Trees or standing timber are measured in inches of diameter at DBH (diameter breast height).
Yeah, Wolfman these guys have it.
DBH is measured at ~ 4.5 feet from the ground. If the tree is on a slope, measure from the uphill side. For me, at 6'4", it's at the bottom of my breastbone.
Like Newbie and sawmill said, there are tables that predict Board feet or cords per tree (pulp or firewood). They vary by different calculations, or "Rules," one of which is saw kerf at the ultimate sawmill. Common are Doyle, Scribner and International 1/4" scales, but vary by area. They will often predict "Form Class," which is the taper typical for Western softwoods, Northern hardwoods, etc.
Around here, marketable hardwood is 10" at the small end. We don't buy/sell pulp much anymore here - that was 3" small end. If making a tally of trees/board feet, you can often predict % of cull too, to get a better/closer estimate.
You can make a D-tape from rope or make calipers. I make one of rope every few years - because I always lose the one I made before......
Since I don't cruise timber often, never made sense to buy a D-tape, but they're not that expensive. Usually, someone just wants me to look at their woodlot for a possible timber cut - so I don't need to real exact. Rope works fine.
Since circumference = Pi*Diameter, a mark on your rope at 50.24" will equal 16" DBH, etc. Typically go even DBH's, 24" class, 26" class, etc.
If you make calipers, you would take couple measurements on each tree, and average. There's also a Biltmore (?, is that right guys?) stick, which is a one-handed way to do it.
They also use a Prism, which is cool, but damned if I remember how to use one. It's more for cruising stands for a general idea of stocking in total.
I looked for my last D-rope - can't find it......
