Aggregate base takeoff: converting SY to tons with compaction factors

Aggregate base is drawn as a thickness in a detail and delivered by the ton. Getting between the two requires two decisions — the area it covers and the density you assume — and both should be written down next to the number.

Step 1: get the area right

Base area is rarely identical to pavement area. Most details show base extending past the back of curb, under the curb itself, and beyond the pavement edge by several inches. On sidewalks it follows the walk. Read the section before assuming the areas match.

• Include base under curb and gutter where the detail shows it.
• Include the specified overbuild beyond the pavement edge.
• Separate heavy-duty from light-duty sections; base thickness usually differs.
• Keep sidewalk and pad base as separate quantities from roadway base.

Step 2: convert thickness to volume

The arithmetic is simple: SY × thickness in feet ÷ 3 = CY of compacted material, then CY × tons per CY = tons. A 4,500 SY lot with 6" of base is about 750 compacted CY, or roughly 1,050 tons at 1.4 tons per CY.

Step 3: state your density and allowances

Crushed aggregate density varies with gradation and source. Working figures commonly land between 1.35 and 1.5 tons per compacted cubic yard; local suppliers can give you a number for their product. Whatever you pick, put it on the line item so a reviewer can adjust rather than guess.

Compaction and loss allowances are separate from density. Material placed loose and compacted to spec consumes more than the theoretical in-place volume, and thin lifts over soft subgrade consume more still.

Section pitfalls that change the number

1. Undercut and replacement: if the geotech requires proof rolling and undercut, that stone is a separate quantity from the structural section.
2. Subbase versus base: two different gradations at two thicknesses are two line items.
3. Geotextile or geogrid shown under the stone, measured in SY.
4. Stabilized subgrade — lime or cement treatment — which replaces stone thickness rather than adding to it.
5. Temporary construction access stone that will be removed or incorporated later.

Checks

• Does base area exceed paving area by roughly the overbuild the detail requires?
• Does base thickness match the section for each pavement type you measured?
• Have you accounted for stone under sidewalks, pads and curb separately?
• Is the geotextile area consistent with the base area where it is required?

Aggregate is heavy, cheap per ton and expensive in aggregate — the risk is not in the unit price but in the volume. Making the area, the thickness and the density visible turns a large number into one that can be checked in a minute.