How grout quantity is worked out
Grout fills the gap around every tile, so the amount you need depends on four things: how big the tiles are, how wide the joints are, how thick the tile is, and how much floor or wall you're covering. The industry rule of thumb is lbs per ft² = ((tile length + tile width) ÷ (tile length × tile width)) × joint width × tile thickness × 14.5, with all tile dimensions in inches. The first part of that expression is really a measure of how much joint edge exists per unit of tile surface — which is why small tiles are so much thirstier than large ones. Swap 12″ tiles for 2″ mosaic and the grout requirement jumps roughly six-fold for exactly the same floor area.
A few things the formula can't know. Unsanded grout and sanded grout have different densities, so a bag of one won't stretch as far as a bag of the other. Tiles with a heavily textured or lugged back, and any tile set on an uneven substrate, swallow extra grout that never shows on the surface. Rectified porcelain laid tight with 1/16″ joints uses very little. Because of that spread, buy at least one bag more than the calculation says, and mix in full bags where you can — part-bags mixed at different water ratios are the usual cause of grout that dries to visibly different shades across a floor. Keep any unopened spare for repairs; a bag from the same batch will match, a fresh one bought a year later probably won't.
This is an estimate. Coverage varies by grout type, brand, tile back profile and how flat your substrate is — check the coverage chart on the bag and confirm with your supplier before buying.