enter 4.7 kΩ and 20 mA the way they are printed on the part instead of translating everything into scientific notation first.
It shows the working
Both equations applied are displayed with your actual numbers substituted in. Useful for homework, for checking yourself, and for pasting into a design note where someone else has to follow the reasoning.
Component rating guidance
The calculator converts dissipation into a suggested standard resistor wattage with 2× headroom, and flags currents and voltages that move a design out of low-voltage hobby territory.
A live Ohm’s law wheel
The classic four-quadrant wheel is drawn as an interactive diagram that highlights which quantities you supplied and which were derived, so the relationship becomes visual rather than memorised.
Fast, private, mobile ready
A few kilobytes of vanilla JavaScript, no libraries, no accounts and no tracking. It loads instantly on a phone at the bench and keeps working with a poor connection.
Step by step
How to use the Ohm’s law calculator
Four steps and about fifteen seconds.
1
Enter your first known value
Type it in and set the unit prefix beside it. Supply voltage and load resistance are the usual starting pair.
2
Enter the second known value
As soon as a second box has a number, the remaining two are solved and marked as calculated.
3
Check the formulas panel
Confirm the calculator used the pair you intended, and read your numbers substituted into each equation.
4
Check power before you buy
Look at the dissipation and the suggested rating, then copy the whole result into your notes or a parts list.
Where it applies
What Ohm’s law tells you in practice
Four quantities, one relationship, and most of the everyday decisions in a low-voltage circuit.
- Sizing an LED series resistor
Subtract the LED forward voltage from the supply, then divide by the target current. A 5 V rail, a 2 V red LED and 20 mA gives 150 Ω.
- Predicting heat in a resistor
Power is what destroys parts, not voltage. A 100 Ω resistor on 12 V dissipates 1.44 W and will scorch a quarter-watt part in seconds.
- Choosing wire and fuses
Divide appliance wattage by supply voltage to get the current a cable must carry, then pick a conductor and fuse rated comfortably above it.
- Estimating battery runtime
Current drawn against pack capacity in amp hours gives an approximate runtime, before efficiency losses and voltage sag are considered.
- Diagnosing a suspect load
Measure voltage across a device and current through it, and the resistance you calculate tells you whether it matches the specification or has drifted.
- Understanding voltage drop
Long cable runs have real resistance. At high current even a fraction of an ohm removes usable volts and converts them into heat in the wire.
Questions
Ohm’s law calculator FAQ
The questions that come up most often once the numbers stop matching expectations.
What is Ohm’s law?
Ohm’s 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. It is written V = I × R, where V is volts, I is amperes and R is ohms.
Rearranged, it gives I = V ÷ R and R = V ÷ I. Pairing it with the power law P = V × I produces the twelve formulas in the table above, which is what makes any two known quantities enough to find the remaining two.
How do I calculate watts from volts and amps?
Multiply them: power in watts equals volts times amps. A 12 V supply delivering 2 A is providing 24 W. If you know voltage and resistance instead, use P = V² ÷ R; if you know current and resistance, use P = I² × R. All three give the same answer for the same circuit.
What resistor do I need for an LED?
Subtract the LED forward voltage from your supply voltage, then divide the remainder by the current you want. For a 5 V supply, a red LED with a 2 V forward drop and a 20 mA target, that is 3 V ÷ 0.02 A = 150 Ω. The nearest common value upward, 180 Ω, is the safe choice.
Check the dissipation too: 3 V across 150 Ω is only 0.06 W, so a quarter-watt resistor is comfortable. Never connect an LED directly across a supply, because its resistance falls as it heats and the current runs away.
Does Ohm’s law work for AC circuits?
It works for purely resistive AC loads such as heaters and incandescent lamps, using RMS values for voltage and current. Once capacitance or inductance is present, resistance is replaced by impedance and voltage and current fall out of phase, so real power becomes P = V × I × power factor rather than a simple product.
This calculator handles the resistive DC case, which covers most bench and hobby work.
Why does my measured current not match the calculation?
Usually because something in the circuit is not ohmic or not what you assumed. Resistor tolerance of 5% shifts the answer immediately. Resistance rises with temperature in most conductors, so a warm component draws less current than a cold one. Supply voltage sags under load, meter leads and breadboard contacts add resistance, and semiconductors do not follow a straight line at all.
What resistor power rating should I choose?
Calculate the dissipation, then choose a standard rating of at least twice that figure. A part dissipating 0.4 W wants a 1 W resistor rather than a half-watt one. Manufacturer ratings assume free air at moderate ambient temperature, so derate further in a sealed box, in a hot environment, or where several warm parts sit close together.
How do I convert milliamps and kilohms in these formulas?
Formulas need base units, so convert first: 1 mA is 0.001 A, 1 µA is 0.000001 A, 1 kΩ is 1000 Ω and 1 MΩ is 1,000,000 Ω. A handy shortcut is that volts divided by kilohms gives milliamps directly, so 5 V across 2.2 kΩ is about 2.27 mA. The unit selector next to each field does all of this for you.
Are Ohm’s law and Kirchhoff’s laws the same thing?
No, they answer different questions and are used together. Ohm’s law relates voltage, current and resistance in a single component. Kirchhoff’s current law says the current entering a node equals the current leaving it, and his voltage law says the voltage rises and drops around any closed loop sum to zero. You use Kirchhoff to work out what voltage or current a component sees, then Ohm’s law to work out the rest.
Solve the circuit before you power it up
Two numbers in, four numbers out, plus the dissipation figure that decides whether the part survives. Keep it open on the bench alongside your multimeter.
Calculate volts, amps, ohms & watts