Underground detention takeoff: chambers or pipe

For a quick check of a bid, what one chamber in the middle of a bed accounts for, with each maker's published stone depths, 40% voids and 1.42 tons per cubic yard of stone. The storage column matches the makers' published installed storage. The outside rows, the end caps and the edge stone add more, so use the calculator for the real layout.

Dig is the chamber's share of the pit, with 18 inches of cover over the chambers; fabric is its share of the top and bottom. For the DC-780 these match StormTech's published 4.2 tons of stone and 5.9 cubic yards of excavation per chamber.

1. The bed. Length = chambers in a row × installed length + the two end caps + the edge stone at both ends. Width = rows × chamber width + the stone between rows + the edge stone on both sides. Depth = stone below + chamber height + stone above. For pipe, the runs' length and outside diameter take the chamber's place.
2. Stone. The bed's volume less what the chambers or pipe take up. Chambers are counted at their own storage, which overstates stone only by their wall thickness; pipe at its outside diameter. Tons = cubic yards × the stone's weight.
3. Storage. What the chambers and end caps hold, or the pipe's inside volume, plus the stone's voids: stone volume × voids (usually 40%).
4. Geotextile. The bed wrapped on every face: 2 × length × width + 2 × (length + width) × depth, plus the overlap allowance, in square yards. Layouts that leave the bottom open, or add a woven strip under an inlet row, change this, so follow the plans.
5. Excavation. The bed plus the working room, dug from finished grade to the bottom of the stone: the cover over the chambers or pipe (which includes the stone above, the way the makers and the plans measure it), their height and the stone below. With sloped sides the pit is a prismoid: depth ÷ 6 × (bottom area + 4 × middle area + top area).
6. Backfill and surplus. Backfill is the excavation less the bed. The bed's own volume is what's left over, before shrink; the haul calculator turns it into loads.

4 rows of 10 StormTech DC-780 chambers, with 9 inches of stone below, 6 above, 6 between rows and 12 around the edge, 18 inches of cover, 1 foot of working room and 1:1 sides:

• Bed: 74.8 × 20.5 ft, 3.75 ft deep (212.9 CY).
• 40 chambers and 8 end caps.
• Stone: 143.7 CY, 204 tons.
• Storage: 1,869 cf in the chambers + 1,552 cf in the stone = 3,420 cf.
• Geotextile: 483 SY with 15% for overlap.
• Excavation 392 CY, backfill 179 CY, surplus 213 CY.

Chamber figures and stone depths. Picking a chamber fills in its maker's published dimensions and the stone depths their layouts use. The plans may call for more stone below (for a soft subgrade) or more cover; enter what the plans show.

Pipe. Storage comes from the inside diameter; the bed and the stone it displaces come from the outside diameter, which on corrugated pipe is several inches larger. The list carries ADS's published diameters, and for N-12 its detention layout's spacing and bedding. The makers don't set the stone above the pipe, so take it from the plans. For corrugated metal pipe, enter the diameters from your supplier.

Stone weight. The calculator starts at 1.42 tons per cubic yard, the figure on StormTech's product sheet. It varies by quarry and by rock, and the stone is bought by the ton, so use your supplier's figure.

Voids. Chamber and pipe makers size systems with 40% voids in clean, uniformly graded stone. Use the figure on the plans if it differs.

• StormTech SC-160LP, SC-310, SC-740, DC-780 and SC-800 Design Manual (ADS, 5/25). adspipe.com. Checked October 3, 2026.
• StormTech MC-3500 and MC-7200 Design Manual (ADS, 5/24). adspipe.com. Checked October 3, 2026.
• StormTech MC-4500 Design Manual (ADS, 7/25). adspipe.com. Checked October 3, 2026.
• Stone weight, 1.42 tons per cubic yard: StormTech MC-7200 Product Sheet (ADS, 9/26). adspipe.com. Checked October 3, 2026.
• Stone voids, 40%: StormTech Technical Note 6.30, Porosity of Structure Backfill (ADS, 2024). adspipe.com. Checked October 3, 2026.
• CULTEC Stormwater Management Systems, Commercial Design Manual, CULG002 (5/25). cultec.com. Checked October 3, 2026.
• Pipe diameters: ADS Drainage Handbook, Specifications (1/26). adspipe.com. Checked October 3, 2026.
• Pipe spacing and bedding: ADS Drainage Handbook §6, Retention/Detention, Table 6-2 (2009). adspipe.com. Checked October 3, 2026.

Pipe sizes in the list: ADS N-12, 12 in (O.D. 14.5 in); ADS N-12, 15 in (O.D. 18 in); ADS N-12, 18 in (O.D. 22 in); ADS N-12, 24 in (O.D. 28 in); ADS N-12, 30 in (O.D. 36 in); ADS N-12, 36 in (O.D. 42 in); ADS N-12, 42 in (O.D. 48 in); ADS N-12, 48 in (O.D. 54 in); ADS N-12, 60 in (O.D. 67 in); ADS HP Storm, 12 in (I.D. 12.2, O.D. 14.5 in); ADS HP Storm, 15 in (I.D. 15.1, O.D. 17.7 in); ADS HP Storm, 18 in (I.D. 18.2, O.D. 21.4 in); ADS HP Storm, 24 in (I.D. 24.1, O.D. 28 in); ADS HP Storm, 30 in (I.D. 30.2, O.D. 35.5 in); ADS HP Storm, 36 in (I.D. 36, O.D. 41.5 in); ADS HP Storm, 42 in (I.D. 42, O.D. 47.4 in); ADS HP Storm, 48 in (I.D. 47.9, O.D. 54.1 in); ADS HP Storm, 60 in (I.D. 59.9, O.D. 67.1 in). Found a figure that's wrong? Tell us at support@aggrado.com.