Pool Concrete Calculator

How the pool concrete calculator works

A concrete pool shell is a box with its top missing: a floor slab and four walls. Enter the inside length, width and average depth of the pool, the thickness of the floor and of the walls, and the calculator takes the outer box and subtracts the water space inside it:

V = (L + 2t_w)(W + 2t_w)(D + t_s) − L × W × D

L, W and D are the inside length, width and average depth, t_w the wall thickness and t_s the floor thickness. Working it this way counts two things a quick estimate misses: the part of the floor slab that runs under the walls, and the four corner blocks where the walls meet. Your waste allowance goes on top.

Worked through

With the metric defaults, an 8 × 4 m pool averaging 1.5 m deep with 200 mm floor and walls has an outer plan of 8.4 × 4.4 = 36.96 m². The floor is 36.96 × 0.2 = 7.39 m³ and the walls (36.96 − 32) × 1.5 = 7.44 m³, so the shell is 14.83 m³ in place, or 15.57 m³ with 5% waste. Floor area times thickness plus perimeter times depth times thickness would give 13.6 m³, about 8% short. In US units, the default 26 × 13 ft pool averaging 5 ft deep with 8 in floor and walls comes to 19.63 yd³ in place and 20.61 yd³ with 5% waste.

Average depth

Most pools slope from a shallow end to a deep end. Measure each depth from the floor to the top of the wall, not to the water line: the walls rise above the water to the coping, and that height is concrete too. If the floor falls evenly, the average of the two depths gives the wall area exactly: a pool 1.0 m deep at one end and 2.0 m at the other has an average of 1.5 m. A pool with a hopper, a steep drop or a separate deep area needs its sections worked out one at a time, because the average then no longer describes the walls.

Sprayed concrete and rebound

Most concrete pools are shot rather than poured, and some of the material bounces off the surface. The US Army Corps of Engineers' Standard Practice for Shotcrete (EM 1110-2-2005) says rebound for conventional dry-mix shotcrete, the process pool builders usually call gunite, "can be expected to be at least 20 percent of the total material passed through the nozzle" in the best of conditions, and that wet-mix shotcrete rebounds somewhat less. The American Concrete Institute's Guide to Shotcrete also notes that wet-mix normally has less rebound and waste, and lists swimming pools among its standard uses.

If a fifth of what goes through the nozzle ends up on the ground, the nozzle has to deliver the in-place volume divided by 0.8, which is 25% more. The result gives that figure for dry-mix: 18.54 m³ for the default metric pool. For wet-mix shotcrete or cast concrete, enter your own waste allowance, and ask the contractor what they allow. The American Shotcrete Association's article on gunite and shotcrete pools adds that rebound must be cleared out rather than left in the shell, since rebound left in place is one of the causes of cracking it lists.

Thickness and steel

The floor and wall thicknesses are inputs because they are design decisions, not rules of thumb. As the American Shotcrete Association's pool FAQs put it, pool wall thickness is part of the pool engineer's design, and a thicker wall can need more reinforcing steel. Soil, groundwater, the height of retained ground and the pool's shape all change the answer, so take both thicknesses from the drawings.

The shell weight is given at about 2,400 kg per cubic metre, which matters for access, crane lifts and any pool built on or near a structure. The default metric shell weighs about 35.6 tonnes.

Checking a quote

Contractors quote shotcrete in different ways: by the volume placed, by the volume delivered, or as a price for the shell. Compare their volume with the two figures here. A delivered volume close to the in-place shell leaves nothing for rebound, and a gunite quote well above the 20% rebound figure is worth asking about. The dimensions they price from should match the drawings, including any benches, steps or a raised beam at the top of the wall, which this calculator leaves out.

What this tool does not do

It does not design the shell, size the reinforcement, allow for steps, benches, beams or coping, or cover curved and freeform pools, which need their shapes worked out separately. It gives the concrete for a rectangular shell and a way to check a quote.

Using this calculator with other BuildMetricLab tools

The pool volume calculator gives the water the shell will hold, and the pool tile calculator covers the finish inside it. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.