What this calculator does
Find out how much grout to buy for your tile job, in weight and in bags. Grout fills the joints, the gaps between tiles. So the amount you need depends on how much of the surface is joint and how deep those joints are.
Small tiles with wide joints can take several times as much grout as large tiles with tight joints over the same floor. That's why the single coverage figure on a bag is only a starting point.
This grout calculator first works out what share of the surface is joint, from the tile length, tile width and joint width. It multiplies that by the joint depth and the area to get the volume of grout. Then it turns the volume into a weight, using a typical density you can change. Finally it adds extra for waste and rounds up to whole bags. It works in US units (square feet, inches and pounds) or metric (square meters, millimeters and kilograms).
Where it's used
Why this math matters outside the classroom:
- Bathroom floors. A homeowner tiling 100 ft² with 12 in tile and 1/8 in joints finds 112.5 in³ of joint to fill, about 7 lb of grout.
- Joint share. For 12 × 12 in tile with 0.125 in joints, (12 + 12) × 0.125 ÷ 144 = 0.0208, so about 2% of the floor is grout.
- Metric bags. Tilers grouting 10 m² of 300 mm tile with 3 mm joints, 10 mm deep, fill 2 liters of joint, about 3.2 kg of grout.
- Comparing designs. Designers choosing between mosaic and large-format tile compare grout use, since smaller tiles mean far more joint to fill and clean.
- Volume practice. Students treat each joint as a thin rectangular prism and add the volumes, then convert 1,728 in³ to 1 ft³.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 7.G.B.6. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose US or metric units and enter the area you're tiling.
- Enter the tile's length and width, the joint width you plan to use and the joint depth. The depth is usually the tile's thickness.
- Check the grout density and bag size against your product, and set a waste allowance.
- Read how many bags to buy, with the weight and joint volume behind it.
What the inputs mean
- Grout joint width
- Use your spacer size. Rectified porcelain (tile cut to very exact edges) is often set with 1/16 in joints. Handmade or uneven tile needs wider ones. Many makers suggest unsanded grout for joints of 1/8 in or less and sanded grout above that.
- Joint depth (usually the tile thickness)
- Grout normally fills the joint to the full tile thickness. If thin-set (the tile adhesive) squeezes up into the joints and isn't cleaned out, the real depth is less.
- Grout density (cured, as packed)
- This turns volume into weight. Cement grouts are commonly around 100 lb/ft³ (1.6 kg/L) as packed in the joint. Epoxy and urethane grouts differ. If your product publishes a coverage chart, compare its figure with the result.
- Grout density
- About 1.6 kg per liter (1,600 kg/m³) is a typical planning figure for cement grout.
- Extra for waste
- Covers grout that hardens in the bucket, gets wiped off in cleaning or is lost when you mix small batches.
Formula and method
joint share = (tile length + tile width) × joint width ÷ (tile length × tile width) volume = area × joint share × joint depth weight = volume × density × (1 + waste ÷ 100) bags = weight ÷ bag size, rounded up
In a grid, each tile comes with one joint along its length and one along its width. So the joint area for one tile is (length + width) × joint width. Divide that by the tile's own area to get the fraction of the surface that's grout.
In US units, the area is turned into square inches (× 144), so the volume comes out in cubic inches. Dividing by 1,728 then gives cubic feet. In metric, one square meter of surface times one millimeter of depth is exactly one liter.
Worked example
100 ft² of 12 × 12 in floor tile with 1/8 in (0.125 in) joints, 3/8 in (0.375 in) deep, grout at 100 lb/ft³, 10% waste and 25 lb bags.
- Joint share: (12 + 12) × 0.125 ÷ (12 × 12) = 3 ÷ 144 = 0.020833.
- Volume: 100 × 144 × 0.020833 × 0.375 ÷ 1,728 = 0.065104 ft³ (112.5 in³).
- Weight: 0.065104 × 100 × 1.1 = 7.1615 lb.
- Bags: 7.1615 ÷ 25 = 0.2865, rounded up to 1.
Result: You need about 7.2 lb of grout. One 25 lb bag covers the job with plenty to spare, or a 10 lb bag if your product comes in that size.
Understanding the result
The weight is the handy number for choosing between bag sizes. The joint share tells you how much grout the design uses. It's about 2% for large tile with tight joints, and 10% or more for mosaics.
If the result is close to a whole bag, buy the extra one. A bag of the same color from a different batch can dry to a slightly different shade.
Assumptions and limitations
- Tiles are rectangles laid in a grid or a brick-style offset (running bond), with one joint width throughout.
- Joints are filled to the full depth you enter.
- The density is a typical planning value unless you replace it with your product's figure.
- Hexagonal, irregular and mosaic tiles on mesh have joints shaped differently. Use the product chart for those.
- Movement joints at walls and corners, where one surface meets another, are filled with sealant, not grout. They aren't counted.
- Brands differ in density and recommended joint sizes. Their coverage charts come first.
Frequently asked questions
How much grout do I need per square foot?
It depends almost entirely on the tile size and joint width. 12 × 12 in tile with 1/8 in joints at 3/8 in deep takes roughly 0.07 lb per ft². A 2 × 2 in mosaic with the same joints takes about 0.4 lb per ft², over five times as much.
Why does tile thickness matter?
The joint is as deep as the tile is thick. So a 3/8 in floor tile holds half again as much grout per foot of joint as a 1/4 in wall tile with the same joint width.
Should I use sanded or unsanded grout?
Many makers recommend unsanded grout for narrow joints, around 1/8 in or less. They suggest sanded grout for wider joints, where the sand helps stop shrinking and cracking. Some polished stone and glass tile need unsanded grout to avoid scratches. Follow the tile and grout makers' guidance.
Can I mix only part of a bag?
Usually yes, if you measure the powder and water in the same proportions given on the bag. Mixing small batches keeps grout from setting in the bucket before you use it.
How this calculator is checked
Method: Grid joint geometry: joint volume per unit area = (L + W) × joint width × depth ÷ (L × W); weight from an editable typical density, with whole-bag rounding. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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