Asphalt Density Explained: Why 145 lbs/ft³?
The default density used in nearly every online asphalt calculator is 145 lbs per cubic foot (≈ 2,322.7 kg/m³). Here's where that number comes from, when to use it, and when to ask your supplier for a more accurate value.
What "Density" Means for Asphalt
Density is mass per unit volume. For asphalt, it depends on three things:
- Aggregate: the rock used (limestone, granite, recycled).
- Binder: bitumen, typically 4–6% of total weight.
- Air voids: small empty spaces after compaction, usually 3–7%.
The Standard Reference: 145 lbs/ft³
This figure represents typical dense-graded hot mix asphalt (HMA) at design air voids. It's published in countless industry references and used as the working assumption in most calculators because it's accurate to within ~3% for the great majority of mixes.
When to Override the Default
- Stone Matrix Asphalt (SMA): denser, often 150 lbs/ft³ or higher.
- Open-graded friction course (OGFC): lower density, ~125 lbs/ft³.
- Porous asphalt: 100–110 lbs/ft³.
- Asphalt millings (RAP): 120–130 lbs/ft³ compacted.
- Cold mix: variable, 130–140 lbs/ft³.
Density in Metric
- 1 lb/ft³ ≈ 16.018 kg/m³
- 145 lbs/ft³ ≈ 2,322.7 kg/m³
- Equivalently: 2.32 g/cm³ or 2.32 tonnes/m³
Effect of Compaction
Loose asphalt out of a truck is around 130 lbs/ft³. Rolled and fully compacted, it rises to 145+ lbs/ft³. That's why the 10% waste buffer matters — your tonnage order is for loose material, but your design thickness is for the compacted final surface.