Units and conversions every civil estimator should know cold
A working reference for civil takeoff units: LF, SF, SY, CY, tons, acres and EA, with the conversions and density factors used daily on sitework bids.
6 min read
Key takeaways
- SF × inches ÷ 324 = CY. It is the most-used conversion in sitework.
- SY × inches ÷ 36 = CY, for paving and base sections.
- Tonnage needs a density; asphalt ≈ 110 lb/SY-inch, aggregate ≈ 1.4 tons/CY.
- Match your unit to the bid form's unit, not to your preference.
Unit errors are the quietest way to blow a bid. The measurement is right, the trace is right, and the number is off by a factor of nine because someone reported square yards where the form asked for square feet. Here is the working set for civil and sitework takeoff.
Length and area
| Conversion | Factor |
|---|---|
| Feet to yards | ÷ 3 |
| SF to SY | ÷ 9 |
| SF to acres | ÷ 43,560 |
| SY to acres | ÷ 4,840 |
| Acres to SF | × 43,560 |
| Station to feet | × 100 |
Area and thickness to volume
| Given | To CY | Example |
|---|---|---|
| SF and inches | SF × in ÷ 324 | 3,000 SF at 4" = 37 CY |
| SF and feet | SF × ft ÷ 27 | 3,000 SF at 0.33' = 37 CY |
| SY and inches | SY × in ÷ 36 | 1,200 SY at 6" = 200 CY |
| Acres and feet | acres × ft × 1,613 | 2 ac at 0.5' = 1,613 CY |
324 comes from 12 inches per foot times 27 cubic feet per cubic yard, divided by 1 — worth remembering as a single number rather than deriving it each time.
Volume to weight
Tonnage always requires a density, and densities vary by material and source. Use these as planning values and replace them with supplier-confirmed numbers before bidding.
| Material | Typical density | Common conversion |
|---|---|---|
| Hot mix asphalt | ≈ 145 lb/CF | ≈ 110 lb per SY per inch |
| Crushed aggregate base | ≈ 135 lb/CF | ≈ 1.4 tons/CY compacted |
| Sand / bedding stone | ≈ 100 lb/CF | ≈ 1.35 tons/CY |
| Riprap | ≈ 125 lb/CF | ≈ 1.65 tons/CY |
| Topsoil | ≈ 85–100 lb/CF | ≈ 1.2 tons/CY loose |
| Concrete | ≈ 150 lb/CF | ≈ 2.0 tons/CY |
Earthwork factors
- Bank (in-place) volume is what the grading plan computes.
- Loose volume is bank × swell factor (typically 1.15–1.30) — it drives truck counts.
- Compacted volume is bank × shrink factor (typically 0.85–0.95) — it drives import quantities.
- State which basis a volume is on. "5,000 CY" without a basis is ambiguous.
Choosing the reporting unit
- Read the bid form first; report in the unit it asks for.
- Where no form exists, use industry convention: pipe in LF, paving in SY or tons, concrete in CY or SF, structures in EA.
- Keep the measured unit and the reported unit both visible so the conversion is auditable.
- Never convert twice — a value already in tons should not be re-factored from CY.
The habit that prevents most unit errors is simply writing the unit next to every number, every time, from the first measurement through the final export. Unlabeled numbers are how a 9x error survives three reviews.
Doing this work in TakeoffAI? Unit-aware conditions.
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