🧱 Deck Mud Calculator

What Is Deck Mud?

Deck mud — also called floor mud, dry pack, or a mortar bed — is a stiff, sandy mix of sand and portland cement with just enough water to hold together when squeezed. It's used to build the sloped base under shower floors and to level subfloors before tiling. Because it contains no lime and very little water, it packs hard and won't slump on a slope.

Standard Deck Mud Formula

The classic deck mud ratio is 5 parts sand to 1 part portland cement by volume. A 4:1 mix is a bit richer and stronger; 6:1 is leaner and more economical for thicker fills.

How Much Sand and Cement?

In a dry-pack mix the cement and water fill the gaps between sand grains, so the finished bed volume is roughly equal to the loose sand volume. The calculator above sizes the sand to your bed volume and the portland cement to your chosen ratio.

Deck Mud Calculator: Square Feet to Cubic Feet

Deck mud is sold and mixed by volume (cubic feet), but you measure your shower pan or floor in square feet. Converting between the two is the whole job, and it only takes one number: your average bed thickness.

That divide-by-12 is just inches converted to feet. A 1-inch-thick bed is 1/12 of a foot deep, so every square foot of floor takes 0.083 cubic feet of mud. Double the thickness and you double the material.

The coverage chart below is the shortcut most tile setters keep in their head — how many square feet one cubic foot of deck mud covers at each common thickness, and how many bags that works out to.

Use the average, not the maximum — but on a pan that drains to a point, the average is not the midpoint. A bed running 1-1/4 in at the drain and 1-3/4 in at the perimeter averages 1.58 in, not 1.5 in, because far more of the floor sits out near the thick perimeter than sits near the drain. The midpoint rule under-orders by about 5%. Here is the arithmetic, and the calculator’s Sloped shower pan mode does it for you.

Not sure of your floor area? Measure it first with our Square Footage Calculator, then come back and switch the calculator above to "Total square feet."

Deck Mud by Shower Pan Size

Most residential shower pans fall into a handful of standard footprints. These figures assume a 1-1/2 inch average bed and include no waste — add about 10% for screeding and packing.

Sand bags in the last column are 50 lb (about 0.5 cubic foot each); portland is the standard 94 lb bag, which holds a full cubic foot. Note how little cement a 5:1 mix actually needs — a single bag of portland covers every pan on this list, which is why buying one bag and several bags of sand is usually the cheapest route.

Remember the second bed. A traditional liner-style shower pan gets two mud beds — a pre-slope under the liner and the finish bed on top — so double these numbers if you're building one. See the shower pan section below.

Why the Midpoint Rule Under-Orders a Sloped Pan

Almost every deck mud guide tells you to average the thickness at the drain and the thickness at the perimeter. For a pan draining to a point that is wrong, and it is wrong in the direction that leaves you short.

The reason is area, not arithmetic. A pan is screeded along lines running from the drain out to the perimeter. Take any one of those lines: the mud starts at the drain thickness and rises steadily to the perimeter thickness. But the floor does not have equal area at every point along it. Near the drain the ring of floor is tiny; out at the wall it is long. Most of the pan is thick mud, and the midpoint rule quietly assumes the thick half and the thin half are the same size.

Integrating a straight slope over those radial sectors gives an average of t_drain + ⅔(t_perimeter − t_drain), which is the formula above. The run cancels out, so it holds whatever the shape of the pan — a 3 × 3 ft corner and an 8 × 4 ft alcove use the same factor.

What it costs you

The shortfall is always (perimeter − drain) ÷ 6, so it grows with the slope and with the run to the wall. On a small pan it is a bag; on a big one it is a trip back to the yard with a bed already going off.

The exception: linear and trench drains

A trench drain along one wall changes the geometry completely. The bed slopes one way across a rectangle, so the thin half and the thick half really are the same size, and the midpoint rule is exactly right. The calculator asks which kind of drain you have and applies the matching formula — point drains get the two-thirds rule, trench drains get the midpoint.

