How much grout do I need? Formula, coverage and examples

Grout is the cheapest material on a tile job and the easiest one to get wrong. Buy too little and the second batch mixed halfway across the floor can dry a slightly different shade. Buy far too much and a bag sits in the garage until it goes hard. The good news: "how much grout do I need?" is pure geometry — the size of the joints times how many of them there are. Here is the formula, with worked examples for a floor, a shower and a mosaic.

What decides the amount

Four numbers, all of which you know before you start:

• Tile size — small tiles mean many more joints per square foot
• Joint width — the spacer size, such as 1/16", 1/8" or 3/16"
• Joint depth — a full joint is grouted to the tile's face, so this is the tile thickness
• Grout type — mixed cement grout weighs roughly 100–125 lb per cubic foot depending on sand content, and epoxy is heavier

The formula

Think of each tile plus half of the joint around it as one repeating cell:

cell = (tile width + joint) × (tile length + joint)
joint area = cell − tile width × tile length
cubic inches per sq ft = joint area × depth ÷ (cell ÷ 144)
pounds = sq ft × cubic inches per sq ft × density ÷ 1,728

The one mistake to avoid: include the joint in the cell. Dividing by the bare tile area over-counts how many tiles fit in a square foot. The last step divides by 1,728 because there are 1,728 cubic inches in a cubic foot.

Use the density for the grout you are buying. Planning figures: unsanded about 100 lb/ft³, sanded about 110 lb/ft³, epoxy about 125 lb/ft³.

Worked example 1 — a kitchen floor

A 120 sq ft floor in 12 × 24 in porcelain, 3/8 in thick, with 1/8 in joints and sanded grout. Grout goes between the tiles, so work from the area including the tile waste allowance — 10% for a straight lay, making 132 sq ft.

• Cell: 12.125 × 24.125 = 292.5 sq in; joint area = 292.5 − 288 = 4.5 sq in
• Per sq ft: 4.5 × 0.375 ÷ (292.5 ÷ 144) = 0.83 cubic inches
• Pounds: 132 × 0.83 × 110 ÷ 1,728 = about 7 lb

One 25 lb bag covers it with plenty to spare — at this size and joint, a 25 lb bag of sanded grout stretches to roughly 470 sq ft.

Worked example 2 — a subway-tile shower

90 sq ft of 3 × 6 in wall tile, 1/4 in thick, with tight 1/16 in joints. Joints under 1/8 in call for unsanded grout (sand will not pack into them and can scratch glazed or polished faces). With 10% waste, 99 sq ft:

• Per sq ft: about 1.10 cubic inches
• Pounds: 99 × 1.10 × 100 ÷ 1,728 = about 6.3 lb

Notice that this small tile with a 1/16 in joint uses about as much grout per square foot as 12 × 12 tile with a joint twice as wide (1.11 cubic inches): there are simply more joints.

Worked example 3 — a mosaic floor

Mosaics are where people run short. 40 sq ft of 2 × 2 in mosaic, 1/4 in thick, 1/8 in joints, sanded grout, 44 sq ft with waste:

• Per sq ft: about 4.1 cubic inches — five times the 12 × 24 floor
• Pounds: 44 × 4.1 × 110 ÷ 1,728 = about 11.5 lb

A 25 lb bag covers only about 95 sq ft of this mosaic, against about 355 sq ft of 12 × 12 tile at the same joint.

Quick coverage guide (25 lb bag)

From the same formula, at a full-depth joint:

• 2 × 2 mosaic, 1/4" thick, 1/8" joint, sanded: about 95 sq ft
• 6 × 24 plank, 3/8" thick, 1/8" joint, sanded: about 285 sq ft
• 12 × 12, 3/8" thick, 1/8" joint, sanded: about 355 sq ft
• 3 × 6 subway, 1/4" thick, 1/16" joint, unsanded: about 395 sq ft
• 12 × 24, 3/8" thick, 1/8" joint, sanded: about 470 sq ft

These run slightly conservative: for 12 × 12 × 3/8 at 1/8", Laticrete's published 25 lb sanded grout coverage chart lists about 388 sq ft against this model's 355, and for a 2 × 2 mosaic about 100 sq ft against 95. The chart printed on your grout bag is the final word.

Tips that save a second trip

• Round up to whole bags, and buy one batch. Check that every bag carries the same lot number so the colour matches.
• Sanded vs unsanded: unsanded for joints under 1/8"; sanded for 1/8" and wider — the default for most floors. The joint width itself comes from the tile; the tile spacer size guide shows the three-times-variation rule, and the grout calculator turns the joint into pounds and bags.
• Epoxy is sold in kits, not by the pound, so convert the result to kit sizes.
• Mix only what you can use before it starts to stiffen; the bag gives the working time.
• Silicone, not grout, at changes of plane — inside corners and where the wall meets the tub or floor need a flexible sealant.

Our app: Smart Tile Calculator

Smart Tile Calculator is a free Android app that works out the whole setting stack from one measurement: tile and boxes with the right waste for the pattern, thinset by trowel notch, grout by the exact joint formula above, and backer board, waterproofing, spacers and sealer. It picks the default grout depth from the tile thickness, flags sanded grout in a joint that needs unsanded, and shows its working for every number. The densities and examples here are the app's own defaults.

It is on Google Play for Android only, and the calculators also run in any browser at smarttilecalculator.com. Still counting tiles? See how much tile you need. Insulating the room first? See what R-value insulation you need.

Still counting the tiles themselves? How much tile do I need covers boxes, waste for each pattern and the thinset.