How to calculate it yourself
Bags needed = (tile area in ft²) ÷ (coverage per bag from chart) × 1.10 waste, rounded up. For natural stone or back-buttered tile, multiply by 1.30 instead. Pre-mixed thinset in plastic tubs (sold for small projects) covers about 30% less per pound than dry mix because of the water content.
Common scenarios
Bathroom floor (40 ft², 12×12 tile, ¼" trowel)
40 ÷ 85 × 1.10 = 0.52 bags. Buy 1 bag of 50-lb modified thinset ($25-35). Plenty of leftover for the threshold transition or a future repair.
Kitchen floor (180 ft², 18×18 tile, ½" trowel)
180 ÷ 35 × 1.10 = 5.7 bags. Buy 6 bags of premium polymer-modified thinset ($30-45/bag). Add another 1-2 bags if back-buttering.
Shower walls + floor (90 ft², mixed tile)
Walls (60 ft² of 6×6 tile, ¼" V-notch): 60 ÷ 100 = 0.6 bags. Floor mosaic (16 ft², ³⁄₁₆" V-notch): 16 ÷ 110 = 0.15 bags. Total: 1 bag covers it all comfortably with leftover for the ledger and curb.