Formula
Joint volume per unit area is (tile length + tile width) ÷ (tile length × tile width), times the joint width, times the joint depth. In inches that product is multiplied by 144 to land on cubic inches per square foot. In millimetres the same product is already litres per square metre. Weight is volume times density. Order weight adds the waste. Bags round up.
in³/ft² = ((L + W) ÷ (L × W)) × joint × depth × 144
order = volume × density × (1 + waste ÷ 100)
bags = ceil(order ÷ bag)
Small tiles use more grout because that fraction is larger: more joint length in each square foot. A 1 × 1 mosaic is nothing like a 24 × 24. Depth starts equal to the tile thickness. A joint under 1/8 in points at unsanded grout. A joint of 1/8 in or wider points at sanded grout. Cement density starts at 0.064 lb/in³, about 1.77 kg/L, so a 12 × 12 × 3/8 tile with a 1/8 in joint lands on Custom’s Polyblend Plus chart of 348 ft² per 25 lb bag. Epoxy starts at 0.045 lb/in³, about 1.25 kg/L, so the same tile lands on the CEG-Lite chart of 187 ft² per 9.5 lb unit. Both are editable. The chart on the product you bought wins.
Worked examples
12 × 12, 1/8 in joint, 3/8 in deep, 100 sq ft, 0.064 lb/in³. 1.125 in³/ft², 7.20 lb, 7.92 lb with 10 percent waste, one 10 lb bag or one 25 lb bag. A 25 lb bag covers about 347 sq ft.
3 × 6 subway, 1/16 in joint, 5/16 in deep, 40 sq ft. 1.406 in³/ft², 3.60 lb, 3.96 lb with waste, one 10 lb bag. Unsanded.
12 × 24, 1/8 in joint, 3/8 in deep, 200 sq ft. 0.844 in³/ft², 10.80 lb, 11.88 lb with waste, two 10 lb bags or one 25 lb bag.
300 × 600 mm, 3 mm joint, 10 mm deep, 10 m². 0.15 L/m², 1.5 L, 2.66 kg, 2.92 kg with waste, one 5 kg bag.
Reference
Density checked against Custom Building Products coverage charts. Change it when your bag disagrees.
| Grout | Starting density | Chart it was set to |
|---|---|---|
| Cement, sanded or unsanded | 0.064 lb/in³ | Polyblend Plus, 348 ft² per 25 lb |
| Epoxy | 0.045 lb/in³ | CEG-Lite, 187 ft² per 9.5 lb |
Both rows are a 12 × 12 × 3/8 in tile with a 1/8 in joint. Coverage may vary with the install.
Questions
How much grout do I need for 100 sq ft?
A 12 × 12 tile with a 1/8 in joint and a 3/8 in depth is 1.125 cubic inches of joint per square foot. At 0.064 lb per cubic inch that is 7.20 lb, and 10 percent waste makes 7.92 lb, one 10 lb bag. A 25 lb bag covers about 347 sq ft at those settings, before waste.
How many square feet does a 25 lb bag of grout cover?
It depends on the tile and the joint. On this page’s 12 × 12 example a 25 lb bag covers about 347 sq ft. Custom’s Polyblend Plus chart lists 348 sq ft for a 12 × 12 × 3/8 tile with a 1/8 in joint. Smaller tile and a wider joint use the bag much faster. The chart on your bag wins.
How much grout for a backsplash?
A 3 × 6 subway tile, 1/16 in joint, 5/16 in deep, over 40 sq ft is 3.60 lb, or 3.96 lb with 10 percent waste. That is one 10 lb bag. The joint is under 1/8 in, so unsanded grout is the usual choice.
Sanded or unsanded grout for 1/8 inch joints?
Sanded. Custom and the usual TCNA practice put sanded grout at 1/8 in and wider, and unsanded below that. A hybrid or fine-aggregate grout can cross that line if the label says so.
How much grout for mosaic tile?
A 1 × 1 mosaic has far more joint length per square foot than a 12 × 12 tile. The (length + width) ÷ (length × width) term is what grows. Use the mosaic preset and the joint the sheet was made with. The bag chart still wins.
How do I calculate grout for a shower?
Add the wall areas and the floor area, then run them as one area if the tile and the joint match. A different tile on the floor is a second pass. This page does not size a waterproofing membrane.
How long does mixed grout last?
Use the pot life on the bag. Cement grout is mixed in the amount you can place in that time. Epoxy follows the product label and the SDS. This page does not give a working time.
Sources
- Custom Building Products, Polyblend Plus Sanded Grout coverage chart. A 12 × 12 × 3/8 in tile with a 1/8 in joint is 348 ft² per 25 lb bag. The 7 lb column is the same chart scaled. The starting cement density of 0.064 lb/in³ is the value that lands on that row. The published 0.070 lb/in³ guess covered about 317 ft² and was replaced.
- Custom Building Products, CEG-Lite 100% Solids Commercial Epoxy Grout coverage chart. The same tile and joint is 187 ft² per 9.5 lb unit (Part A + Part B). The epoxy density of 0.045 lb/in³ lands on that row. A 19 lb unit is double the coverage.
- TCNA Handbook and ANSI A108.10, for the general practice that sanded grout is used at about 1/8 in and wider and unsanded grout below that. The product label can say otherwise.
- NIST SP 811. 0.45359237 kg/lb and 16.387064 cm³/in³ turn 0.064 lb/in³ into about 1.772 kg/L.