Coverage depends on tile size and trowel size
Thinset coverage is never one fixed number. A smaller trowel notch spreads less mortar and usually covers more square footage per bag than a deeper notch. Larger tile also tends to need a heavier mortar bed than smaller tile, which reduces real coverage.
That is why a useful thinset mortar calculator combines tile-size and trowel choices rather than just dividing area by a single generic coverage figure. The result is still an estimate, but it is much closer to the real bag count than a flat rule of thumb.
Bags = ceil(Area / Coverage per bag) + Waste bags
Coverage per bag depends on tile size, trowel notch, substrate flatness, and the specific mortar product you choose.