How to calculate it yourself
Grout volume (in³) ≈ (linear feet of joint × joint width × tile thickness × 12). Linear feet of joint per ft²: 24 ÷ tile width + 24 ÷ tile length. Most retail bags list "covers X ft² of [tile size] at [joint width]" — read the label to confirm. A 10-lb bag of standard sanded grout covers roughly 50-100 ft² of 12×12 with ⅛" joints.
Common scenarios
Bathroom floor (40 ft², 12×12 tile, ⅛" joint)
40 ft² × 0.07 lbs/ft² = 2.8 lbs. Buy one 10-lb bag minimum (smallest standard size). Leftover is fine — tightly seal the bag and it lasts 1-2 years dry.
Kitchen backsplash (22 ft², 3×6 subway, 1/16" joint)
Tighter joints need less grout. About 22 ft² × 0.06 lbs/ft² = 1.3 lbs. A 10-lb bag is overkill — many subway-tile projects use a smaller pre-mixed grout tub (3.5-5 lbs) instead.
Mosaic shower floor (16 ft², 2×2, ⅛" joint)
Mosaics use 3× more grout than 12×12 because of the joint density. 16 × 0.20 lbs/ft² = 3.2 lbs. Buy one 10-lb bag and have plenty for the wall mosaic in the same shower.