Shotcrete Calculator

1. Enter the surface area to be sprayed in square feet. For a curved shell or slope, use the developed (unrolled) area, not the flat footprint.
2. Enter the finished thickness in inches, exactly as the field is labelled.
3. Set the rebound allowance: 5 to 15 percent for most wet-mix work, higher for dry-mix and overhead. Add an optional price per cubic yard to estimate material cost.
4. Read the result: shotcrete to supply in cubic yards is the ordering number, because it already includes the rebound that never sticks to the wall.

Here is what each result line means:

A shotcrete calculator converts the area and thickness of a sprayed-concrete job into the volume of material you need to supply, allowing for rebound. It answers the question every shotcrete estimate starts with: how much concrete do I batch or order to end up with the thickness I want on the wall?

Shotcrete is concrete or mortar sprayed pneumatically onto a surface at high velocity. The wet-mix process pumps ready-mixed concrete to the nozzle; the dry-mix process, sometimes called gunite, adds water at the nozzle. Either way, some of the material bounces off instead of sticking, and that rebound is the reason supply always exceeds the in-place volume. This tool is an estimating aid for walls, pool and spa shells, slopes, tunnels and repairs. It is not a mix-design or structural tool: it gives the volume to supply, not the strength, reinforcement or thickness your project requires. If you still need the mix proportions, size them with the concrete mix calculator, or work out the bags of cement with the cement calculator.

The in-place volume is simply the area times the thickness. Because thickness is entered in inches, the tool divides it by 12 to get feet, then multiplies by the area in square feet to get cubic feet. It then applies the rebound allowance as an overage on that in-place volume, because rebound is extra material you must supply but never ends up on the wall.

1. Convert thickness from inches to feet by dividing by 12 (a 3 inch layer is 0.25 feet).
2. Multiply the surface area by that thickness to get the in-place volume in cubic feet.
3. Multiply by one plus the rebound fraction to get the supply volume (10 percent rebound means multiply by 1.10).
4. Divide cubic feet by 27 for cubic yards, and multiply by 0.0283168 for cubic meters.

The rebound loss line is just the difference between the two volumes, so you can see how much material the allowance is adding.

Suppose you are spraying a retaining wall 100 square feet in area to a finished thickness of 3 inches, using wet-mix shotcrete with a 10 percent rebound allowance.

Calculation: thickness is 3 / 12 = 0.25 ft. In-place volume = 100 x 0.25 = 25 cu ft. Adding 10 percent rebound gives 25 x 1.10 = 27.5 cu ft to supply. In cubic yards that is 27.5 / 27 = 1.02 cu yd, the in-place volume is 25 / 27 = 0.93 cu yd, and the rebound loss is about 0.09 cu yd. At 150 pounds per cubic foot the supplied material weighs about 2.06 short tons.

The lesson: order against the supply figure, not the in-place figure. If you batch only the 0.93 cubic yards that stays on the wall, you run short the moment rebound starts falling to the ground.

Process: Wet-mix vs Dry-mix

The dry-mix (gunite) process generally throws off more rebound than wet-mix, because water is added at the nozzle and the stream hits the surface harder and drier. The American Shotcrete Association puts typical rebound at 5 to 15 percent, averaging about 10 percent of the material by weight, with dry-mix at the higher end.

Surface Position

Orientation matters: vertical placement generally rebounds less than overhead work, and floors shot from above lose the least. Overhead and dry-mix work together can push losses toward the 10 to 30 percent range that ACI 506R cites for shotcrete.

Nozzle Operator and Air

A skilled nozzle operator holding the right distance and angle, with correct air pressure, is the single biggest control on rebound. Poor technique or wrong air can double the waste.

Thickness and Reinforcement

Thin layers over congested mesh or rebar rebound more because material ricochets off the bars before the paste can build up. Thicker, well-consolidated layers capture more of the stream.

Overspray and Seepage

Wind-blown overspray and water seeping through the substrate both reduce how much material adheres; controlling water is essential or the shotcrete will not bond at all.

Both processes place the same sprayed concrete; they differ in where water is added, which changes rebound, dust and control. Modern equipment has narrowed the gap, so the choice often comes down to access, volume and contractor expertise.

