Deck Mud Calculator & Coverage Chart (Bags per Sq Ft)

Deck Mud Calculator (Shower Pan & Mortar Bed)

Enter the floor size, drain position and slope to get the volume of deck mud and the bags to buy.

Scope: This tool estimates mortar bed material only. It does not design waterproofing, membranes, drains, curbs or the subfloor. Follow the TCNA Handbook / ANSI A108 methods and your membrane and drain manufacturer instructions.

Deck mud coverage chart (bags per square foot)

Enter the floor area in square feet and the bed thickness to get the exact number of bags. For a quick check, the chart below shows how many square feet one bag covers on a flat bed of uniform thickness, with no waste allowance. A sloped shower pan uses the average thickness, which the calculator works out for you.

Square feet covered per bag of floor mud on a flat bed (no waste)
Bed thickness	80 lb bag covers	60 lb bag covers
0.75 in (19 mm)	10.6 sq ft	8.0 sq ft
1 in (25 mm)	7.9 sq ft	6.0 sq ft
1.25 in (32 mm)	6.3 sq ft	4.8 sq ft
1.5 in (38 mm)	5.3 sq ft	4.0 sq ft
2 in (51 mm)	4.0 sq ft	3.0 sq ft

Yields used: 80 lb ≈ 0.66 ft³ (18.7 L) and 60 lb ≈ 0.50 ft³ (14 L), from the QUIKRETE Floor Mud data sheet. Other brands differ, so check the yield printed on your bag and add a waste allowance (10% is common).

How this calculator works

Rise = slope x distance from drain to farthest wall. Average thickness = thickness at drain + rise / 3 (center drain, level perimeter) or + rise / 2 (linear drain); flat bed = thickness. Volume = area x average thickness x (1 + waste / 100). Bags = round up(volume in liters / yield per bag).

Worked example

A 5 ft x 3 ft shower with a center drain, 1 in at the drain, 2.5 ft to the farthest wall and a 1/4 in per foot slope rises 0.625 in, so the bed is 1.625 in at the wall and 1.21 in on average. That is 1.51 ft³, or 1.66 ft³ (47 L) with 10% waste: 3 bags of 80 lb floor mud at 18.7 L each.

Assumptions to check

The floor is a rectangle and the perimeter is level; the slope runs from every wall down to the drain.
A center-drain pan is modeled as a pyramid and a linear-drain pan as a wedge. Off-center drains are covered by entering the distance to the farthest wall.
Only the mortar bed is counted. Pre-slope under a liner, curbs, membranes, thinset and tile are not included.
Bag yield must come from the product label. The default is the QUIKRETE Floor Mud 80 lb figure.

Read the calculation and QA methodology →

FAQ

How much slope does a shower pan need?

A slope of 1/4 in per foot (about 2%) toward the drain is the common requirement and is the figure on the QUIKRETE Floor Mud data sheet. Check your local code and your membrane and drain maker instructions.

How do I calculate deck mud by square feet?

Multiply the floor area in square feet by the average bed thickness in feet to get cubic feet, add waste, then divide by the bag yield. Example: 15 sq ft at an average of 1.21 in (0.1 ft) is about 1.5 ft³, or 3 bags of 80 lb floor mud with 10% waste.

Why is the average thickness lower than the thickness at the wall?

The bed is thinnest at the drain and thickest at the walls. With a center drain and a level perimeter the volume works out like a pyramid, so the average is the drain thickness plus one third of the rise. A linear drain slopes one way, so the average is the drain thickness plus half the rise.

How many square feet does a bag of deck mud cover?

On a flat bed 1 in thick, an 80 lb bag of floor mud (about 0.66 ft³) covers about 7.9 sq ft and a 60 lb bag (about 0.50 ft³) about 6 sq ft. At 2 in thick the coverage halves to about 4 and 3 sq ft. These figures use the QUIKRETE Floor Mud yields; check your bag and add waste.

Can I mix my own deck mud instead of buying bags?

Yes. Site-mixed deck mud is commonly about 4 to 5 parts sand to 1 part portland cement by volume. Use the volume result to size the batch, and follow the TCNA / ANSI method your installation uses.