Mixing Deck Mud: Batch Sizes and Water

Deck mud is mixed by volume, not weight, which makes batching easy: fill any container five times with sand and once with portland cement. Consistency between batches matters more than hitting a lab-perfect ratio.

The Snowball Test

Water is where most first-timers go wrong. Deck mud should look almost dry — barely damp sand with a grey tint. Add water a little at a time, mix thoroughly, then squeeze a handful into a ball. If the ball holds its shape and doesn't drip or crumble, it's right. If water runs out between your fingers, it's too wet: add more dry sand and cement in ratio rather than trying to wait it out.

A too-wet bed slumps out of its slope, shrinks as it dries, and ends up soft. There's no rescuing it after the fact, so err dry — dry pack that's slightly too stiff still packs fine under a float.

Which Sand?

Use a clean, sharp masonry sand or washed concrete sand. Play sand works but its rounded, fine grains pack less densely, giving a slightly weaker bed. Avoid anything with clay, silt, or organic material — it prevents the cement paste from bonding to the grains.

Skip masonry cement entirely for floor beds. It contains lime, which makes mortar sticky and workable on walls but leaves a floor bed softer than straight portland. Type I/II portland cement is what you want.

Deck Mud vs. Wall Mud vs. Thinset vs. Self-Leveler

Four materials get confused constantly because they all sit between the subfloor and the tile. They aren't interchangeable:

The critical distinction: only deck mud can hold a slope. Self-leveler does exactly what its name says — it finds level and stays there, which is the opposite of what a shower floor needs. Thinset is an adhesive, not a filler; trying to build depth with it causes shrinkage cracking and a hollow-sounding floor.

The lime in wall mud is what makes it cling to a vertical surface without sagging. That same lime makes it softer underfoot, so it doesn't belong in a floor.

How to Pack a Deck Mud Bed: Step by Step

Dry pack is forgiving to place and unforgiving to fix. The sequence below covers a bonded mud bed over a concrete or plywood subfloor.

1. Confirm the subfloor is solid. Plywood should be at least 1-1/8 inch total thickness with no flex between joists. A bed packed over a bouncy floor will crack no matter how well it's mixed.
2. Set the drain height. For a shower, the drain flange establishes your thinnest point. Everything else slopes up from it, so get this right before mixing anything.
3. Install the bond coat or cleavage membrane. A bonded bed goes over a slurry of thinset or a portland/water paint. An unbonded bed floats over a slip sheet (roofing felt or poly) with wire lath — the choice depends on the assembly.
4. Establish your perimeter height. Snap a line or set screeds around the walls at the finished perimeter height. This is what you'll screed to, so measure from the drain and confirm your slope before you start.
5. Check the slope math. Shower floors need 1/4 inch of fall per foot of run. A 30-inch run from wall to drain means the perimeter sits 5/8 inch higher than the drain.
6. Mix a batch. Blend the sand and cement dry until the color is uniform, then add water gradually to snowball consistency. Mix only what you can place in about 45 minutes.
7. Dump and rough-spread. Pile the mud in and rake it roughly to shape with a trowel or your hands, keeping it slightly proud of the finished height.
8. Pack it down. Compact the mud firmly with a wood or magnesium float — this is what gives dry pack its strength. Loosely placed mud stays crumbly.
9. Screed to the slope. Ride a straightedge from the perimeter screeds toward the drain, cutting the surface to a consistent plane. Fill the low spots it reveals, pack again, and re-cut.
10. Float the surface flat. Work a float in circles to close the surface. Aim for flat and uniform, not glassy — a slightly open texture bonds better with thinset.
11. Check with a level. Lay a level from the drain outward in several directions. Every path should fall evenly toward the drain with no flat spots or dips that hold water.
12. Let it set. Give the bed at least 24 hours before tiling — longer in cool or humid conditions. Keep foot traffic off it entirely for the first day.

Ready for the next step? Our Shower Tile Calculator handles the tile, thinset, and grout that go over your finished bed.

