Example 1
Rz = 50 Ω.
Resistors in Parallel Calculator
Type R1 [Ω] and R2 [Ω]. The calculator computes Rz = R1·R2/(R1+R2): two 100 Ω resistors are 50 Ω, and 60 Ω and 40 Ω are 24 Ω. In parallel the voltage is the same and currents add.
Series: Rz = R1+R2. Ohm's law: U = I·R.
Result
Resistance R1 (Ω) and Resistance R2 (Ω). The result shows up here.
How it works
Parallel equivalent resistance obeys 1/Rz = 1/R1 + 1/R2, or Rz = R1 R2 / (R1+R2). At 100 Ω and 100 Ω you get 50 Ω. At 60 Ω and 40 Ω: 2400 / 100 = 24 Ω. At 10 Ω and 10 Ω you get 5 Ω. Rz is always smaller than the smaller of the pair, because current has two paths.
The form has two fields: R1 and R2 in ohms, both positive. The result is in ohms. Type 2.2 kΩ as 2200. A comma and a period mean the same R: 47,5 and 47.5.
R = 0 will not run: the reciprocal of zero does not exist. Both fields need a positive number before 50 Ω appears. In parallel the voltage on both is the same, and currents add: I1 + I2 = Iz.
Series Rz = R1 + R2 grows. Here Rz drops. Ohm's law U = I R already takes one R, for example this Rz. This calculator combines two paths; it does not compute watts.
Two identical R values give half: 100 || 100 = 50. A very large R2 barely changes Rz, because 1/R2 is small. Three branches take two steps: first a pair, then that Rz with the third.
Type 100 and 100, click Calculate, and check 50 Ω. Then 60 and 40: 24 Ω. The calculator computes product over sum; it does not lay out a bridge.
How to use
- In the first field enter R1 in ohms, for example 100. Both R values must be positive.
- In the second field enter R2 in ohms, for example 100 or 40.
- Click Calculate. Two 100s give 50 Ω, because 1/Rz = 1/100 + 1/100. 60 and 40 give 24 Ω.
- Zero in R1 or R2 is rejected. Both fields need a positive number.
- For the sum R1+R2, open resistors in series. One R for U = I R is on Ohm's law.
Formula
Rz = R1·R2 / (R1 + R2)
R1 > 0, R2 > 0.
Letters of two parallel resistors
In parallel Rz = R1·R2/(R1+R2). Two 100 Ω resistors give 50 Ω. 60 Ω and 40 Ω give 24 Ω. Shared voltage, currents add.
- Rz
- Parallel equivalent [Ω]. Always smaller than the smaller of R1 and R2. Two equals give half.
- R1
- First branch [Ω], for example 60. Current in it is U/R1.
- R2
- Second branch [Ω], for example 40 Ω. R1·R2/(R1+R2) gives 24 Ω.
Real-life examples
Example 2
Rz = 24 Ω.
Example 3
Rz = 5 Ω.
Example 4
Rz = 132 Ω.
Example 5
Rz = 500 Ω.
Example 6
Rz ≈ 32 Ω.
Example 7
Rz = 3 Ω.
Example 8
Rz ≈ 91 Ω.
Example 9
Rz = 1 Ω.
Example 10
Rz = 100 Ω.
Ways to use this calculator
- You check two 100 Ω resistors: 50 Ω.
- You compute 24 Ω from 60 Ω and 40 Ω.
Frequently asked questions
How much Rz with two 100 Ω?
Equivalent resistance is 50 Ω. 1/Rz = 1/100 + 1/100, or 100×100 / 200. Two equal resistors in parallel give half.
How much at 60 Ω and 40 Ω?
You get 24 Ω. The product 60×40 over the sum 100. Rz is smaller than 40 Ω, the smaller branch.
Which units do I type?
Both resistances R1 [Ω] and R2 [Ω], both positive. Result Rz [Ω].
How is this different from series?
In parallel 1/Rz = 1/R1 + 1/R2 and Rz is smaller than each. Series adds: Rz = R1 + R2.
Does a comma in 47.5 work?
Yes. 47.5 and 47,5 are the same R [Ω]. A comma and a period mean the same value.
Is the voltage the same?
Yes. In parallel both branches sit on the same U [V]. Currents I = U/R add, which is why Rz is smaller.
Why is Rz smaller?
Because current has two paths. More current at the same U means a smaller equivalent resistance.
Where is series?
On the series-resistors page. There a sum. Here the product over the sum.
What about R = 0?
Zero ohms is a short. The calculator cannot form a clean Rz then, because you divide by R1+R2 with a zero branch.
How much at 10 Ω and 10 Ω?
You get 5 Ω. Two equal resistors in parallel always give half of one.
Knowledge sources
The formula is the school one. Units follow SI; NIST SP 330 and BIPM define the measures, not your result.
Page updated in 2026.