Concrete Base Calculator: Slab Concrete and Gravel Base

Quick answer

Updated 2026-10-22 · Uses the same slab and gravel functions as our concrete slab calculator and gravel calculator. Base depth from IRC R506.2.2 and Quikrete project instructions; densities from Missouri and Ohio DOT. No built-in prices.

How do I use the concrete base calculator?

1. Enter the slab length, width and thickness. For round or odd shapes, split them into rectangles and add the results.
2. Keep 5–10% waste on the concrete for spillage and an uneven base.
3. Enter the base depth you want after tamping. The calculator scales it up to the loose depth you spread.
4. Replace the density with the tons-per-yard figure your quarry or yard gives for the exact product (crusher run, #57 stone, sand).
5. Order the concrete in yards or bags and the base in loose yards or tons, whichever unit the supplier sells by.

What formulas does the calculator use?

It runs two existing TrueToolCalc functions on the same footprint: the slab volume and the gravel volume and weight.

How much concrete and gravel base for common slab sizes?

A 12×12 ft slab 4 in thick on a 4 in base takes 1.96 yd³ of concrete and 2.22 yd³ of loose gravel: the loose base order is 1.14 times the concrete order at these settings, so the base is not a small extra. Each row was computed with Calcs.concreteSlab and Calcs.gravel from this site's calcs.js.

Reading the table: on a 4 in slab with a 4 in base, every 100 ft² adds about 2.31 tons of gravel and 2.47 yd³ of soil to haul away, on top of 62 bags of concrete.

How much does the base depth change the gravel order?

Each extra inch of compacted base on a 12×12 ft slab adds 0.56 yd³ of loose gravel, about 0.83 tons. Going from Quikrete's 3 in slab base to a 6 in base doubles the order.

Thick bases go down in layers: Colorado DOT caps each compacted layer of aggregate base at 6 in, so an 8 in base is two lifts.

Loose yards or compacted density: which ton figure is right?

Both give similar tons only if each density is paired with its own volume. Loose yards × 1.5 t/yd³ gives 2.31 tons for a 10×10 ft, 4 in base; compacted yards × Ohio DOT's 2.0 t/yd³ in place gives 2.47 tons, 7% more. Mixing them up (compacted yards × a loose density) is the mistake that leaves you short.

What this shows: the Ohio DOT compacted-in-place figure is about 7% heavier than the 1.25 loose factor with 1.5 t/yd³, and with MoDOT's 1.40 loose density the gap to its own compacted factor is 19%. A 1.25 factor with 1.40 t/yd³ implies a compacted density of only 1.75 t/yd³, so the DOT numbers suggest a dense-graded base compacts more than 5-to-4. If you order by weight, the compacted-density column (C or D) is the safer figure; if you order by loose yards, add a margin or raise the factor toward 1.4.

Worked example: a 12×12 ft slab on a 4 in base

A 12×12 ft slab 4 in thick on a 4 in compacted base needs 1.96 yd³ of concrete and 2.22 yd³ (3.33 t) of loose gravel, with 3.56 yd³ of soil dug out.

Result looks wrong? Check these

• Gravel looks like too little: check that the density matches the volume type. 1.5 t/yd³ is a loose figure; a compacted base weighs about 2.0 t/yd³ in place.
• Supplier quotes compacted yards: set the loose/compacted ratio to 1 so you do not count the shrinkage twice.
• Concrete order looks high for 2×4 forms: 2×4 lumber is 3.5 in deep, so a slab screeded to the forms needs 12.5% less than a true 4 in.
• Base wider than the slab: if you run the base past the edges, enter the base footprint in a second run and use only its gravel lines.
• Thickness typed in feet: both depths are in inches.

When is a gravel base required under a slab?

Under the International Residential Code a base is required when the slab is below grade: section R506.2.2 calls for 4 in of clean graded sand, gravel, crushed stone, crushed concrete or crushed blast-furnace slag passing a 2 in sieve, except on well-drained or sand-gravel Group I soils. R506.1 sets a 3.5 in minimum slab floor, and R506.2.1 limits compacted fill to 24 in of clean sand or gravel or 8 in of earth unless approved. Outdoor flatwork such as patios and walks is outside that section, but Quikrete's patio and walkway guides still use a 4 in compacted gravel subbase, noting that it improves drainage on clay soil. Your local building department has the final word.

Frequently asked questions

Sources

• International Code Council, 2021 International Residential Code, Section R506 Concrete Floors (On Ground): R506.1 3.5 in minimum, R506.2.1 fill, R506.2.2 4 in base course and Group I exception (retrieved 2026-10-22).
• Quikrete, Pouring & Finishing Concrete Slabs: excavate about 7 in (3 in gravel base + 4 in concrete), spread 3–4 in of gravel and tamp (retrieved 2026-10-22).
• Quikrete, Building a Concrete Patio: about 5 in of compactible gravel tamped to 4 in; 4 in subbase + 3.5 in concrete with 2×4 forms (retrieved 2026-10-22).
• Quikrete, Concrete Mix No. 1101 data sheet: 80 lb bag yields 0.60 ft³ (retrieved 2026-10-15).
• Ohio Department of Transportation, 2005 Construction and Material Specifications, Item 304 Aggregate Base, 304.07: crushed stone and crushed gravel 4,000 lb/yd³ compacted in place (retrieved 2026-10-22).
• Missouri DOT Engineering Policy Guide, 450 Estimate Factors (archived 2009 version): Type 1 aggregate base 0.058 tons/yd²/inch compacted; gravel and crushed stone based on 1.40 tons/yd³ loose; statewide preliminary estimates only (retrieved 2026-10-22).
• Colorado DOT, 2023 Standard Specifications, Division 300, 304.04 and 304.06: 6 in maximum compacted layer, 95% of maximum density (retrieved 2026-10-22).

Related calculators & guides

• Concrete slab volume in yards
• Gravel in yards and tons
• Bags of premixed concrete
• Bags vs ready-mix slab cost
• Footing volume
• 4 inch vs 6 inch slab

Last updated 2026-10-22 · About TrueToolCalc and how we check numbers · Found an error? Use the contact page.