Ohm's Law Calculator
Calculate voltage, current, resistance, or power using Ohm's Law (V = I × R). Enter any two values to find the others.
This calculator implements the full Ohm's Law and Joule's Law relationship between voltage (V), current (I), resistance (R), and power (P) -- four quantities where any two determine the other two. Enter Voltage and Current (the two fields already filled by default) and the solver derives Resistance from R = V / I and Power from P = V x I. Enter a different pair instead -- say Voltage and Resistance, or Current and Power -- and the solver switches formulas automatically, always deriving the two fields you left at 0. A field left at 0 is treated as "not yet provided": zero volts, zero amps, and zero ohms are all degenerate readings that describe nothing about a circuit, so the solver never treats a blank field as a real measurement. Because whichever two values you type stay exactly as typed -- Voltage never moves the Current you entered, and Current never moves the Voltage you entered, no matter how far you drag either slider -- the calculator only ever recomputes the two fields it is solving for. Raising Voltage while Current stays fixed always raises both Resistance (R = V / I grows with V) and Power (P = V x I grows with V); raising Current while Voltage stays fixed always raises Power the same way. If more than two fields are filled, the calculator uses the first two in V-I-R-P order and recalculates the rest, and the Status field tells you which formulas it used.
Inputs
R = V / I — calculated from the values you entered. Clear one of those to edit this instead.
P = V × I — calculated from the values you entered. Clear one of those to edit this instead.
The circuit, live
12 volts pushes 2 amps through 6 ohms, dissipating 24 watts — the bulb is glowing brightly.
Results
Voltage (V)
12
The V-I-R-P wheel
Using V and I — the highlighted formulas are the ones being applied.
- V = I × R
- V = P / I
- V = √(P × R)
you entered
- I = V / R
- I = P / V
- I = √(P / R)
you entered
- R = V / I←
- R = V² / P
- R = P / I²
calculated
- P = V × I←
- P = V² / R
- P = I² × R
calculated
Ohm's law
- 1.Divide volts by amps
This is the definition of resistance rather than a consequence of it: an ohm IS one volt per amp. Dividing the two you measured gives the resistance that must be present.
- 2.Multiply volts by amps for power
A volt is a joule per coulomb and an amp is a coulomb per second, so their product is joules per second — watts. The charge unit cancels, which is why the multiplication is dimensionally allowed at all.
Where the units go
- volts ÷ amps A→Ω
- volts × amps C→W
Georg Ohm published the proportionality between current and voltage after measuring it on wires of varying length, using a thermocouple for a steady source. The work was poorly received at first — his own ministry considered it insufficiently rigorous.
How to Use This Calculator
- Enter any two of the four values — voltage (V), current (A), resistance (Ω), or power (W) — and leave the other two at 0.
- The calculator solves for the two you left blank using Ohm's Law (V = IR) and Joule's Law (P = VI).
- Review the calculated voltage, current, resistance, and power, plus the formulas used to derive them.
- Check the status field to confirm the values were solved (it prompts for a second value if only one was entered).
- Use for basic circuit analysis, resistor selection, and power dissipation calculations.
How the result changes with Voltage
| Voltage | Voltage (V) |
|---|---|
| 24 | 24 |
| 84 | 84 |
| 156 | 156 |
| 216 | 216 |
What each input means
- Voltage
- Volts. Leave at 0 to solve for it.
- Current
- Amps. Leave at 0 to solve for it.
- Resistance
- Ohms. Leave at 0 to solve for it.
- Power
- Watts. Leave at 0 to solve for it.
How this is calculated
Worked example, using the default values
- Calculate resistanceR = V / IV = 12 V, I = 2 A = 6 Ω
- Calculate powerP = V × IV = 12 V, I = 2 A = 24 W
Engine last updated .
Frequently Asked Questions
Why doesn't entering a voltage change the current I already typed?
The current field is one of the two values you supplied, so the solver treats it as known and never recalculates it. Ohm's Law only lets you solve for the two remaining unknowns -- here, Resistance and Power -- from the two you provide; changing Voltage recalculates those two, but the Current field you typed stays exactly as entered until you clear it or type a different number.
What happens if I enter three or four values instead of two?
The calculator uses the first two values in Voltage-Current-Resistance- Power order as the known pair and recalculates the rest from those, ignoring what you originally typed in the extra fields. The Status/Formulas Used field explains which two values were treated as known and which were recalculated, so you can see exactly what happened instead of getting a silently contradictory answer.
Why do Resistance and Power both increase when I raise Voltage?
With Current held fixed, Resistance follows R = V / I and Power follows P = V x I -- both formulas have Voltage in the numerator, so a higher Voltage directly raises both results. This holds across the calculator's full 0-240V range as long as Current stays at whatever value you entered.
Why do I need to leave two fields at 0 for the calculator to work?
Ohm's and Joule's laws only pin down the system once exactly two of the four electrical quantities are known -- with only one known value, infinitely many combinations of the other three would satisfy the equations, so the calculator has nothing to solve for and shows the "waiting for a second value" status until you provide a second measurement.
Can I use this for AC circuits with reactance, not just DC?
No -- the formulas here (V = IR and P = VI) describe purely resistive DC circuits or the real-power relationship in an AC circuit at unity power factor. An AC circuit with inductance or capacitance also has reactance and impedance, which change the V-I relationship and require the phase angle between voltage and current, neither of which this calculator accounts for.
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