How much grout for 100 square feet of 12x24 tile?

A 12 × 24 puts less joint into every square foot than a 12 × 12, so it ought to be the cheaper floor to fill. Whether it actually is comes down to one word on the carton: rectified.

CUSTOM Building Products' Technical Bulletin TB85 sets the floor, and it is not a matter of taste. "ANSI defines the requirement for grout joints in ceramic and natural stone tile installations in A108.02 Section 4.3.8. This section states that under no circumstances should the grout joint be less than 1/16" (1.6 mm)." Above that: "TCNA recommends a minimum 1/8" grout joint for rectified tile and minimum 3/16" grout joint for calibrated tile." A 12 × 24 always has a side longer than 15 in, so it sits in that bracket by definition, and the hairline joint a small tile can carry is simply not on offer.

Push both minimums through the geometry, at 3/8 in thick, per 100 sq ft:

Joint	Grout
1/8 in — rectified minimum	4.7 lb
5/32 in — rectified plus 1/32 in warp	5.9 lb
3/16 in — calibrated minimum	7.0 lb

The identical floor in 12 × 12 at a 1/8 in joint takes 6.25 lb. So a rectified 12 × 24 saves you about 25%, while a calibrated one costs 12% more than the small tile it was meant to beat. End to end within its own bracket the swing is 49% — bigger than anything the change of tile size buys you.

The middle row is TB85 as well: "if the edge warping of a rectified tile is 1/32", the minimum grout joint is 1/8" + 1/32" = 5/32"." A quarter more grout than the headline figure, for a tile that still counts as rectified.

This page starts you at 3/16 in because that is the number you cannot be under without knowing more about your tile. Do not pinch the joint to save a bag, either — the same bulletin is blunt about the result: "grout that is placed in joints that are too narrow will be powdery and generally discolored."