Compaction Lift Thickness Calculator

Lift thickness is one of the few construction numbers where public specifications agree almost exactly. Four states, four documents, one answer: 8 inches loose. Everything below was read in the document itself, not in a summary of it.

Rock changes the numbers, upward. TxDOT builds rock embankment in layers of 18 in. or less and never over 2½ ft, and requires the layer to be deeper than the largest rock in it. MaineDOT allows compacted rock layers to 3 ft. Caltrans ties the layer to the stone: where the material is over 50% rock larger than 8 inches, the loose layer must not exceed the maximum size of rock in it.

Because that is the depth the roller has to work through. Compaction energy falls off with depth, so a thick lift compacts at the top and stays loose at the bottom — and a nuclear gauge sitting on the surface will happily tell you it passed. The soft layer shows up later, as a dip in a road or a settled trench patch, two winters after anyone could still fix it cheaply. Proof rolling exists precisely to find what the gauge missed.

This is also why the equipment sets the ceiling and not the material. INDOT's provision is the clearest statement of it in print: 9 inches loose behind a ten-ton roller making five passes, and 4 inches in the same soil the moment the only thing that fits is a rammer. Same dirt, less than half the lift.

Loose to compacted is a real number and it is bigger than most people expect: eight inches of ¾ inch crushed base spread on the ground is under six inches once it is rolled. For aggregates these were measured directly. For natural soils the study publishes two separate steps — compacted to bank, then bank to loose — and the combined figure below is those two multiplied together, which is arithmetic and not a measurement of ours.

Aggregate values measured in Table 4.11 of WHRP G22-05 (WisDOT, December 2023). Soil values derived as (1 + compacted-to-bank) × (1 + bank-to-loose) − 1 from Table 4.10 of the same report. The clay spread is wide because clay is: the study found some eastern Wisconsin clays already at or above their Proctor maximum dry density while still in the ground.

Compacted thickness of one lift

compacted per lift = loose lift / (1 + expansion % / 100)

A lift shrinks as it is rolled. Spreading 8 in of ¾ in crushed base leaves about 5.7 in in place at the measured 41.5% midpoint — which is why a 24 in fill takes five lifts and not three.

Number of lifts

lifts = ceiling( total compacted thickness / compacted thickness of one lift )

What you actually spread

compacted per lift (adjusted) = total / lifts · loose per lift = compacted per lift × (1 + expansion % / 100)

Rounding up the lift count means each lift ends up slightly thinner than the maximum. That is the number to mark on the stakes.

Compacted-to-loose expansion, aggregates

measured directly: ¾ in base 36–47%, 1¼ in 34–52%, 3 in 34–45%, breaker run 27–30%, select crushed 27–29%, granular backfill 38–52%, structure backfill 35–48%, RAP 35–46%, RCA 37–60%

Compacted-to-loose expansion, natural soils

derived: (1 + compacted-to-bank) × (1 + bank-to-loose) − 1

The study publishes the two steps separately, not the combined figure. Multiplying the published ranges gives sand with gravel 29–37%, clean sand 13–19%, silty sand 3–46%, silt 42%, clay 20–72%. It is arithmetic on published values, not a measurement of ours — treat the spread as real.

Volumes, when an area is entered

compacted yd³ = area (ft²) × total (in) / 12 / 27 · loose yd³ = compacted yd³ × (1 + expansion % / 100)