Spiral Window Balance Replacement

A quick learner from Texas says:
March 7, 2018
I need to replace 4 window balances, but I've found sizing them is really difficult. I believe it is 3/8 in. across, but do I measure the interior spring or the plastic covering as shown in the attached picture? Also the length is an issue. Will it be 24 in., the length of the interior spring, or 25 in., the length of the covering plastic sleeve? It's a single hung balance with 2 pins. Also the window itself is about 20lbs. From this description can you recommend a replacement?
User submitted photos of a window balance.
A quick learner from Texas says:
March 9, 2018
And what's difficult about choosing the correct balance is on the old balance the Plastic covering is 25 in. long
the circular spring is 24 in. long. and
the inner most cork screw is 26 in. long.

So which measurement do I us?
Profile picture of Paul
Paul from SWISCO responded:
March 12, 2018
Thanks for contacting us. The length of the cover is what you want to go with. If it's 25", I'd go with 25". You also want to go by the diameter of the outer tube, not the inside spring. If it's 5/8", go with our S600-25.
25" Series 600 Spiral Tilt Window Balance
25" Series 600 Spiral Tilt Window Balance
S600-25
$8.95
25" Series 900 Spiral Balance
25" Series 900 Spiral Balance
S900-25
$8.70
How to replace a tilt window spiral balance.
How to replace a tilt window spiral balance.
VIDEO
WATCH >
How to Identify a Replacement Spiral Balance
How to Identify a Replacement Spiral Balance
VIDEO
WATCH >
How to choose the proper spiral balance tip color
How to choose the proper spiral balance tip color
GUIDE
READ >
How to adjust a spiral balance on a tilt window
How to adjust a spiral balance on a tilt window
GUIDE
READ >
Reply to Thread
Use this form to ask a follow-up question or to share your experience regarding this subject.
(your email address will be kept private and is only used to inform you when a response has been made)
Current wait time is 3-4 business days
We promise that one of our experts will personally get back to you as soon as they are able. Thank you very much for your patience and understanding. If your inquiry is time-sensitive, please feel free to reach us at (888) 991-1929 or at help@swisco.com.