The calculation
A 20 × 5 ft area at 3 inches
Measure only the part that will receive trap rock. This 100 sq ft layer is 25 cu ft (100 × 3 ÷ 12) or 0.93 cu yd (25 ÷ 27). Using Dresser’s 2,600 lb/cu yd average, its estimated loose ordering weight is 2,407 lb, or 1.20 US short tons. The volume is the same if another supplier’s product weighs more or less.
Finished cu ft = area (sq ft) × finished depth (in) ÷ 12
Planned cu yd = finished cu ft × (1 + optional extra ÷ 100) ÷ 27
US short tons = planned cu yd × supplier lb/cu yd ÷ 2,000
Supplier coverage suggestions are useful for shopping, but a range such as “2–3 inches” cannot yield one exact square-foot figure. Choose a finished depth first, then compare the estimate with the yard’s product guidance.
Name the product
“Trap rock” is a rock type, not one gradation
USGS groups basalt and diabase under trap rock. A quarry then crushes and screens that rock into different products. Dresser’s current schedule separately lists 3/4 in rock, 3/4 in Class 5 base, and drain-field stone. Ask for the product name and size range, not only the geological name.
Open-graded crushed rock
Little fine material leaves spaces between pieces. Check the actual product specification for a drain trench, an open-graded base, or a contained decorative surface. It may move under tires if used as an unsupported driveway wearing course.
Minus or Class 5 base
A range of coarse and fine particles is intended to compact into a firmer base. FHWA distinguishes dense-graded structural bases from open-graded drainage layers. Ask the yard for its compaction guidance and loose delivery weight.
Drain-field or erosion stone
Larger screened rock serves a different design. Dresser lists a 1–4 in drain-field product and still larger erosion sizes. Use the project specification for size and thickness; a 3 in decorative-layer example should not be copied to those applications.
Reference
Coverage changes with finished depth
These are geometric yields at the 2,600 lb/cu yd example weight, before any order extra. One cubic yard always contains 27 cu ft; the square feet per short ton depend on bulk weight. For another product, change the calculator field.
| Finished depth | Sq ft per cu yd | Sq ft per short ton | 100 sq ft needs |
|---|---|---|---|
| 2 in | 162 | 124.6 | 0.62 cu yd / 0.80 ton |
| 3 in | 108 | 83.1 | 0.93 cu yd / 1.20 tons |
| 4 in | 81 | 62.3 | 1.23 cu yd / 1.60 tons |
Rounded table values are for estimating. Check the chosen stone size against the layer depth; larger rock may need a deeper layer or a different construction detail.
Source notes and practical limits
The Dresser Trap Rock 2026 product schedule supplies the 1.3-short-ton/cu-yd starting value and shows separate gradations. Fick’s 3/4–1 in listing uses 2,700 lb/cu yd, illustrating why the field is editable. The FHWA granular-base guide explains how grading and fines affect base stability and drainage. The calculator estimates material volume; it cannot select a structural pavement design or predict the exact weight of a wet, graded delivery.
Trap rock questions
How much trap rock do I need for 100 square feet?
At a finished depth of 3 inches, 100 square feet requires 25 cubic feet, or 0.93 cubic yard, before any optional order extra. Using Dresser Trap Rock’s average 2,600 pounds per cubic yard, that volume is about 1.20 US short tons. Ask your supplier for the loose bulk weight of the exact gradation you are buying.
How many square feet will one ton of trap rock cover?
At 3 inches deep and an example loose bulk weight of 2,600 pounds per cubic yard, one US short ton covers about 83 square feet by volume. At 2 inches it covers about 125 square feet; at 4 inches, about 62. Product weight, finished depth, grading, and uneven ground change the estimate.
How much does a cubic yard of trap rock weigh?
There is no single weight for every trap-rock gradation. Dresser Trap Rock lists an average of 1.3 US short tons (2,600 pounds) per cubic yard. Fick Supply lists 2,700 pounds per cubic yard for its 3/4- to 1-inch trap rock. Enter the weight given by your seller before converting cubic yards to tons.
Is clean trap rock the same as trap-rock driveway base?
No. Clean crushed trap rock has little fine material and is sold for uses such as drainage and decorative beds. Minus or Class 5 base contains a range of smaller particles that can compact into a firmer layer. Dresser lists these as separate products. Select the gradation for the intended layer, then use its own supplier weight.
Can I use trap rock in a French drain?
A clean, open-graded trap-rock product can be used where the drain design calls for that size and grading. A dense-graded driveway base containing fines is a different product and can obstruct the open spaces that carry water. For a pipe trench, use the Drain Rock Calculator to account for pipe displacement and the actual stone envelope.