Gravel calculator for a concrete slab

The gravel layer under a concrete slab is easy to underrate because it disappears the moment the concrete is poured and never gets looked at again, yet it does more to determine how the slab performs over the following years than almost any other single decision in the project. A slab poured directly onto bare, unprepared soil will move with that soil, cracking or heaving as it freezes, thaws, dries or absorbs water unevenly across the footprint over successive seasons. The gravel base interrupts that relationship: it drains water away from directly under the slab and it gives the concrete a stable, even surface to rest on, so the soil's own movement matters far less once a properly built base sits between the two layers. This is also why skipping the base entirely to save a delivery fee is a false economy: the cost of the gravel is small relative to the cost of breaking out and repouring a slab that has cracked from below, and a base that was never there cannot be added back in afterward without removing the concrete above it first.

Depth and material for a slab base are chosen with both of those jobs in mind at once, drainage and load support, which is why crusher run rather than a rounded or decorative stone is the standard material here. Its angular particles lock together under compaction into a dense structural layer, while the voids between properly graded particles still let water pass through rather than pooling against the underside of the slab after rain. A base that is too shallow fails on the load support side first, showing up as cracking under concentrated weight; a base on soil that was never properly drained fails on the water side instead, showing up as heaving in freeze prone climates where trapped moisture expands. Checking local frost depth guidance before finalizing the base thickness is worth the extra few minutes, since the right depth for a mild climate is often not enough where the ground freezes hard every winter. If the same slab needs drainage gravel along one edge to carry water away from the site, the drainage gravel calculator handles that separate zone with its own area and depth figures.

Compaction matters more for a slab base than for almost any other gravel application on this site, because once concrete is poured on top there is no practical way to go back and fix a soft spot underneath it without breaking out the slab itself. The base should be compacted in lifts, thinner layers compacted one at a time rather than one thick layer compacted all at once, since a single deep lift tends to compact well near the surface while leaving the bottom portion loose. That loose bottom layer then settles slowly under the slab's own weight over months or years, which is exactly the slow, uneven settling a good base is supposed to prevent in the first place, and the cost of fixing it after the fact is far higher than compacting it properly in lifts up front.

A detail worth deciding before you order rather than after: the base generally should extend a short distance beyond the slab's own edges, not stop flush with the formwork, because the perimeter of a slab is where edge support matters most and where a base that ends exactly at the visible boundary is most likely to erode or settle unevenly first. Measuring the base area slightly larger than the planned slab footprint and entering that larger figure above, rather than the slab's own dimensions, builds that margin in from the start instead of discovering the shortfall once the forms are already set.

Once the gravel volume is settled, it is worth running the slab itself through the concrete calculator or the more specific concrete slab calculator so the two material orders, base and pour, are planned together rather than as separate afterthoughts arriving on different schedules, and the paver base calculator is a useful cross check if part of the same site transitions from a poured slab to a paved surface using a related base depth that should stay consistent across the boundary. It is worth confirming with the concrete supplier or crew what base depth and compaction they expect to see before the pour, since some crews will refuse to pour onto a base they consider inadequate, and finding that out the day of the pour costs far more time than confirming it during planning.