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Calculator Group

Concrete and Site Work Estimating

Published July 19, 2026

The 8 calculators in this group

Site work is designed in one unit and delivered in another. A slab, a base course, a driveway are all planned as a volume — so many cubic yards to fill a footprint at a given depth — but the truck that shows up prices and weighs its load, because gravel, sand and asphalt are all sold by the ton. Getting from one unit to the other requires a density figure specific to the material, and it is the single most common place a site-work order comes back wrong: right on volume, wrong on the weight the volume actually turns out to be.

The second conversion this group shares is less obvious and just as costly to miss: dug soil does not stay the size of the hole it came out of. Undisturbed ground compacts tightly; the moment it is excavated, air gets mixed into it and the same material takes up noticeably more space in a pile or a truck bed than it did in the ground. A hole and the pile it produces are measured by two different numbers even though nothing was added or removed, and confusing the two is how a haul-away order comes up short on truck capacity.

A cubic yard in the design and a cubic yard on the truck are not the same weight

Gravel, sand and asphalt each carry their own density, and none of them convert to weight at the same rate. A cubic yard of compacted gravel base weighs meaningfully more than a cubic yard of sand, which in turn weighs differently than the same volume of hot asphalt mix, and a delivery ordered in tons using the wrong material's density figure either shorts the job or leaves a costly amount sitting unused in the driveway. The volume you design to is fixed by the footprint and the depth; the weight you order is that volume passed through a density conversion specific to whatever is actually being delivered.

That is why the gravel calculator and the sand calculator both report a weight figure alongside the volume rather than leaving that conversion to be done by hand at the supplier's counter — it is exactly the kind of unit-mismatch error that is invisible until the truck is already there and light, or heavy, by a fraction that matters.

Digging a hole makes a bigger pile than the hole was

Soil in the ground is compressed by everything sitting on top of it, and once excavated it loosens, typically taking up somewhere around a quarter to a third more space than it did undisturbed — the swell factor. That means a hole measured at, say, 10 cubic yards produces a pile, or fills a haul-away truck, closer to 12 or 13 cubic yards once it's out of the ground, and ordering disposal capacity to match the hole's volume rather than the swelled volume is a reliable way to need a second truck.

The reverse matters just as much on backfill: soil put back into an excavation and compacted returns most of the way toward its original density, which is why backfill volume calculations use a different factor than removal volume does, even for the identical dirt. Confusing swell with backfill compaction, or applying one factor where the other belongs, is a subtler version of the same mistake and shows up as either too much spoil hauled away or not enough fill left to finish the job.

The order this group runs in: excavate, base, reinforce, place

Excavation comes first for an obvious reason — nothing else in this group has a footprint to work against until the ground is opened up and cleared to the right depth. Base material comes second, compacted in place, because a slab poured onto loose or unprepared soil settles unevenly as that soil consolidates later under load, cracking a surface that had nothing wrong with the concrete itself.

Reinforcement goes in third, after the base is compacted and before concrete arrives, because rebar or mesh has to be positioned and supported within the forms before there is anything to place it inside. Placing concrete is last, and it has to be last, because pouring before the base is properly compacted just moves the settling problem underneath a slab that is now much harder to fix than the gravel was.

Rebar is a lapped grid, not a length total

A rebar quantity is not simply the slab's perimeter and a few interior runs added up — it is a full two-way grid, bars running each direction at a set spacing, and every bar that isn't long enough to span the whole dimension in one piece needs a lap splice where two bars overlap enough to transfer load between them. That overlap is additional material beyond the slab's raw dimensions, not a rounding allowance, and skipping it in a hand calculation is a common way a rebar order comes up short by exactly the length of however many splices the job needed.

Bar spacing itself is a structural decision in the same way stud or joist spacing is elsewhere on this site — tighter spacing means more bars and more laps for the identical slab, and it is set by load and slab thickness rather than by habit, which is worth confirming against the actual project requirement before assuming a default spacing is close enough.

Pavers and poured concrete solve the same footprint with different arithmetic entirely

A poured slab and a paver patio can cover the identical footprint and still be two completely different calculations. Poured concrete is a volume problem — area times thickness, converted to cubic yards, then to the bags or truckloads that deliver that volume. Pavers are a unit-count problem, the same modular-counting logic that governs brick and block elsewhere on this site: how many individual units, of a stated size, fill the footprint once the pattern and its cut waste are accounted for.

The base beneath both is where the projects converge again — both need a compacted gravel base, sized the same volume-to-weight way as any other bulk material in this group — which is why the paver calculator and the concrete slab calculator share a base-preparation logic even though the material going on top of that base could not be more different.

FAQ

Do I need the excavation calculator before or after the concrete slab calculator?

Before. Excavation volume and the swell factor determine how much spoil has to be hauled away, and that number needs to be settled while the hole is still being planned, not after a slab quantity has already been worked out. The slab calculator's inputs — footprint and thickness — feed the excavation depth, so the two are run close together, but excavation and haul-away planning should happen first.

My driveway is asphalt, not concrete. Do I still need the gravel calculator?

Yes — asphalt pavement sits on a compacted gravel base the same way a concrete slab does, and that base is calculated the same volume-to-weight way regardless of what finished surface goes on top of it. Run the gravel calculator for the base course first; the asphalt mix itself is a separate order again, since asphalt's own density is different from the base gravel underneath it.

Should rebar be ordered before or after the concrete volume is settled?

After the slab's footprint and thickness are settled, since rebar spacing and grid size are derived from those dimensions, but before the concrete is actually poured, since the rebar has to be placed and supported inside the forms before the pour happens. Slab volume and rebar quantity are worked out from the same dimensions but the rebar physically goes in first.

I'm doing a paver patio, not a poured slab. Do I need the concrete slab calculator at all?

Only if there is also a poured concrete footing or edge restraint involved, which some paver installations use and others don't. For the base itself, the gravel calculator covers the compacted stone most paver systems are laid on, and it is worth reading through an actual patio build to see how base preparation and surface material stay separate calculations even on the same job.