easy to calculate when you dont have right ohms

Um, I apologize for the smart remark answer... Using math is the correct way, changing the value of a modern film resistor is almost impossible.

The thought come to mind, as a boy I had a beat-up Tripplett meter that had a pop up dual meter housing. When changing the battery, the circuits were exposed, a 2 watt (big) carbon dog-bone resistor was trimmed to tolerance with a large notch in its center, painted the color of the 3d place "dot"...

That image remained in my mind when a resistor was needed that I could not double up. So I filed one to fit.

To our readers, any resistor that has changed value should be removed, there in no telling how much more it has changed, if it will change with heat/cool or be noisy...
Yah, I have to get back into my Zenith 53, I relied on those old resistors. They were initially fine, then after a few hours of operation, they have changed value and change when ever the radio warms up and the radio looses sensitivity. It a good radio for about 10 minutes  

A project when I get "caught-up"...

I got a bargain on 1% 2 watt metal film resistors so I can hit the OEM values. I intend to paint over them so they will "agree" with the Zenith markings... They are as big as the 1/2 watt resistors in the radio... So they will look and reach the existing terminals

Chas

Thanks Chas! Of course you are absolutely right for non RF applications, I forgot about that method.

Tolerances of just common radios and amplifiers are not usually considered because of parts tolerances used (5-20%) 

Question follows, are you allowing for current, or is doing that, reducing wattage of resistor or voltage?

I think it gets more complicated when working with AC and RF. That may need some extra thought.

Were not talking design considerations or measurements tolerances, when its not used in critical situations. Just my opinion. Previous thoughts just popped up while posting.

Of course your talking about the old style carbon composition resistors, right?
I have almost non of that type, but a lot more of the ceramic, film and wire wounds.

But another point would be, filing an old out of tolerance resistor would bring it back into tolerance.

Use a 1.8K carbon resistor and file a notch in the center, test, keep filing until value is found. Seal with clear nail polish or a bit of clear lacquer...

I used to have a program in a plugin card for my former HP-41 Calculator. Best calculator at the time.
Now I have search to come up with a program but I will make my own I guess on my HP 48SX.

OK here's the fun part.

I need to replace a resistor at my bench project but I don't have whats needed, what do I do?

I found a resistor that's almost close enough but not within the percentage of the value needed.

I need a goofy value of  1.93Kohms

I have on hand a 2.5Kohm resistor. Since it's higher in value than whats needed but close I will use it.

So I need to use another unknown value resistor to be connected in parallel to approach the replacement Value.
 So to find what I need is to use this formula and do some implementations.

So Here we go:
1.  required value?  = Rt = Required Total value. = 1.93Kohms

2.  known found close value resistor = R1 = 2.5Kohm

3.  search for this value at the store or stash. = R2

Formula to solve problem?

R2 = (R1 * Rt) / (R1 - RT)

 
 2.5KOHM (R1) X  1.93KOHM (RT)
---------------------------------------
 2.5KOHM (R1)  -  1.93KOHM   (RT)

=   4,825,000
     ---------
      570.00

=  8.464KOhms
OR  
=  8.5KOHMS
(IF IT'S NOT THAT TIGHT OF A TOLERANCE)