ARDEX X 77 Coverage Calculator: How Many 40 lb Bags?

ARDEX X 77 MICROTEC coverage

Source: ARDEX Americas Technical Data Sheet: ARDEX X 77 MICROTEC Fiber Reinforced Tile and Stone Mortar, content updated 2026-09-10, read 2026-10-26.

How the calculation works

bags = area × (1 + waste ÷ 100) ÷ coverage per bag, rounded up

The manufacturer gives a coverage range for each trowel size because the real figure depends on the tile back, the substrate and how much mortar ends up in the ridges. This calculator divides by the low end of the range, so the number it tells you to buy should not leave you short. If you back-butter large tiles, plan for coverage at or below the low end.

Worked example

A 150 sq ft floor with a 1/2" x 1/2" square notch trowel and 10% extra: 150 × 1.1 = 165 sq ft. At the low end of the coverage range (55 sq ft per bag) that is 165 ÷ 55 = 3 bags, so buy 3. That is 120 lb of powder.

ARDEX X 77 MICROTEC key specs

What X 77 is for

ARDEX X 77 MICROTEC is a fiber-reinforced, polymer-modified cement mortar for setting tile and stone. You mix it with plain water. ARDEX pitches it at large format and large, heavy porcelain tile, including very large thin porcelain panels, on floors and walls, inside or outside. It also sets quarry, ceramic and glass tile in areas that are not submerged, and most natural stone that is not sensitive to moisture. The numbers on this page come from the ARDEX Americas technical data sheet, content updated 2026-09-10, which is the version ARDEX links from its X 77 product page.

The data sheet stresses three working properties: an open time of up to 60 minutes, a pot life of 3 hours, and sag resistance that ARDEX describes as double the ISO requirement. Those are the reasons contractors reach for it on big tiles and on walls, where a mortar that slumps or skins over too fast makes trouble.

How the coverage numbers work

ARDEX prints one figure per trowel, not a range: 110 sq ft per 40 lb bag with a 1/4 x 1/4 inch square notch, 82.5 sq ft with a 1/4 x 3/8 inch square notch and 55 sq ft with a 1/2 x 1/2 inch square notch. The calculator uses those three numbers exactly as printed. The data sheet adds that coverage depends on the surface profile, density and porosity, and says all its data come from a partial lab mix at 70°F. On a real job you lose mortar to a rough slab, to back-buttering, to the bucket and to mortar that skins over before you set tile into it. That is why the waste box starts at 10%. For large format tile that you back-butter, 15 to 20% is a safer allowance.

X 77 covers more per pound than many 50 lb thin-sets because it takes a lot of water: 8 1/4 to 8 3/4 US quarts for each 40 lb bag. One bag therefore makes a large volume of mortar. Do not try to stretch it further by adding more water. The data sheet is blunt that extra water weakens the mortar and lowers its strength.

Mixing it

Put the water in the bucket first, then add the powder while mixing with a paddle on a heavy-duty 1/2 inch drill running at 650 rpm or more. Mix for about 2 to 3 minutes until there are no lumps. For small batches, ARDEX allows 2 parts powder to 1 part water by volume, mixed by hand with a margin trowel. Stir again just before you apply it. If the mortar starts to stiffen inside its 3-hour pot life, the data sheet says you may retemper it with the drill and paddle.

ARDEX also asks you to keep dust down: empty each bag gently, and use a gutter-hook vacuum attachment with a bucket vacuum and HEPA extraction while mixing.

Substrates and limits

The data sheet lists concrete cured at least 28 days, cement backer board, existing tile that is sound and well bonded, approved waterproofing and uncoupling membranes, interior plywood floors, non-aluminum metal indoors (call ARDEX first), and properly prepared non-water-soluble adhesive residue on concrete primed with ARDEX P 4. Some of these need priming. Existing tile and other non-porous surfaces can be primed with P 4, skim coated with a scratch coat of the mortar, or, indoors in dry areas only, set with X 77 mixed with ARDEX E 90 admix. Drywall gets a PVA primer.

