Thinset Calculator
Count thinset mortar bags from your tiled area and trowel notch size — the notch your tile size calls for sets the coverage per bag.
The trowel decides the coverage
Thinset is not spread flat — it is combed into ridges by the trowel's notches, and the ridges collapse into the bed under the tile. A bigger notch lays down more mortar, so coverage per bag falls as tiles get bigger: that is the whole calculation. The figures here are the mid-points of the coverage charts the major manufacturers (MAPEI, Custom Building Products, LATICRETE) publish for a 50-lb bag.
Bags = tiled area ÷ coverage per bag for your notch · 1/4 in notch ≈ 80 sq ft per 50-lb bag
Example: 100 sq ft of 12 in tile on a 1/4 × 1/4 in trowel needs 100 ÷ 80 = 1.25 bags exactly — 2 bags with the 10% allowance in, or three 25-lb bags if you prefer the lighter size.
| Trowel notch | For tile | Coverage per 50-lb bag |
|---|
| 3/16 in V-notch | Mosaics, small wall tile | ≈ 95 sq ft |
| 1/4 × 1/4 in square | To 12 in | ≈ 80 sq ft |
| 1/4 × 3/8 in square | 12–16 in | ≈ 60 sq ft |
| 1/2 × 1/2 in square | 16 in and up, large format | ≈ 40 sq ft |
Two habits keep real coverage near the chart: hold the trowel at a consistent 45 degrees (a flatter angle starves the bed), and back-butter large or warped tiles, which uses extra mortar the chart does not include — stay at the top of the waste range for large format work. Coverage also assumes a flat substrate; a wavy floor being flattened with the bed can double consumption in the low spots.
Frequently asked questions
What coverage do I actually need under a tile?
The standard is at least 80% mortar contact for dry interiors and 95% for wet areas and exteriors, with no hollow corners. The notch sizes exist to achieve that: if a lifted test tile shows stripes instead of a full bed, go up a notch size.
Modified or unmodified thinset?
Polymer-modified thinset is the default for most tile work — better bond and slight flexibility. Unmodified is specified between impervious layers (such as over a membrane with porcelain) where a modified mortar cannot dry. The bag counts are the same; the choice is about the system, not the quantity.
How far does one bag really go on a first tile job?
Plan on the chart figure, buy to the calculator's count, and expect the first room to run a little heavy while your trowel angle settles. An unopened spare bag returns to the store; a half-tiled floor at 6 pm does not forgive.
Can I use thinset to level a floor dip?
No — thinset is a bonding bed, not a filler; past about 1/4 in it shrinks and cracks. Dips belong to self-leveling compound or a patching product, applied before the tile work starts.
CalcCoverage