Pre-Slope, Liners, and Two-Bed Shower Pans

If you're building a traditional shower pan with a sheet liner, you need two separate mud beds — and this is the single biggest reason people run short on material.

The Pre-Slope (First Bed)

The bottom bed goes directly on the subfloor and slopes at 1/4 inch per foot toward the drain's weep holes. Its job is to make sure any water that reaches the liner runs to the drain rather than pooling. Skipping the pre-slope leaves standing water trapped under the liner — the classic cause of a shower that smells and eventually rots the floor beneath it.

The Liner

A PVC or CPE membrane drapes over the pre-slope and up the walls, clamped into the two-piece drain. Protect the weep holes with a layer of gravel or the plastic guards that ship with the drain so mud can't seal them shut.

The Finish Bed (Second Bed)

The top bed goes over the liner at the same 1/4 inch per foot and gives you the surface you tile. It should be at least 1-1/4 inch thick at the drain so it has enough body to resist cracking.

Budget accordingly: a 3 × 3 ft pan built this way needs roughly 2.2 cubic feet of mud total, not 1.1. Run the calculator once for each bed, or simply double the thickness figure.

Topical Membranes: One Bed Instead of Two

Modern bonded waterproofing systems put a paint-on or sheet membrane over a single sloped mud bed. That halves the mud, removes the pre-slope, and lowers the total build height — handy when you're matching an existing floor. Follow the manufacturer's instructions closely; these systems have their own drain assemblies and cure times.

7 Deck Mud Mistakes to Avoid

• Mixing it too wet. The most common error by far. Wet mud slumps out of its slope and cures soft. Aim for barely damp — snowball consistency.
• Skipping the pre-slope. A liner laid flat on the subfloor traps water permanently. Always slope the bed beneath a liner.
• Not packing hard enough. Dry pack gets its strength from compaction, not water. Mud that's raked into place and floated lightly stays crumbly and breaks up under tile.
• Using masonry cement or premixed mortar mix. Both contain lime for wall work. For a floor bed use straight portland cement, or a bagged sand/topping mix.
• Guessing the slope. Eyeballing it produces flat spots that hold water. Set your perimeter height from actual drain-to-wall measurements at 1/4 inch per foot.
• Ordering off the maximum thickness. Sizing a sloped bed off its thickest point overbuys badly. Use the average of the drain and perimeter thicknesses.
• Tiling too soon. A fresh bed is fragile. Give it a full 24 hours minimum, and don't kneel on it before then — a knee-shaped dent means cutting the section out and repacking.

Frequently Asked Questions

How do I calculate deck mud by the square foot?

Multiply the floor area in square feet by the average bed thickness in inches, then divide by 12 to get cubic feet. For example, a 3 × 3 ft shower pan (9 sq ft) at an average 1.5" thickness needs about 1.1 cubic feet of deck mud. Enter your numbers above for an exact sand and cement count.

How many square feet does a cubic foot of deck mud cover?

It depends on thickness. One cubic foot covers 12 square feet at 1 inch, 8 square feet at 1-1/2 inches, and 6 square feet at 2 inches. The coverage table above lists every common thickness.

How many bags of deck mud do I need per square foot?

At a 1-1/2 inch average bed, one 60 lb bag of sand/topping mix covers about 4 square feet — roughly 2.5 bags per 10 square feet. At 1 inch it stretches to about 6 square feet per bag. Each 60 lb bag yields about 0.5 cubic foot.

Is deck mud the same as floor mud?

Yes. "Deck mud" and "floor mud" both refer to the same dry-pack mortar — sand and portland cement at roughly 5:1. "Wall mud" or "fat mud" is different: it adds lime so it sticks to vertical surfaces.

How thick should a mud bed be?

A shower pan mortar bed should be at least 1.25 inches at the drain and slope up about 1/4 inch per foot toward the walls, so the average thickness is usually 1.5–2 inches. For leveling a subfloor, beds typically run 1/2 inch to 2 inches.

