# What is resistance tolerance?

by hover
Tags: resistance, tolerance
 P: 338 Say I have a 20 ohm resistor with a tolerance of 5%. What does it mean to have a tolerance of 5%?? Thanks
Mentor
P: 38,915
 Quote by hover Say I have a 20 ohm resistor with a tolerance of 5%. What does it mean to have a tolerance of 5%?? Thanks
The value is guaranteed to be between 0.95 * R and 1.05 * R.
Mentor
P: 38,915
 Quote by berkeman The value is guaranteed to be between 0.95 * R and 1.05 * R.
Another way to say it is +/- 5%

P: 338

## What is resistance tolerance?

 Quote by berkeman The value is guaranteed to be between 0.95 * R and 1.05 * R.
So would it make perfect sense to say that for this example that the value of this resistor is guaranteed to be between (.95)(20) and (1.05)(20) which is 19 ohms and 21 ohms? Is that the correct idea?

thanks
Mentor
P: 38,915
 Quote by hover So would it make perfect sense to say that for this example that the value of this resistor is guaranteed to be between (.95)(20) and (1.05)(20) which is 19 ohms and 21 ohms? Is that the correct idea? thanks
Exactly. 5% of 20 Ohms is 1 Ohm.
 P: 338 Just one more question Say you put 2 of these resistors in series. The resistance will jump to 40 ohms but what happens to the tolerance? Will it double or stay the same?
Mentor
P: 38,915
 Quote by hover Just one more question Say you put 2 of these resistors in series. The resistance will jump to 40 ohms but what happens to the tolerance? Will it double or stay the same?
You can figure that one out for yourself, using the equation for the total resistance.

You know that the smallest you can get is 19+19 Ohms, and the largest is 21+21 Ohms. What tolerance does that indicate.

Now do it for the parallel resistor case.... Do you get a similar or different answer...?
P: 338
 Quote by berkeman You can figure that one out for yourself, using the equation for the total resistance. You know that the smallest you can get is 19+19 Ohms, and the largest is 21+21 Ohms. What tolerance does that indicate. Now do it for the parallel resistor case.... Do you get a similar or different answer...?
Oh ok I see. Just do 2(19)/40 and 2(21)/40 and you get the same tolerance of 5% I believe. In other words, whether in series or parallel, the tolerance doesn't change.

Thanks!

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