Tile Grout & Mortar Calculator

The grout formula

Joint length per sq ft = 288 × (L + W) ÷ (L × W) inches, with L and W in inches
Grout (lb) = area × joint length × joint width × tile thickness × 0.0284 × 1.1

The 0.0284 is the density of sanded grout in pounds per cubic inch, and the 1.1 adds 10% for waste. For 12-inch tile the joint length is 48 inches per square foot; for 6-inch tile it doubles to 96, which is why small formats eat so much more grout.

Worked examples

100 sq ft, 12-inch tile, 1/8 inch joint, 3/8 inch deep → 100 × 48 × 0.125 × 0.375 × 0.0284 × 1.1 = 7.0 lb
200 sq ft, 6-inch tile, same joint → 200 × 96 × 0.125 × 0.375 × 0.0284 × 1.1 = 28.1 lb
100 sq ft, 18-inch tile, same joint → 100 × 32 × 0.125 × 0.375 × 0.0284 × 1.1 = 4.7 lb
100 sq ft, 12-inch tile, 1/4 inch joint → double the width, so 14 lb

Thinset comes from the trowel, not the tile

Mortar use is set by the notch that spreads it. The calculator uses midpoints of published 50 lb bag coverage: 1/4 × 1/4 inch covers about 95 sq ft, 1/4 × 3/8 inch about 63.5, 1/2 × 1/2 inch about 44.5, and a 3/4 × 9/16 inch U-notch about 36. Bags = area × 1.1 ÷ coverage, rounded up.

The notch also has to suit the tile. Up to 8-inch tile takes the quarter-inch square, up to 17-inch the 1/4 × 3/8, up to 24-inch the half-inch square, and anything larger the U-notch. A smaller notch than the tile needs risks hollow spots; a larger one wastes mortar and can push tiles out of plane.

What the estimate leaves out

Grout is clamped to a minimum 1/16 inch joint and 3/16 inch depth, so a very thin tile with a hairline joint still gets a usable figure rather than zero. Bag counts assume 25 lb grout bags and 50 lb thinset bags; a 10 lb grout bag means buying more units for the same weight.

Uneven substrates, thick-bed installs, back-buttering large-format tile and trowel ridges that collapse in hot weather all add mortar. Order one spare bag for a small job and 10% extra for anything over 200 sq ft.

Frequently Asked Questions