Case 1
- What are you looking for?
- Power P — from V and I
- Voltage (V)
- 12V
- Current (I)
- 2A
- Resistance (R)
- 6Ω
24.000000
Open with these valuesOhm's Law
24.000000
Result: 24.000000Ohm's law ties three quantities together: V = I × R. Pick what you are after and enter the other two — resistance is V ÷ I, current is V ÷ R, voltage is I × R. Power comes from the same circuit as P = V × I, so 12 V at 2 A is 6 Ω and 24 W.
Worked examples
Case 2
- What are you looking for?
- Resistance R — from V and I
- Voltage (V)
- 230V
- Current (I)
- 10A
- Resistance (R)
- 6Ω
23.000000
Open with these valuesCase 3
- What are you looking for?
- Voltage V — from I and R
- Voltage (V)
- 12V
- Current (I)
- 0.5A
- Resistance (R)
- 100Ω
50.000000
Open with these valuesHow it's calculated
V = I × R, I = V ÷ R, R = V ÷ I, P = V × I
- StepChoose which of the four quantities you want the calculator to work out.
- StepFill in the two fields that quantity needs — the third is then ignored.
- ResultRead the answer above: volts, amperes, ohms or watts, depending on your choice.
What this number means
Ohm's law says that the current through a conductor is proportional to the voltage across it, with the resistance as the constant of proportionality: V = I × R. Rearranged, the same single relation answers three questions. Resistance is voltage divided by current, R = V ÷ I. Current is voltage divided by resistance, I = V ÷ R. Voltage is current times resistance, V = I × R. Power is the fourth quantity of the same circuit and follows from the definition of the volt as one watt per ampere: P = V × I. A 12 V supply pushing 2 A therefore sees 6 Ω and delivers 24 W. Two divisions have a limit. Resistance from a current of zero and current from a resistance of zero both divide by zero, and this page shows no result rather than an infinite one — a resistance measured at no current is not enormous, it is simply undetermined. Note that power here is built from voltage and current. If you know current and resistance instead, let the calculator find the voltage first, enter it, and then ask for the power.
The third field is ignored
Each choice uses exactly two of the three inputs. Whatever stands in the remaining field does not enter the calculation.
Zero in the denominator gives no answer
Resistance from zero current, or current from zero resistance, divides by zero. The page then shows no result, because the quantity is undetermined rather than infinite.
One circuit, four numbers
Any two of voltage, current and resistance fix the third, and the power follows. A 12 V, 2 A circuit is always 6 Ω and 24 W.
Commonly misread
I know current and resistance, so I can ask for the power directly.
Power here is P = V × I. Let the calculator find the voltage from I and R first, enter it, then ask for the power.
Doubling the current doubles the power at fixed resistance.
It quadruples it. At fixed resistance the voltage doubles too, and power is the product of both.
Ohm's law holds for every component.
It holds for ohmic conductors, where resistance stays constant. Diodes, lamps and semiconductors change resistance with voltage or temperature.
Reference table
| Looking for | From | Formula | Example |
|---|---|---|---|
| Voltage V | I and R | V = I × R | 2 A × 6 Ω = 12 V |
| Current I | V and R | I = V ÷ R | 12 V ÷ 6 Ω = 2 A |
| Resistance R | V and I | R = V ÷ I | 12 V ÷ 2 A = 6 Ω |
| Power P | V and I | P = V × I | 12 V × 2 A = 24 W |
| Resistance R | V and I | R = V ÷ I | 230 V ÷ 10 A = 23 Ω |
| Power P | V and I | P = V × I | 230 V × 10 A = 2300 W |
Questions
What is Ohm's law?
It states that the current through a conductor is proportional to the voltage across it, written V = I × R. The resistance R is the constant of proportionality, measured in ohms.
How do I calculate resistance from voltage and current?
Divide the voltage by the current: R = V ÷ I. A 12 V supply drawing 2 A means 12 ÷ 2 = 6 Ω.
How is power related to Ohm's law?
Power is voltage times current, P = V × I, because one watt is one volt-ampere. The same 12 V and 2 A circuit therefore uses 24 W.
Why does the calculator return nothing sometimes?
Two of the four formulas divide: resistance by the current and current by the resistance. If that divisor is zero the result is undetermined, so no number is shown.
Does Ohm's law apply to alternating current?
In the same form only for purely resistive loads. With coils or capacitors the resistance is replaced by impedance, which also depends on frequency.
Sources and last check
Information, not professional advice.