Grid targets are a ruler built into the paper. Each grid line is labeled in MOA or MIL for your exact distance. Read your widest holes directly off the edges, subtract, and you have your group size in the angular units your scope uses — no calipers, no conversion math. You get separate horizontal and vertical spreads, which is more diagnostic than a single group size number because it tells you what's actually happening.
For center-to-center group size, subtract the caliber-specific bullet correction printed on the target. 37 calibers from .17 HMR to .50 BMG. Any distance from 1 to 1,400 yards or 1,280 metres. Free PDF download.