How the thinset calculator works
Trowel size is the hidden variable
Thinset quantity is really trowel geometry: a 1/4" notch spreads ~25 lb per 100 sq ft, a 1/2" notch spreads ~60. Tile size dictates the trowel (bigger tiles need fuller coverage to prevent lippage and hollow spots), and the trowel dictates the bag count. Under-troweling large tiles is the leading cause of cracked tile over slab movement.
Modified vs unmodified
Modified thinset (polymer-enhanced) is the general-purpose standard for cement board and most interior jobs. Unmodified pairs with waterproof membranes (Schluter-style) that provide their own bonding layer. The membrane or board manufacturer's spec wins every argument — using modified over some membranes voids their warranties.
Mix discipline
Thinset has a working window of 30–45 minutes and a pot life that shortens in heat. Mix small batches (half bags), let it slake 10 minutes, and never retemper (re-wet) stiffened thinset — the polymer is spent. The discipline that feels wasteful is what keeps every tile bonded like the first.