Example 1
U = 12 V, I = 0.5 A → R = 24 Ω.
Resistance from Ohm's Law calculator
Type voltage U [V] and current I [A]. The calculator computes resistance R = U/I. 12 V at 0.5 A is 24 Ω. 230 V at 10 A is 23 Ω. Zero amperes does not divide.
Voltage from Ohm: U = I·R. Power: P = U·I.
Result
Voltage U (V) and Current I (A). The result shows up here.
How it works
Resistance from Ohm's law is R = U/I. At 12 V and 0.5 A you get 24 Ω. At 230 V and 10 A you get 23 Ω. An LED at 5 V and 0.02 A is 250 Ω. 12 V and 24 A is 0.5 Ω. Same Ohm triplet, only the unknown is R, not U.
The form has two fields: voltage U in volts and current I in amperes. The result is in ohms. Convert milliamps: 20 mA is 0.02. A comma and a period mean the same I: 0,5 and 0.5.
I must not be zero, because you do not divide by zero. Typed 0 in I is rejected. Zero volts at nonzero I gives R = 0. Both fields need a number before 24 Ω appears.
The full Ohm page computes U = I·R. Here you invert the formula and solve for ohms. Series and parallel already combine two R values; the calculator uses measured U and I and returns one R.
Once you have R, power is P = U·I. At 12 V and 0.5 A that is 6 W. Wire resistance from ρ, l, and S is a different page: material and geometry there, a U and I measurement here.
Type 12 and 0.5, click Calculate, and check 24 Ω. Then 230 and 10: 23 Ω. The calculator divides two numbers; it does not check a breaker or a wire gauge.
How to use
- In the first field enter voltage U in volts, for example 12 or 230.
- In the second field enter current I in amperes, for example 0.5. Type 20 mA as 0.02.
- Click Calculate. The calculator divides U by I. 12 V and 0.5 A give 24 Ω. 230 V and 10 A give 23 Ω.
- I = 0 is rejected, because the calculator does not divide by zero. Zero volts at nonzero I gives R = 0.
- Need voltage from I and R? Open Ohm's law U = I·R. Here it stays R from U and I.
Formula
R = U / I
I must not be zero.
R, U, and I on the Ohm card
The unknown here is resistance: R = U/I. 12 V and 0.5 A give 24 Ω. 230 V and 10 A give 23 Ω. Zero amperes does not divide.
- R
- Resistance [Ω] from U/I. 12 / 0.5 = 24 Ω. 5 V and 0.02 A is 250 Ω.
- U
- Measured drop [V]. 12 or 230. Zero volts at nonzero I gives R = 0.
- I
- Measured current [A]. Type 20 mA as 0.02. I = 0 is rejected.
Real-life examples
Example 2
U = 230 V, I = 10 A → R = 23 Ω.
Example 3
LED: R = 250 Ω.
Example 4
R = 90 Ω.
Example 5
U = 460 V, I = 20 A → R = 46 Ω.
Example 6
Starter: 12 V, 150 A → R = 0.08 Ω.
Example 7
USB charger: 5 V, 2 A → R = 2.5 Ω.
Example 8
24 V string: 24 V, 4 A → R = 6 Ω.
Ways to use this calculator
- You compute 24 Ω from 12 V and 0.5 A.
- You check a heater: 230 V and 10 A is 23 Ω.
Frequently asked questions
What is R at 12 V and 0.5 A?
Resistance is 24 Ω. You divide 12 V by 0.5 A, the first example on the page.
What about 230 V and 10 A?
You get 23 Ω. 230 V mains at ten amperes is heater territory, not 12 V electronics.
Which units do I type?
Voltage U [V], current I [A]. Result R [Ω]. Type 20 mA as 0.02 A.
What if I = 0 A?
The calculator refuses zero amperes. R = U/I at zero current does not exist, because you do not divide by zero.
How is this different from U = I R?
Here you solve R = U/I. There you compute voltage from current and resistance. Same Ohm triple, different unknown.
Does a comma in 0,5 work?
Yes. 0.5 and 0,5 are the same current I [A]. A comma and a period mean the same value.
Where is wire resistance from ρ?
On the wire-resistance page. Here R comes from U and I, not from length, area and resistivity.
Can R come out as a fraction?
Yes. 12 V and 24 A give 0.5 Ω. An ohm does not have to be a whole number.
Where is P = U I?
On the electric-power page. 12 V and 0.5 A is 6 W. Here you compute resistance, not power.
What about 5 V and 0.02 A?
Resistance is 250 Ω. Five volts at 20 mA is the order of a small electronics resistor.
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.