For the water content that drives strength on the wet-mix side, check the water cement ratio calculator before you fix the mix.

Pool and Spa Shells

Gunite and shotcrete pools are sprayed to a shell thickness, so estimate the developed surface area times thickness, then add rebound, to order the batch.

Retaining Walls and Slopes

Soil nailing, slope stabilisation and retaining walls use sprayed concrete over mesh or nails; size the material from the face area and the specified thickness.

Repairs and Tunnels

Spalled concrete repairs and tunnel linings are classic shotcrete jobs. For a repair, use the patch area and depth; for a lining, use the developed area of the section.

1. Ordering the In-place Volume

The most common error is batching only the concrete that stays on the wall. Rebound is real material you must supply, so always order against the supply figure that includes it.

2. Using a Flat Footprint for a Curved Surface

A pool shell or slope has more surface than its plan outline. Use the developed (unrolled) area or you will under-order.

3. Mixing Up Feet and Inches

Entering thickness in feet instead of inches shrinks the result twelvefold. Keep area in square feet and thickness in inches, as the fields are labelled.

4. Using One Rebound Figure for Every Job

A 10 percent allowance suits efficient wet-mix walls but understates dry-mix overhead work, which can lose far more. Match the allowance to the process and position.

5. Treating Material Cost as Installed Cost

The cost line here is material only. Placed shotcrete also carries pumping, labour, mesh and nozzle-crew costs, which usually dwarf the material.

The geometry is exact; the real variable is the rebound allowance, which depends on the mix, the process, the position and the nozzle operator rather than on arithmetic.

Rebound is the material that bounces off the receiving surface instead of sticking, mostly coarse aggregate with some sand and cement. It is wasted, so you must supply more concrete than ends up on the wall. The American Shotcrete Association estimates it at 5 to 15 percent, averaging about 10 percent by weight.

Add 5 to 15 percent for efficient wet-mix work and more for dry-mix or overhead spraying, where ACI 506R cites losses of 10 to 30 percent. The allowance is extra material you supply but never place, so order against the supply figure this tool gives, not the in-place volume.

They are the same family. Gunite is the older trade name for the dry-mix process, where dry material is conveyed to the nozzle and water is added there. Wet-mix shotcrete pumps already-mixed concrete to the nozzle. Both spray concrete at high velocity; dry-mix generally rebounds a little more.

Use the developed surface area of the shell, not the plan outline, because the curved walls and floor hold more area than the footprint. Multiply that area by the shell thickness, then add a rebound allowance. Enter those figures above to get the cubic yards to supply.

Wet-mix generally rebounds less than dry-mix, because the concrete arrives at the nozzle already mixed and wetted. Modern dry-mix equipment and predampening narrow the gap, but for the same job wet-mix usually wastes less material.

About 4,050 pounds, or roughly 2 short tons, for normal-weight material at 150 pounds per cubic foot. The tool shows the estimated weight of the supplied volume so you can plan delivery, pumping and access.

Because rebound never reaches the wall. The in-place volume is the finished layer, while the supply volume adds the material that bounces off during spraying. The difference is the rebound loss line, and it is why you always batch more than the geometry alone suggests.

No. The optional cost line is for material only, at the price per cubic yard you enter. Installed shotcrete also carries pumping, labour, mesh, admixtures and rebound disposal, which usually cost more than the concrete itself. Treat the figure as a material estimate.

It depends on the application and the specification: thin repairs may be 1 to 2 inches, pool shells and walls 3 to 6 inches, and structural linings more. Use the thickness from your plans or engineer; this tool estimates volume, not the required thickness.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• American Shotcrete Association, rebound and dry-mix efficiency (ASA; rebound 5 to 15%, average 10% by weight; dry-mix higher than wet-mix).
• American Shotcrete Association, reasonable rebound and ACI 506R losses (ASA citing ACI 506R-05; approximate shotcrete losses 10 to 30%; vertical less than overhead).
• American Shotcrete Association, about shotcrete (ASA; shotcrete as a process, wet-mix and dry-mix, dry-mix sometimes called gunite).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete, unit weight and ordering context).