From our test site.
Dimensions 20 x 1 x 17 "
Billable weight: 1.625 lb
Dimensional weight: 2.0481927 UPS will round off to 3lb
Package weight was 1.6lbs
UPS will use the billable weight 3 lb (Dimensional weight) except for UPS Ground where Dimensional weight does not apply.
The UPS API will provide the dimensional wight and the actual weight pricing on your website.