Ohm's Law Calculator
Enter any two of voltage V, current I, resistance R and power P to compute the other two via V = IR and P = VI. All six combinations with substituted working — pairs naturally with the equivalent resistance tool.
Try an example
Enter any two of V, I, R and P to compute the other two.
How to use
- 1
Enter two values
Fill in any two of V, I, R and P that you already know.
- 2
Read the other two
The remaining two are computed from V = I × R and P = V × I automatically.
- 3
Read the working
The Working panel shows the substituted formulas, with one-click copy.
Features
- Any two of V, I, R, P determine the other two (all six combinations supported)
- Substituted formulas shown for whichever pair you enter
- Clear notices for under/over-filled inputs, negative values and zero division
- Completes circuit calculations with the resistance tool; inputs are never transmitted
Use cases
Check circuit homework
Verify hand-worked currents and voltages in Ohm’s law problems.
Check power budgets
Compute power from a component’s voltage and current to size the supply.
Chain with resistance
Feed the R from the resistance tool in to get the full circuit current.
Details
Ohm’s law states voltage = current × resistance (V = I × R), and power is P = V × I. Combining them means any two of V, I, R and P determine the other two — six substitution patterns in total (V = IR, I = V/R, R = V/I, P = V²/R, P = I²R, P = VI).
Fill in two of the four fields and the tool computes the other two, showing the substituted formulas it used. Combined with the equivalent resistance tool — find R for the whole circuit first — entering R plus the supply voltage V yields the current and power in one go. Round results with the significant figures tool.
Conditions: these relations apply to DC with a single resistive load (AC and phase are not modeled). Combinations that require dividing by zero (such as a nonzero voltage with zero resistance) are rejected with an error. Enter voltage, current, resistance and power as values of 0 or more. Inputs are never transmitted; everything is computed locally in your browser.
FAQ
What is Ohm’s law?
It is the fundamental relation voltage = current × resistance (V = I × R). Any two of V, I and R determine the third, and combining it with P = V × I means any two of the four values determine all of them.
Why do I need the power P?
Power (watts) is the rate at which a component consumes energy — it sizes power supplies and estimates heat. It has three equivalent forms: P = VI, P = I²R and P = V²/R, all giving the same value.
Does this work for AC circuits?
The tool covers the basic DC relations with a resistive load. AC adds RMS values and power factor (phase), which need separate consideration.
What happens if I fill three or more fields?
The tool reports an error. Only two of the four values are independent, so a third can contradict them. Clear every field you want computed.
Related guide
GuideHow to Use Ohm’s Law: Voltage, Current, Resistance and Power
Learn to use Ohm’s law V = IR to find voltage, current and resistance, plus the three power formulas P = VI, P = I²R and P = V²/R — fully substituted examples including an LED resistor selection. Written for physics students and electronics beginners.
GuideHow to Find the Equivalent Resistance: Series and Parallel Circuits
Learn the two rules of equivalent resistance — series adds up (R = R₁ + R₂ + …) and parallel combines reciprocals (1/R = 1/R₁ + 1/R₂ + …) — with fully substituted examples, why parallel shrinks the total, and how to tackle mixed circuits.
Related Tools
Equivalent Resistance Calculator
Compute the equivalent resistance of resistors in series or parallel with fully substituted working. Handles 2 to 4 resistors and cross-checks the two-resistor parallel shortcut R₁R₂ ÷ (R₁ + R₂) — handy for physics circuits.
Significant Figures Calculator
Count significant figures, round to a chosen precision, and apply the multiplication/division or addition/subtraction rules. Switch between half-up and JIS Z 8401 half-even rounding; scientific notation is shown alongside.
Half-Life Calculator
Compute the remaining amount after any elapsed time with N = N₀ × (1/2)^(t/T). Includes carbon-14 dating (5730 years) and drug-metabolism examples, plus the remaining percentage, half-lives elapsed and the decay constant λ.
Electricity Cost & Energy Calculator
Turn power (W, or A × V) and usage time into energy in kWh and electricity cost per day, month and year. The rate is editable for your own tariff, and the tool converts between kWh, J, MJ and kcal.
All processing happens in your browser. Your files are never uploaded.
Verified: All five two-value combinations (V,I / R,I / P,V / P,R / I,R), over-fill, negative and zero-division errors and the English page are covered by browser tests
Did you know?
The Ohm’s law triangle makes V = I × R memorable: cover the value you want with a finger — hiding I leaves V ÷ R, hiding R leaves V ÷ I, and both division forms appear automatically.