Over membranes the rule is tighter. Floors over membranes are interior only, and the mortar bed may be no thicker than 3/8 inch. In all other cases the limit is 1/2 inch. A 1/2 x 1/2 inch trowel collapses well under that once the tile is beaten in, but it is worth knowing if you are tempted to build up a low spot with mortar. ARDEX lists separate pre-leveling products for that job.

X 77 is fine for dry or damp substrates with no standing water, for showers and other intermittently wet areas, for exteriors except over plywood, and for properly balanced freshwater pools. It is not for saltwater pools, not for glass tile in pools or other submerged areas, and not for moisture-sensitive stone or agglomerate tile. For those, ARDEX points to its 100% solids epoxy adhesive or to rapid-setting mortars such as X 32, S 28 and N 23.

Temperature and timing

Substrate and air must be at least 50°F (10°C) during installation and cure. If the floor has radiant heat, turn it off 48 hours before, during, and at least 48 hours after the installation. At 70°F, grout floor tile after 24 hours and wall tile after 8 hours. Cold and humid conditions stretch those times, and forced drying is not recommended. Adjustment time is 20 to 30 minutes, so you can still nudge a tile into line shortly after setting it.

Standards and strength

ARDEX tests X 77 to ANSI A118.15 and A118.11. The data sheet reports more than 400 psi shear strength on porcelain mosaics after 28 days, more than 250 psi after freeze-thaw cycling, more than 450 psi on glazed wall tile, and more than 150 psi for quarry tile on plywood. Under ISO 13007 it is classed C2 (improved adhesion), T (slip resistance on walls), E (extended open time) and S1 (deformable).

Buying and storing it

X 77 comes in 40 lb (18 kg) bags in gray or white, and keeps 12 months unopened in a cool, dry place out of the sun. ARDEX says it is meant for trained professional contractors and is not sold through home improvement centers, so you will usually find it at tile and flooring distributors. Because the bag is 40 lb rather than 50 lb, compare price per square foot, not price per bag, when you weigh it against other mortars.

FAQ

How much does a 40 lb bag of ARDEX X 77 cover?

ARDEX's data sheet gives 110 sq ft (10.2 m²) per bag with a 1/4 x 1/4 inch square notch, 82.5 sq ft (7.6 m²) with a 1/4 x 3/8 inch square notch and 55 sq ft (5.1 m²) with a 1/2 x 1/2 inch square notch. It adds that coverage depends on the surface profile, density and porosity, so rough or thirsty substrates and back-buttered tile will use more.

Why does X 77 cover more per bag than a 50 lb thin-set?

X 77 takes a lot of water, 8 1/4 to 8 3/4 quarts for a 40 lb bag, so one bag makes a larger volume of mortar than most 50 lb bags mixed with 5 to 7 quarts. ARDEX's 110 sq ft at 1/4 x 1/4 inch is still a lab figure. The calculator uses it as printed, so keep at least 10% extra on the order, and more for large format tile that needs back-buttering.

Which trowel should I use for large format tile with X 77?

ARDEX leaves trowel choice to the tile type and size and asks for enough depth to cover the tile uniformly, following TCNA guidance on mortar transfer. For large format porcelain that usually means the 1/2 x 1/2 inch notch, which is why the calculator starts there. Remember the bed limit: no more than 3/8 inch over membranes and 1/2 inch elsewhere.

Can I use X 77 in a pool or shower?

Yes for showers, which the data sheet lists as intermittently wet interior areas, and yes for properly balanced freshwater pools and other submerged areas. It is not for saltwater pools or for glass tile in pools; ARDEX points those to its 100% solids epoxy adhesive. It is also not for moisture-sensitive stone or agglomerate tile.

How long before I can grout over X 77?

At 70°F (21°C) the data sheet says 24 hours for floor tile and 8 hours for wall tile. Cold substrates and high humidity stretch that time, and the substrate and air must stay at 50°F (10°C) or warmer while the mortar cures. ARDEX lists its FL, FH, FG-C, WA and DG 1 grouts to follow it.

ARDEX and ARDEX X 77 MICROTEC are trademarks of their owner. This page is an independent estimate based on the published data sheet; check the bag label before mixing.