Grout calculator

How much grout do you need?

The amount of grout you need is the volume of the joints, and that depends on four sizes: the tile's length and width, its thickness (the joint depth) and the joint width. Small tiles have more joint per square foot than large ones, so they use more grout.

Joint volume per unit of area = (L + W) ÷ (L × W) × joint width × depthL, W = tile length and width; depth = tile thickness; same units throughout

This is the same formula grout makers publish. MAPEI's Ultracolor data sheet, for example, gives consumption in kilograms per square meter as (A + B) ÷ (A × B) × thickness × joint width × 1.5, with all sizes in millimeters and 1.5 standing for that grout's density in kilograms per liter.^[1] Its own table checks out: a 20 × 20 mm tile, 4 mm thick, with 2 mm joints needs 0.1 × 4 × 2 × 1.5 = 1.2 kg/m².^[1]

Grout calculator worked example (and a check against the metric formula)

For the defaults, 120 sq ft of 12 × 24 in tile, ⅜ in thick, with ⅛ in joints, both ways of working agree:

1. In inches: (12 + 24) ÷ (12 × 24) × 0.125 × 0.375 = 0.00586 cu in per sq in; × 144 = 0.8438 cu in per sq ft.
2. Volume: 120 sq ft × 0.8438 ÷ 1,728 = 0.0586 cu ft.
3. Weight: 0.0586 cu ft × 93.6 lb/ft³ = 5.48 lb; with a 10% allowance, 6.03 lb, one 25 lb bag.
4. Metric check: A = 304.8 mm, B = 609.6 mm, thickness 9.525 mm, joint 3.175 mm → 0.2232 kg/m² = 0.0457 lb per sq ft; × 120 = 5.49 lb. The tiny difference is rounding 1.5 kg/L to 93.6 lb/ft³.

The unit conversions come from NIST: one pound per cubic foot is 16.01846 kg/m³ and one square foot is 0.09290304 m², so 1.5 kg/L (1,500 kg/m³) is 93.64 lb/ft³.^[2]

Grout per square foot by tile size and joint width

Grout per square foot rises in proportion to the joint width and the tile thickness and falls as tiles get bigger. Pounds for 100 sq ft of ⅜ in thick tile at 93.6 lb/ft³, before any allowance:

Thicker tile needs proportionally more: ½ in thick tile uses a third more grout than ⅜ in, at the same joint. Mosaics and subway tile use several times what large-format tile does.

How much area a bag of grout covers

A bag covers its weight divided by the pounds needed per square foot. With 12 × 12 in tile, ⅜ in thick and ⅛ in joints, the joints hold 1.125 cu in per sq ft, so a 25 lb bag at 93.6 lb/ft³ fills about 410 sq ft. Halve the tile size to 6 × 6 in and the same bag covers half the area.

Planning the rest of the job

Grout is a small line in a tile budget. Start with the area from the square footage calculator, count tiles and boxes with the tile calculator, and put the whole job into the bathroom remodel cost calculator if it is part of a bath. For context, the Census Bureau's 2023 American Housing Survey put the median owner-occupied bathroom remodel at $5,000 and the median flooring job at $3,000.^[3] A tile job big enough to finance raises two questions. A home equity loan pays out one lump sum while a HELOC lets you draw as the work proceeds,^[4] and the interest is deductible only if the work substantially improves the home rather than repairing it.^[5] The home improvement loan calculator compares the payments.