Grout Calculator

How it works

Grout fills the joints between tiles, so the amount needed is the volume of those joints. Each tile, with the joint along one long and one short side, forms a repeating module (tile length + joint) × (tile width + joint); the joint takes joint × (length + width) of that module. That share of the tiled area, times the joint depth, is the joint volume.

Multiplying the volume by how many pounds of grout fill a cubic foot of joint gives the weight. Manufacturers publish coverage charts rather than that figure, so the defaults are values that reproduce CUSTOM Building Products' published charts: 113 lb per cubic foot matches its Polyblend Sanded Grout chart (TDS-129, published 3/22/2023) to within about 1%, and 107 lb its Polyblend Non-Sanded Grout chart (TDS-130, published 3/22/2023) to within about 1.3%. Other grouts differ; the coverage chart on your product's data sheet is the better check.

Small tiles and wide joints use far more grout, because there is more joint per square foot. CUSTOM's technical bulletin TB89 (2016) notes that a joint planned at 1/8 in that comes out 1/32 in wider needs 25% more grout; the joint width has a large effect.

Formula

joint share = j × (L + W) ÷ ((L + j) × (W + j))      (L, W, j tile and joint sizes)
volume (cu ft) = area (sq ft) × joint share × depth (in) ÷ 12
grout (lb)  = volume × lb per cu ft × (1 + waste % ÷ 100)
bags        = grout ÷ bag size, rounded up
coverage per bag = area × bag size ÷ grout without waste

Example

100 sq ft of 12 in × 12 in tile, 3/8 in thick, with 3/16 in joints: the joints take 0.1875 × 24 ÷ (12.1875 × 12.1875) = 3.03% of the surface, a volume of 100 × 0.0303 × 0.375 ÷ 12 = 0.095 cu ft.

At 113 lb per cubic foot for sanded grout that is 10.7 lb, so one 25 lb bag covers about 234 sq ft, the figure in CUSTOM's Polyblend Sanded Grout chart (TDS-129, 2023) for this tile and joint. With a 10% allowance, 11.8 lb: 1 bag.

Assumptions and limitations

• Joints are full depth and uniform in width. CUSTOM's TB89 bulletin points out that mortar left in the joints reduces grout use and that variations in tile size, edges and alignment change it.
• The pounds-per-cubic-foot defaults are values fitted to CUSTOM's Polyblend coverage charts (TDS-129 and TDS-130, both published 3/22/2023), not figures printed on a data sheet, and are labelled typical. Other brands, epoxy grouts and single-component grouts have different coverage, given on each product's technical data sheet.
• CUSTOM's 2023 data sheets list Polyblend Sanded Grout for joints from 1/8 in to 1/2 in and Polyblend Non-Sanded Grout for joints up to 1/8 in. The calculator does not check the joint width against the grout type.
• The waste allowance is a typical figure. Grout lost to clean-up, mixing more than can be used and porous or textured tile all add to the real figure.
• The result is an estimate. It is not a substitute for the grout manufacturer's instructions.

Frequently asked questions