Rock Calculator — Quantity, Weight, and Cost
Scope: estimate rock volume, weight, and cost from surface coverage (area × depth) or a known volume. Outputs align with the calculator: yd³/m³, tons/tonnes, and cost. Good for pea gravel, crushed rock, and decorative stone.
Inputs (with typical values)
- Calculation mode: By Area or By Volume
- Units: Imperial (ft, in, yd³, tons) or Metric (m, cm, m³, tonnes)
- Area length, width (plan dimensions)
- Depth: compacted thickness (e.g., 2–4 in decorative, 4–6 in driveways)
- Volume: if you already have yd³ (Imperial) or m³ (Metric)
- Rock bulk density: Imperial input in lb/yd³ (typical 2,600–3,000 lb/yd³), Metric input in kg/m³ (typical 1,500–1,800 kg/m³)
- Wastage/voids allowance (%): 3–10% common; more for irregular areas or spillage
- Cost per unit mass: $/ton (US ton) or $/tonne (Metric)
Method (formulas)
- Area mode: area_ft2 = length × width; depth_ft = depth/12; vol_ft3 = area_ft2 × depth_ft; vol_m3_from_area = vol_ft3 × 0.0283168466
- Volume mode (Imperial): vol_m3_from_volume_yd3 = volume × 0.764554858
- Pick vol_m3 based on mode and units, then adjust: vol_m3_adj = vol_m3 × (1 + voidPct/100)
- Imperial density handling: lb_per_m3 = density(lb/yd³)/0.764554858; mass_kg = lb_per_m3 × vol_m3_adj × 0.45359237
- Metric density handling: mass_kg = density(kg/m³) × vol_m3_adj
- Weight: tons(US) = mass_kg/907.18474; tonnes = mass_kg/1000
- Cost: Imperial = tons × unitCost; Metric = tonnes × unitCost
Worked example (matches the tool)
Imperial, By Area: 20 ft × 10 ft at 3 in depth; density 2,700 lb/yd³; 5% allowance; $45/ton.
- Area = 20 × 10 = 200 ft²; depth = 3/12 = 0.25 ft; vol = 200 × 0.25 = 50 ft³
- Convert to m³: 50 × 0.0283168466 = 1.416 m³
- Add 5%: 1.416 × 1.05 = 1.4868 m³ ≈ 1.49 m³
- Density conversion: lb/m³ = 2700/0.764554858 = 3532.8; mass_kg = 3532.8 × 1.4868 × 0.45359237 ≈ 2387 kg
- Tons (US) = 2387/907.18474 ≈ 2.63 tons; Cost = 2.63 × $45 ≈ $118.4
Note: The live calculator simplifies rounding and may present yd³, bag counts, and cost with 2 decimals. Always match density and cost to your supplier.
Assumptions and pitfalls
- Depth is compacted thickness; add allowance for grading irregularities.
- Bulk density varies by rock type and moisture; confirm with quarry.
- Do not double-apply waste; keep allowance consolidated in the voids/waste field.
- Driveways/traffic areas commonly need 4–6 in placed and compacted in lifts.
- Bag estimates are approximate; bulk delivery pricing is usually per ton/tonne.
Sanity checks
- Coverage rule-of-thumb: 1 yd³ covers about 100 ft² at 3 in. If your result deviates heavily, recheck depth units.
- Weight check: many crushed rocks land near 1.3–1.5 tons per yd³ (US). If far off, verify density input.
Cost breakdown (planning)
- Direct material = unit mass price × computed tons/tonnes
- Add-ons (not in calculator): delivery, small-load fees, fuel surcharge, sales tax, and contingency (5–10%)
Planning estimate only. Validate against drawings, specs, and supplier quotes before ordering.