About this calculator
Grout is sold by weight, but what you need depends on the volume of the joints: how long they are, how wide, and how deep. Small tile has far more joint per square foot than large tile, so a mosaic backsplash can take more grout than a whole floor of 24 in tile. This grout calculator works out that joint volume from your tile size, tile thickness and joint width, turns it into pounds, and rounds up to the bags or boxes sold in stores.
It also tells you which kind to buy. Sanded grout is for joints of ⅛ in and wider; unsanded grout is for narrower joints and for polished stone or glass that sand would scratch. The drawing shows a patch of your tile with every joint to scale, and a section through one joint so you can see the width and depth the grout has to fill.
How to use it
- Measure the length and width of the area. For an L-shaped room or a backsplash with a window, split it into rectangles and add up the results, or use the square footage calculator.
- Enter the tile size and its thickness from the box label. The joint is as deep as the tile is thick.
- Enter the joint width, the size of your spacers: 1/16 in for rectified tile, ⅛ in for most floor tile, ¼ in or more for handmade or rustic tile.
- Leave the grout type on automatic unless your tile maker asks for a particular one, and enter your local prices to see the cost.
The formula
Joint length per sq ft (in) = (L + W) ÷ (L × W) × 144
Joint volume per sq ft (cu in) = joint length × joint width × tile thickness
Grout (lb) = joint volume ÷ 1,728 × 110 lb per cu ft × area × (1 + extra%)
Bags = grout ÷ 25 lb (sanded) or 10 lb (unsanded), rounded up
L and W are the tile’s length and width in inches. Cement grout weighs about 110 lb per cubic foot of joint, which we worked out from Custom Building Products’ Polyblend coverage chart: a 25 lb bag of sanded grout covers 348 sq ft of 12 × 12 × ⅜ in tile with ⅛ in joints, and this formula gives 349. Across the rest of the sanded and non-sanded charts it comes within about 5%. For that tile and joint, 100 sq ft takes 7.2 lb, or 7.9 lb with 10% extra.
Worked example
These are the calculator’s starting values and the results they produce:
| Area length | 10 ft |
|---|---|
| Area width | 10 ft |
| Tile length | 12 in |
| Tile width | 12 in |
| Tile thickness | 0.375 in |
| Grout joint width | 0.125 in |
| Grout type | Pick by joint width (sanded from ⅛ in) |
| Extra for the float and sponge | 10 % |
| Sanded grout, 25 lb bag | $25.00 |
| Unsanded grout, 10 lb bag | $20.00 |
Tips before you buy
- Doubling the joint width doubles the grout. The same 100 sq ft of 12 × 12 tile takes about 7.9 lb with ⅛ in joints and 15.8 lb with ¼ in joints.
- Buy all the grout for one room at once, from the same color and batch, and mix it the same way every time so the color stays even.
- Mix only what you can use in about 30 minutes, and work small sections: pack the joints diagonally with a rubber float, then sponge off the haze with as little water as you can.
- Don’t grout the gap at the walls, at a tub or at changes of plane. Those are movement joints and get a matching caulk instead.
- Keep a little leftover grout and the color name, so you can repair a joint later.
Frequently asked questions
How much grout do I need for 100 square feet?
For 12 × 12 in tile ⅜ in thick with ⅛ in joints, about 7.2 lb, or 7.9 lb with 10% extra: one 25 lb bag of sanded grout. Smaller tile and wider joints take more; with 6 × 6 × ¼ in tile and ⅛ in joints it is about 9.5 lb before the extra.
How many square feet does a 25 lb bag of grout cover?
It depends on the tile and the joint. With ⅛ in joints a 25 lb bag covers about 260 sq ft of 6 × 6 × ¼ in tile, 350 sq ft of 12 × 12 × ⅜ in tile and 700 sq ft of 24 × 24 × ⅜ in tile. With ½ in joints and 12 × 12 tile it covers only about 87 sq ft.
Should I use sanded or unsanded grout?
Use sanded grout for joints ⅛ in and wider (up to ½ in); the sand keeps wide joints from shrinking and cracking. Use unsanded grout for joints narrower than ⅛ in, and for polished marble, glass or metal tile that sand could scratch.
How much grout do I need for a subway tile backsplash?
A 30 sq ft backsplash of 3 × 6 in subway tile ¼ in thick with 1/16 in joints needs about 2.1 lb of grout, or 2.4 lb with 10% extra: one 10 lb box of unsanded grout, with plenty left for repairs.
Sources
Areas and volumes are standard geometry. The published figures this calculator relies on come from these sources; other defaults (waste allowances, typical densities and prices) are common trade figures worth checking with your supplier.
- Custom Building Products Polyblend sanded grout data sheet TDS-129: sanded grout for joints ⅛–½ in, 25 lb bag coverage chart (used for 110 lb per cu ft of joint)
- Custom Building Products Polyblend non-sanded grout data sheet TDS-130: non-sanded grout for joints up to ⅛ in, 10 lb box coverage chart