Example 1
I = 0.5 A, R = 100 Ω → U = 50 V.
Ohm's Law Calculator
Enter any two of voltage, current and resistance. The calculator fills the third from U = I·R. 0.5 A through 100 Ω is 50 V. 12 V across 6 Ω is 2 A. 24 V at 3 A is 8 Ω.
Need resistance from voltage and current: R = U/I. Power next door: P = U·I.
Result
Any two of I, R or U. The result shows up here.
How it works
Ohm's law ties current, resistance, and voltage in one product: U = I·R. At 0.5 A and 100 Ω you get 50 V. At 1 A and 12 Ω you get 12 V, a typical car rail. At 2 A and 47 Ω you get 94 V. At 10 A and 23 Ω you get 230 V, household order of magnitude, still just two field numbers, not a model of the whole installation.
The first field is current I in amperes, the second is resistance R in ohms. The result is in volts. If you have 2.2 kΩ, type 2200, not 2.2: the field does not know the kilo prefix. 20 mA is 0.02 A; at 1000 Ω that is 20 V, as on the page. A comma and a period are the same current, so 0,5 and 0.5 both run.
This calculator computes U from I and R and does not invert the formula. You know voltage and current and want ohms? Open the R = U/I page. Series and parallel live next door too: here you already type one R, you do not build a network from several resistors.
Zero amperes is rejected, because U = 0·R says nothing about the circuit, and the sibling R = U/I does not exist at zero current. Zero ohms will not run either: a homework line would give U = 0, a real wire always keeps some resistance. A negative I is current direction, not a unit mix-up.
Once you have U, power is next door as P = U·I. At 50 V and 0.5 A that is 25 W. There is one Ohm triplet, only the unknown changes. I = U/R at a fixed 230 V is a different page: there current falls as R rises. Here you type I and R and get U.
Type 0.5 and 100, click Calculate, and match 50 V. Then 3 A and 4 Ω: 12 V again, a different pair on the same rail. The unit switch does not rewrite ohms into another resistance scale; you stay with amperes, ohms, and volts.
How to use
- In the first field enter current I in amperes, the way an ammeter would show it. For example 0.5 or 10. Convert milliamps first: 20 mA is 0.02.
- In the second field enter resistance R in ohms. 100 means one hundred ohms. Type 2.2 kΩ as 2200, not as 2.2, because the field does not know the kilo prefix.
- Click Calculate. The calculator multiplies I by R and shows voltage U. 0.5 A and 100 Ω give 50 V. 2 A and 47 Ω give 94 V.
- Zero amperes or zero ohms is rejected, because that circuit says nothing useful. A negative I is current direction, not a unit mix-up.
- Need ohms from voltage and current? Open the R = U/I page. Power after U is P = U·I. Here it stays voltage from I and R.
Formula
U = I · R
SI units: V, A, Ω. I and R from the fields, U on the page.
U, I, and R at 0.5 A and 100 Ω
This calculator multiplies current by resistance. 0.5 A × 100 Ω = 50 V, the first example pair, not a model of the whole installation.
- U
- Voltage in volts. The calculator result: 0.5 A × 100 Ω = 50 V. At 1 A and 12 Ω you get 12 V, a car rail.
- I
- Current in amperes, first field. Type 20 mA as 0.02. Zero amperes is rejected, because 0·R says nothing about the circuit.
- R
- Resistance in ohms, second field. 2.2 kΩ is 2200, not 2.2. Zero ohms will not run either.
- Ω
- The unit of R on the label. The field does not know the kilo prefix, so 100 Ω is typed as 100.
- V
- The unit of U on the page. 10 A and 23 Ω give 230 V, household order, still just I times R.
Real-life examples
Example 2
U = 12 V, R = 6 Ω → I = 2 A.
Example 3
U = 24 V, I = 3 A → R = 8 Ω.
Example 4
I = 0.02 A, R = 1000 Ω → U = 20 V.
Example 5
I = 3 A, R = 4 Ω → U = 12 V.
Example 6
I = 0.1 A, R = 220 Ω → U = 22 V.
Example 7
I = 5 A, R = 0.5 Ω → U = 2.5 V.
Example 8
Small current, series resistor.
Example 9
Large current, low resistance.
Example 10
1 mA through 10 kΩ → 10 V.
Ways to use this calculator
- You check voltage on a resistor at a known I and R.
- You compare a 12 V / 1 A homework line with this calculator.
Frequently asked questions
How many volts at 0.5 A and 100 Ω?
U = 0.5 × 100 = 50 V. 1 A and 12 Ω is 12 V. 10 A and 23 Ω is 230 V. Always I times R.
Which units do I type?
Amperes and ohms. The result is in volts. 2.2 kΩ is 2200 Ω in the field, not 2.2. 20 mA is 0.02 A.
What about I = 0 A?
The calculator rejects zero amperes. U = 0·R says nothing about the circuit, and R = U/I does not exist at zero.
Can R be zero?
On paper U would be 0. A real circuit always has some resistance, so zero ohms is rejected here.
Where do I get R from U and I?
On the resistance-from-Ohm page. Here you compute U, there R. Same triplet, different unknown.
How do I get power after U?
P = U·I on the power page. 50 V and 0.5 A is 25 W. This calculator stops at volts.
Series and parallel resistors?
Series: a sum. Parallel: product over sum. Sibling pages. The calculator uses one R already.
How does I fall when R rises at 230 V?
That is I = U/R, not this card. Here you type I and R and get U. The unknown sits on the other side of the formula.
Does a comma in 0.5 A work?
Yes. 0,5 and 0.5 are the same current. The calculator does not require a period.
Is 10 A and 23 Ω really a wall socket?
It is the 230 V order of magnitude, not a model of the installation. The calculator multiplies two numbers and does not check a breaker.
Knowledge sources
Ohm’s law in this calculator is U = I × R or R = U/I. Units follow SI; NIST SP 330 and BIPM define the volt, ampere and ohm.
Page updated in 2026.