How to measure for grout
Grout fills the gaps, so what matters is how much gap there is. That comes from three things you can measure: the tile size, the tile thickness and the joint width. The area of the floor is only the multiplier at the end.
Measure a real tile with a tape, because a tile sold as 12 inches may be 11 and 7/8. Check its edge for thickness, since a floor tile can be half as thick again as a wall tile. For the joint, use the size of the spacers you will actually set with, and if you are grouting tile someone else laid, measure a few joints and take the average.
This is why a small area of mosaic can use more grout than a large floor. A 2 inch mosaic has six times as many joints per foot as a 12 inch tile. Measure the area with the square footage calculator if it is not a plain rectangle.
A worked example
Here is 100 square feet of 12 inch tile, 3/8 inch thick, with 1/8 inch joints. These lines come from the same formula the calculator above runs.
100 sq ft of 12 × 12 in tile, 3/8 in thick, 1/8 in joints
- Joint per square foot: 1.125 cubic inches
- In pounds: 0.065 lb per sq ft
- For 100 sq ft: 6.5 lb
- With 10% extra: 7.2 lb, so 1 bag of 25 lb
One bag would cover 384 square feet by this working. MAPEI prints 383 for the same tile and joint, and Custom prints 348, which shows how close the method is and how far brands differ. Widen the joint to 1/4 inch and the grout doubles to 14.3 pounds. To count the tile itself, use the tile calculator.
Grout for 100 square feet
Read across to see the joint and down to see the tile. The spread is the whole reason a single coverage figure on a listing is no use to you.
| Tile | 1/8 in | 3/16 in | 1/4 in | 3/8 in |
|---|---|---|---|---|
| 2 × 2 in mosaic, 1/4 in thick | 28.6 | 43.0 | 57.3 | 85.9 |
| 4 × 4 in, 1/4 in thick | 14.3 | 21.5 | 28.6 | 43.0 |
| 3 × 6 in subway, 5/16 in thick | 17.9 | 26.9 | 35.8 | 53.7 |
| 12 × 12 in, 3/8 in thick | 7.2 | 10.7 | 14.3 | 21.5 |
| 12 × 24 in, 3/8 in thick | 5.4 | 8.1 | 10.7 | 16.1 |
| 24 × 24 in, 3/8 in thick | 3.6 | 5.4 | 7.2 | 10.7 |
Mistakes that waste a bag
- The wrong grout for the joint. Sanded grout is rated for 1/8 inch and up. In a thinner joint the sand does not fit, and non-sanded is the right call.
- Trusting a coverage number from a listing. One listing showing 768 square feet matches only 24 inch tile at 1/8 inch. Your job may be a quarter of that.
- Ignoring thickness. Thicker tile means deeper joints. Going from 1/4 to 3/8 inch adds half as much grout again.
- Mixing bags one at a time. MAPEI asks you to dry-blend all units before use, because colour can shift between bags.
- Edge shape. LATICRETE warns that coverage varies considerably with cushion-edge or wave-edge tile, and those joints hold more than the square-edged sum says.
Questions
How much grout do I need for 100 square feet?
With 12 by 12 inch tile, 3/8 inch thick, and a 1/8 inch joint, you need about 7.2 pounds including 10 percent extra. Smaller tile needs more: 4 inch tile with the same joint and thickness takes about three times as much.
How much grout per square foot?
It ranges from under 0.1 pound per square foot for large tile with thin joints to over half a pound for small mosaics. There is no single figure, because a square foot of mosaic holds far more joint than a square foot of large tile.
Does joint width change how much grout I need?
Yes, directly. Doubling the joint from 1/8 to 1/4 inch doubles the grout. MAPEI's chart shows the same thing: one bag covers 383 square feet of 12 inch tile at 1/8 inch and 191 at 1/4 inch.
Sanded or unsanded grout?
Use sanded grout for joints of 1/8 inch and wider, and non-sanded for narrower ones. MAPEI's sanded grout is rated for joints from 1/8 to 5/8 inch, and Custom's non-sanded coverage table stops at 1/8 inch. Check the label for your product.
How much does one bag of grout cover?
A 25 pound bag covers about 380 square feet of 12 inch tile at 1/8 inch joints, by MAPEI's chart, and Custom's chart gives about 350. Drop to 4 inch tile and the same bag covers about 200 square feet.
This grout figure is a planning estimate from the size of the joints, using a density we fitted to printed coverage tables. Real coverage shifts with the exact joint, tile edges and how you work, and brands differ by about 10 percent. Check the coverage chart on the label of the product you pick before buying.
Sources and review
- Keracolor S sanded grout technical data sheet, MAPEI, 2026.
- Grout selector card, Custom Building Products, 2025.
- LATICRETE TDS 400 grout data sheet, LATICRETE, 2020.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Joint volume
Cubic inches per sq ft = 144 × (Length + Width) ÷ (Length × Width) × Joint × Thickness
- Length, Width
- Tile size, in inches
- Joint, Thickness
- Joint width and tile thickness, in inches
Worked through: 144 × 24 ÷ 144 × 0.125 × 0.375 = 1.125 cubic inches
Pounds
Pounds = Area × Cubic inches per sq ft ÷ 1728 × 100
- 100
- Pounds of grout per cubic foot, fitted to printed tables
Worked through: 100 × 1.125 ÷ 1728 × 100 = 6.5 lb
The constants, and where they come from
GROUT_POUNDS_PER_CU_FT
Modelled from published dataWeight of grout in the joint, in pounds per cubic foot
Applies to: Cement grout, fitted against two makers' printed coverage tables
Source: Keracolor S Sanded Grout technical data sheet, approximate coverage per 25 lbs, MAPEI, published 2026-02, read 2026-11.
Also from: Grout selector card, Custom Building Products, published 2025-06, read 2026-11.
No maker prints a density. Fitting the joint volume to MAPEI's table gives about 103 and to Custom's about 107, so 100 is close to MAPEI and about 7 percent light against Custom. Coverage differs by brand by about that much.
SUGGESTED_WASTE_PERCENT
Published figureExtra grout for jobsite waste, in percent
Applies to: Tile grouting on site
Source: LATICRETE TDS 400 grout data sheet, LATICRETE, published 2020-02, read 2026-11. This source is about 6 years old, and it is the one the figure rests on.
LATICRETE says it is customary to provide typically 10 percent extra to allow for jobsite waste.
SQ_IN_PER_SQ_FT
DefinitionalSquare inches in one square foot
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
Twelve inches in a foot, and an area is a length times a length, so 12 × 12 = 144.
CU_IN_PER_CU_FT
DefinitionalCubic inches in one cubic foot
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
Twelve inches in a foot, and a volume is three lengths multiplied, so 12 × 12 × 12 = 1728.
What the labels mean
- Modelled from published data:
- derived rather than measured, calibrated against the source named beside it.
- Published figure:
- taken directly from the source named beside it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the grout calculator
- 1
Enter the area you are tiling
- 2
Enter the tile's length, width and thickness
- 3
Choose the joint width, using the spacers you plan to use
- 4
Read the pounds and the bags
- 5
Check the result against the coverage chart on the bag you choose
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- Joints are filled to the full thickness of the tile
- Tiles are square-edged. Cushion-edge or wave-edge tile takes a different amount
- Grout weighs about 100 pounds per cubic foot in the joint, which is our fit and not a maker's figure
- Tile is laid in a regular grid or running bond, which use the same amount of joint
Pro tips
- Blend several bags dry before mixing, which MAPEI asks for, so the colour stays even
- Buy one more bag than you need if the color could be discontinued
- Measure a joint after setting the tile, not before