Three things make the hole bigger than the pool
Ask someone to estimate the dirt from a 16 by 32 pool and they will multiply 512 square feet by an average depth of about five and a half feet, get 2,800 cubic feet, and call it 105 cubic yards. The real number on the default inputs here is 195 bank yards, and the gap is three separate additions that each look small.
Working room. Nobody digs a hole exactly the size of the shell — there has to be space to stand forms, tie steel and get out. Two feet on every side turns 16 by 32 into 20 by 36, which is 720 square feet against 512. That is 41 per cent more footprint before a single extra inch of depth.
Freeboard. The wall keeps going above the water line. The bond beam and the coping detail sit up there, and all of it is excavated. A foot of it adds a foot to the whole footprint.
Floor build-up. The shell floor and whatever base goes under it are below the water, so the hole is deeper than the water is. Another foot, again across the whole footprint.
Two feet of freeboard and floor across 720 square feet is 1,440 cubic feet, or 53 cubic yards, from dimensions that never appear on the pool brochure.
The average depth is not the average of the two ends
A pool floor is usually flat at the shallow end, flat at the deep end, and a straight slope between them. If the flats are different lengths — and they nearly always are, since the shallow flat is where people stand and the deep flat is a small hopper — then the average depth is weighted by length, not by the two end depths.
On the defaults: 14 feet flat at 3.5, 10 feet of slope averaging 5.75, and 8 feet flat at 8.0. That is (14×3.5 + 10×5.75 + 8×8.0) ÷ 32 = 5.33 feet. The midpoint of 3.5 and 8.0 is 5.75, which would have overstated the dig by eight per cent — around 15 cubic yards, or a truck and a bit.
Swell: the dirt gets bigger on the way out
Soil in the ground is bank measure. Once it is dug it is loose measure, and it takes up more room because the digging put air into it. The expansion is the swell factor, and it is entirely a property of the material.
| Material | Typical swell | 195 bank yd becomes |
|---|---|---|
| Sand and gravel | 10 - 15% | 215 - 224 loose yd |
| Common earth, loam | 20 - 25% | 234 - 244 loose yd |
| Clay, wet or dense | 25 - 40% | 244 - 273 loose yd |
| Ripped or blasted rock | 40 - 65% | 273 - 322 loose yd |
Those ranges are what contractors work with rather than measured constants for your yard, which is why swell is an input here. It matters because trucks are charged by the load and loads are counted in loose yards. At 12 loose yards a truck, the difference between sand and clay on this pool is two extra trips.
The general excavation volume calculator covers boxes, trenches and shafts at a single depth, and carries shrink for material being placed and compacted as fill. This page exists because a pool has a sloping floor, a wall above the water and a shell taking up part of the hole afterwards.
The void the shell leaves behind
Once the shell is in, the gap between its outside face and the excavation face gets backfilled. That volume is the excavation footprint minus the shell footprint, times the wall height — 33 cubic yards on the defaults. Two things about it are worth knowing before the spoil pile is hauled away.
The first is that fill compacts. Material dumped loose into a void and then compacted occupies less than it did on the truck, so you buy more than the void measures. The compaction allowance here is an input for the same reason swell is: it depends on the material and on how it is placed.
The second is that keeping enough spoil on site to do the backfill can be cheaper than hauling it all away and buying fill back. Whether that works depends entirely on whether the excavated material is suitable to place against a shell, which is a question for the contractor and sometimes for a soils engineer, not for arithmetic. The calculator shows both numbers side by side so the comparison is at least visible.
What is not on this page
Excavation kills people every year by collapsing on them, and buried gas, electric, water and communications lines are underneath more yards than anyone expects. Call 811 before any digging starts — it is free and it is the one step with no downside. Trench and excavation protection, spoil placement and access are the responsibility of a licensed excavation or pool contractor, and there is no procedure here.
An empty pool can float out of the ground. Groundwater pushing up under an unweighted shell has lifted finished pools clear of their beds, and the season, the water table and the soil all decide whether that is a real risk on a given day. Whether a pool gets emptied, when, and what happens first is a decision for a licensed pool contractor, not something to work out from a web page.
Setbacks, permits, barrier requirements, deck slope, drainage discharge and where equipment may sit are all decided by your building department and, for public or semi-public pools, your health department. Every one of them varies by state, county and city and they change. Take the drawing to them early.
Once the hole exists the rest of the take-off follows: the pool deck and coping calculator for the flatwork round it, the pool surface area calculator for the interior finish, and the pool volume calculator for the gallons that go in it.
Questions people ask
How many cubic yards of dirt come out of a pool?
More than the pool volume, and the multiplier is bigger than most people guess. On a 16 by 32 pool with a 3.5 to 8 foot floor, two feet of working room, a foot of freeboard and a foot of floor build-up, the calculator gives 195 bank cubic yards. The water in that pool is only about 101 cubic yards. The extra comes from the over-dig footprint, from the wall above the water line, and from the shell and base below the floor, and every one of those is invisible on the sales drawing. If your figure is coming out near the water volume, one of those three is missing.
What is the difference between bank yards and loose yards?
Bank measure is the volume the material occupied in the ground, undisturbed. Loose measure is the volume of the same material after it has been dug, when the digging has bulked it with air. The ratio is the swell factor and it belongs to the soil, not to the excavation. It matters because the two numbers get used for different things: the hole is measured in bank yards, but haulage is priced and counted in loose yards, so estimating trucks off the bank number undercounts every time. Compacted measure is a third figure again, smaller than bank, and it is the one that applies to fill being placed and rolled.
How much working room does a pool excavation need?
It depends on the construction method and on who is doing it, which is why it is an input rather than a constant. A gunite pool needs room to set steel, run hose and stand a nozzle operator around the perimeter, and contractors commonly work with a couple of feet. A fibreglass shell needs enough clearance to lower and set the shell and to place backfill evenly as the water goes in. A vinyl liner pool with panel walls needs room to brace panels. The important thing is that the number gets set by the person doing the work, before the machine turns up, because it drives the footprint and the footprint drives everything else on this page.
Can I keep the dirt on site instead of hauling it?
Sometimes, and it can be a large saving because haulage is often the biggest single line in the excavation. What decides it is whether the material is suitable for what you want it to do — backfill against a shell, a raised berm, regrading a lawn — and that is a contractor and sometimes a soils engineer question. Organic topsoil, expansive clay and material full of rock all behave badly as structural backfill. The other constraint is volume: a berm made of 200 loose yards is a substantial landform and neighbours notice. The calculator lets you set the share you keep and shows whether it covers the backfill void.
Why does the calculator ask for the shell wall thickness?
Only to work out the backfill. The wall thickness sets how far the outside face of the shell sits from the inside dimension you entered, which in turn sets how wide the gap between the shell and the excavation face is. That gap times the wall height is the void that has to be filled once the shell is up. It does not affect the excavation volume at all, because the hole is dug to the excavation dimensions regardless of how thick the shell later turns out to be. If you leave it at the default the backfill figure will be roughly right; the excavation figure is unaffected either way.