How to Calculate Mortar for Tile: A Step-by-Step Guide

Mortar for tile in one line

Mortar for tile is area times the waste factor, divided by coverage per bag, rounded up to whole bags. The area is the surface being tiled, the waste factor covers cuts and breakage, and the coverage is what one bag actually spreads at your trowel notch — not the biggest number printed on the bag. The worked example through this guide is a 400 sq ft floor, a 50-lb bag covering 60 sq ft, and 10% waste: 400 times 1.10 is 440.00 sq ft, 440.00 divided by 60 is 7.33 bags exact, and you order 8. Check every step on the mortar-chart page, which runs the same arithmetic on your own numbers.

Step 1: Measure the area

Measure every surface being tiled, not just the floor. A floor is length times width; a wall is height times width, measured wall by wall, then added to the floor. An L-shaped room is split into rectangles, each measured on its own, then added — a single length-times-width guess across an L-shape counts area that is not there. For a shower, add the walls, the floor, and any niche or bench. Openings need a judgement call: small ones — a doorway, a window — are usually left in the figure, because the tile cut around them eats roughly what the opening would have saved. A large opening, a full wall of glass or a wide archway, is deducted outright. A missed wall is a missed set of bags.

Step 2: Find coverage from the bag label and trowel notch

Find the coverage your bag gives at your trowel notch — the notch decides the number. As a rule of thumb for a 50-lb bag: a 1/4 by 1/4 inch square-notch trowel covers roughly 80 to 95 sq ft, a 1/4 by 3/8 inch square-notch covers roughly 60 to 70 sq ft, and a 1/2 inch notch covers roughly 35 to 50 sq ft. A larger notch lays a thicker bed, so each bag spreads over less area. Use the notch your tile size calls for — small mosaics take a small notch, large-format tiles take a large one — and then use the coverage that goes with that notch. The trap is picking the largest coverage figure on the bag while trowelling with a notch that delivers far less. When the bag label gives its own table, trust it over any rule of thumb, and read the row for your notch.

Step 3: Add waste

Add waste before dividing, not after: the waste factor multiplies the measured area, and the order is built on the larger figure. A straight grid layout takes 10% — 400 sq ft becomes 440.00 sq ft — which covers end cuts, a few broken tiles, and mixing loss. A diagonal layout or a complex pattern, herringbone or a room full of angles, takes 15%, because nearly every edge tile is cut and cut tiles cannot be reused elsewhere. Small rooms deserve the higher end too: the cuts are a bigger share of a small area. Waste is the difference between finishing the last row and driving back to the store mid-job.

Step 4: Divide, then round up — never down

Divide the waste-adjusted area by the coverage per bag: 440.00 divided by 60 is 7.33 bags exact. Then round up to the next whole bag, always. Bags are sold whole, and 7 bags cover only 420 sq ft, leaving 20 sq ft of the example floor bare — rounding down saves one bag and strands the last row. The exact figure is information, not an order quantity: 7.33 means 8, 7.01 means 8, and only an exact whole number orders itself. If the exact figure lands just over a whole number, the extra bag is still the right call — an unopened bag usually goes back to the store.

Worked example, start to finish

A 400 sq ft floor, straight grid layout, 1/4 by 3/8 inch square-notch trowel, 50-lb bags covering 60 sq ft each. The measured area is 400 sq ft. Waste at 10% adds 40 sq ft, giving 440.00 sq ft to cover. Exact bags are 440.00 divided by 60, which is 7.33. Rounded up, the order is 8 bags. The check runs backwards: 8 bags times 60 sq ft is 480 sq ft of coverage, which clears the 440.00 sq ft needed. Change one input and the order moves: a diagonal layout at 15% waste needs 460.00 sq ft, which is 7.67 bags exact, still 8 bags; a 1/2 inch notch at 40 sq ft a bag needs 440.00 divided by 40, which is 11 bags exactly; a 1/4 by 1/4 inch notch at 90 sq ft a bag needs 440.00 divided by 90, which is 4.89 bags, rounded up to 5. Area, waste, notch and coverage are the four levers.

Back-buttering and large-format tile

Large-format tiles — 12 by 24 inches and up, or any tile with a side over 15 inches — often need back-buttering: a skim coat of mortar spread on the back of each tile before it is set into the notched bed. Back-buttering uses mortar beyond what the notched bed alone accounts for, so a large-format job ordered on bed coverage alone comes up short. Budget for it before ordering: bump the waste allowance toward 15% or add a bag or two on a large job, and follow the manufacturer guidance. If the job mixes tile sizes, price the large-format areas at their own coverage rather than averaging one figure across the whole room.

Check against the delivered bags

When the bags arrive, check them against the order before opening anything. Count the bags, read the coverage on the label, and multiply bags times coverage: the result must clear the waste-adjusted area, 440.00 sq ft in the example. If the count is short, order the balance before starting, not halfway through a wall — a second trip costs more than the spare bag. Mortar is not grout, and the check is separate: grout fills the joints after the tile is set, and no grout figure belongs in a mortar count.

Common mistakes to avoid

Forgetting waste is the most common mistake: ordering on the bare 400 sq ft gives 6.67 bags exact, 7 ordered, against 440.00 sq ft needed — short before the last row. Using the wrong notch coverage misstates the order by the gap times the area, and the gap between a small notch and a large one can nearly double the bag count. Rounding down strands the final row for the sake of one bag. Counting grout as mortar confuses two different products: grout fills joints, mortar beds tile, and neither substitutes for the other. Measuring only the floor while the walls take tile misses whole surfaces, and buying exactly the exact figure leaves no margin for a single broken tile. Keep the area, coverage, waste and rounded-up order visible, and every figure checks itself.