What's the right sand-to-cement ratio for deck mud?

The standard is 5:1 (sand to portland cement). Use 4:1 for a stronger bed under heavy tile, or 6:1 to stretch material on a thick pour. Avoid masonry cement (it contains lime) for floor beds.

Can I use pre-mixed bags instead of sand and cement?

Yes — a sand/topping mix or a bagged "deck mud" / "sand mix" works well and saves measuring. Each 60 lb bag yields roughly 0.5 cubic foot, so divide your bed volume in cubic feet by 0.5 to get the bag count.

How much water does deck mud need?

Very little — roughly a quart per 5 gallons of dry mix, but go by feel rather than measurement. Add water gradually until a squeezed handful holds its shape without dripping or crumbling. If water runs between your fingers it's too wet; add dry sand and cement in ratio to bring it back.

How much does deck mud weigh?

A packed dry-pack bed weighs about 120 pounds per cubic foot. A 3 × 3 ft pan at 1-1/2 inches is roughly 135 pounds of material — worth knowing before you carry it upstairs one bucket at a time.

How long does a mud bed take to cure before tiling?

Wait at least 24 hours before setting tile, and longer in cool or humid conditions. The bed will feel firm sooner than that, but it dents easily for the first day. Full strength takes about 28 days, which is fine — you can tile long before then.

Do I need a pre-slope under a shower liner?

Yes, always. A liner laid flat on the subfloor traps standing water permanently. The pre-slope is a first mud bed sloped at 1/4 inch per foot so water reaching the liner drains to the weep holes. This means a traditional pan needs two mud beds — budget double the material.

What slope does a shower floor need?

The standard is 1/4 inch of fall per foot from the walls to the drain. On a 30-inch run that puts the perimeter 5/8 inch above the drain. Less than 1/8 inch per foot leaves water pooling; much more than 1/2 inch per foot feels steep underfoot and makes large tile hard to set.

Can I use self-leveling compound instead of deck mud?

Not for a shower pan. Self-leveler finds level by design and cannot hold a slope. It's a good choice for flattening a floor before tile, but any sloped surface has to be built with dry pack.

Can I use play sand for deck mud?

It works, but washed masonry or concrete sand is better. Play sand's fine, rounded grains pack less densely and give a slightly weaker bed. What matters most is that the sand is clean — clay, silt, or organic material stops the cement paste from bonding to the grains.

How do I calculate mud for a sloped shower pan?

Measure the pan, then give the calculator the thickness at the drain and at the perimeter and switch it to Sloped shower pan. It works out the volume-average thickness for you. Do not average the two numbers yourself — on a point drain the true average is (drain + 2 × perimeter) ÷ 3, which is higher than the midpoint.

Is the average thickness just halfway between the drain and the wall?

Only on a linear or trench drain. On a pan that drains to a point, far more of the floor sits out near the thick perimeter than near the drain, so the volume-average is (drain + 2 × perimeter) ÷ 3. A bed running 1-1/4 in to 1-3/4 in averages 1.58 in, not 1.5 in — the midpoint rule leaves you about 5% short.

How much mud does a 3 x 3 ft shower pan need?

At 1-1/4 in over the drain rising to 1-3/4 in at the walls, a 3 × 3 ft pan works out at 1.19 cu ft for the finish bed — about 3 bags of 60 lb sand/topping mix. Add a pre-slope bed underneath and it is 1.44 cu ft in total.

How much extra does the pre-slope bed add?

The pre-slope is the same wedge again: it starts at nothing over the drain flange and rises at the same slope to the wall. On a pan sloping 1/2 in from drain to wall that is about 20% more material than a single-bed pan.

What slope does a shower pan need?

A quarter inch per foot of run from the drain to the wall, which is the standard fall for a tiled shower floor. On a 30 in run that is 5/8 in of rise, so a bed starting at 1-1/4 in over the flange finishes about 1-7/8 in at the wall.