I usually divide the section width by 25.3, so the 355 divided by 25.3=14.03
Here's everything you need to know about the tite if it's a Bridgestone Pillow Dia ( turf tread):
TIRE
SIZE METRIC 355/80D20
CONVENTIONAL
TUBE/TUBELESS TUBE TYPE
ARTICLE NUMBER 344-982
PLY RATING 4
STAR RATING
LOAD INDEX
RIM WIDTH (in.) 11
OVERALL WIDTH (in.) 13.9
OVERALL DIAMETER (in.) 42.4
STATIC LOADED
RADIUS (in.) 19.5
ROLLING
CIRCUM. (in.) 124
ROLLING
CIRCUM. INDEX
BAR HEIGHT
32NDS (in.) 13
NO. OF LUGS
FLAT PLATE (in²)
TIRE WEIGHT (lbs.) 82
MAX. SPEED (mph) 20
MAX LOAD & INFL.
(LBS. @ PSI) 1820 @ 15
One thing I'll forewarn you about this particular tire, is that it's tube type and often Kubota doesn't put tubes in them from the factory. That causes the tire to weather check prematurely, and if loaded with ballast accelerates that condition big time. They'll leek like a seive and it's not the tires fault but the manufacture that failed to put it together right.
On that note, if that happened to you on a new tractor purchase I'd be all over my dealer to get ahold of Kubota and have them make it right even if includes replacement of the tires and of course include the tubes.
You'll have a tough time finding a 388/80-20 tube by the way, and very pricey of you do. Search instead for a combination 15/18R19.5 with the TR218 valve stem and it's a perfect fit. Being close to Soo Michigan you can always cross the bridge as U P Tire in Soo Michigan stocks them.
Al