What cut and fill is
A grading plan tells the contractor where the ground sits today (the existing grade) and where it needs to sit after construction (the proposed or finish grade). Wherever the existing surface is above the proposed, the contractor cuts — excavates the soil down to grade. Wherever the existing is below the proposed, the contractor fills — places soil to raise the ground to grade. The volume of cut and the volume of fill are the two numbers that drive the earthwork bid.
Net (cut minus fill) tells you whether the site balances. Positive net means surplus cut to haul off-site. Negative net means fill shortfall — soil has to be imported. Both directions cost money; designers generally try to balance the grade close to zero net to minimize hauling.
The four methods
1. Average depth (simple) method
The fastest preliminary method. Treat the site as a single rectangle with one average cut depth and one average fill depth. Total cut volume = area × average cut depth, divided by 27 if you want cubic yards from cubic feet. For a 100 ft × 100 ft site with 1 ft of average cut, you get 100 × 100 × 1 = 10,000 ft³ ÷ 27 ≈ 370 CY.
The catch: “average cut depth” here means the average over the entire site, treating fill or zero-depth areas as zero. That’s not the same as the average depth in the cut zone. If only half your site is being cut, the site-wide average cut depth is half of what an engineer would call the local average. Apply the formula consistently or use the grid method.
2. Grid method
Overlay a regular grid on the site plan. For each cell, read the existing elevation and the proposed elevation at the cell's center (or average the four corner elevations for more accuracy). Cell depth = existing − proposed. Positive means cut, negative means fill. Cell volume = cell area × cell depth. Sum all cells for total cut and total fill.
Grid spacing is a tradeoff between effort and accuracy. 20-ft grids are common for sites with moderate elevation change. Tighter grids (10 ft) for steep or complex sites. Sloppier grids (50 ft) for broadly flat sites where elevation change is small.
3. Average end area
Used for cuts that follow a path — roads, ditches, channel excavations. Take cross-sections at regular stations along the alignment. The volume between two consecutive sections is approximatelyV = (A₁ + A₂) / 2 × Lwhere A₁ and A₂ are the cross-section areas and L is the distance between stations. Sum across all section pairs for the total.
4. Prismoidal formula
A refinement of average end area that adds the cross-section at the midpoint:V = L / 6 × (A₁ + 4Aₘ + A₂)Exact for parabolic surfaces. The correction over average end area is typically under 5% on uniform earthwork, larger on transitions from deep cut to shallow cut. Most bid-grade software computes the prismoidal volume automatically.
Shrink and swell
Soil in its undisturbed state (bank volume) expands when excavated and compacts when placed and rolled. A pile of excavated soil in the truck bed occupies more space than it did in the ground — that's swell. The same soil in a structural fill, after compaction, occupies less space than bank — that's shrink.
Hauled volume = bank volume × (1 + swell percent / 100). A 20% swell on 1,000 CY of bank cut yields 1,200 CY in the truck. Trucks measure the loose volume; that's what you bid haul against. Compacted volume = bank volume × (1 − shrink percent / 100). 1,000 CY of bank fill placed at 10% shrink yields 900 CY of finished compacted fill.
Soil type drives the factors. Clay swells more than sand and shrinks more than sand. Wet conditions swell more than dry. Reasonable defaults: 20-25% swell for typical site soil, 5-15% shrink for compacted structural fill. For bidding-grade analysis you want soil-type-specific factors from the geotechnical report; SiteWorx/OS encodes the standard tables.
Hauled cut and import needed
After computing bank cut, bank fill, and the swell factor, the bid line items become:
- Hauled cut (loose):bank cut × (1 + swell). That's the truck volume of excavated soil leaving the site.
- Net (bank): bank cut − bank fill. Positive surplus, negative shortfall.
- Import needed (bank): max(0, bank fill − bank cut). Soil that has to be brought in.
- Compacted placed (bank → in place): bank fill × (1 − shrink). For compaction verification and density testing.
Common pitfalls
Forgetting topsoil strip. The structural cut and fill is what most calculators compute, but real sites strip 4-12 inches of topsoil first and stockpile it for re-spreading. Bid topsoil strip as a separate line item.
Treating swell as if it applies to fill.Swell applies to the soil that's being moved — typically cut soil leaving the site or borrow soil being imported. Bank fill (computed from proposed grade − existing grade) doesn't get a swell factor on its own; the compaction shrink is the relevant adjustment.
Mixing bank and loose volume in the bid. Bid every unit-price line item in consistent volume units. If your unit price for haul is per loose cubic yard, use loose CY. If your import is priced per bank cubic yard, use bank CY. The two are not interchangeable.
Using one grid for the whole site. A 20 ft grid gives the average for that cell. If the cell has a 4 ft cut on one edge and a 1 ft cut on the other, the cell-average underrepresents the volume by missing the variance. Tighten the grid in steep zones.
Ignoring slopes. If your cut has sloped side walls (OSHA 1:1 or 1.5:1 for unstable soils), the open trench volume is larger than vertical-wall math says. The earthwork volume calculator handles trench slope explicitly.
When to move from calculator to software
The grid method works for sites where you can plausibly estimate elevations at each grid node. For real bidding-grade quantities on sites with detailed existing contour maps and proposed grade designs, you want triangulated-surface software that reads the survey directly. SiteWorx/OS does this — it builds the existing and proposed surfaces from PDF or DWG and computes the volume between them by integration, soil-type-specific shrink and swell included.
For preliminary sizing and back-of-the-envelope checks, the calculator gets you 80% of the way there in 10% of the time. For a bid you'll defend, the calculator gives you a sanity-check number; the surface-based software gives you the bid.