Two gravel estimates can start from the same designed trench envelope and differ because they answer slightly different ordering questions. A gross-envelope convention counts the full entered gravel envelope. An OD-cylinder-adjusted convention subtracts the smooth outside-diameter volume of the pipe length inside that envelope. The difference is a model choice, not necessarily an arithmetic error.
Scroll the diagram sideways to read its labels.
A 50 ft quantity comparison
Assume a previously designed ordinary yard-drain material group with a 50 ft gravel envelope, 1 ft width, 1.5 ft height, and 50 ft of selected pipe with a measured 4 in outside diameter. No varying-depth profile, reserve, supplier conversion, or bag rounding is included here. The dimensions are example inputs, not a recommended drain section or pipe size.
| Quantity view | Transparent arithmetic | Volume |
|---|---|---|
| Gross envelope | 50 × 1 × 1.5 | 75.00 ft³ |
| Smooth pipe OD cylinder | π × (2/12 ft)² × 50 | 4.36 ft³ |
| OD-adjusted gravel | 75.00 − 4.36 | 70.64 ft³ |
The gross model keeps the full 75 ft³ in its gravel basis. The adjusted model uses about 70.64 ft³. Neither number yet includes the separate reserve, supplier-volume factor, mass conversion, or whole-bag rounding that can alter an order. If pipe length inside the envelope differs from trench length, the displacement follows the entered pipe length rather than automatically following the trench length.
Read the policy before comparing totals
MaterialNest displays gross envelope and modeled displacement separately, then applies the selected policy to the gravel purchasing path. The subtraction treats the pipe as a smooth cylinder at its actual outside diameter. It does not model corrugations, voids, fittings, aggregate packing, or a product-specific installed yield. A supplier may also quote loose bulk volume or mass on a different basis; record that conversion independently rather than treating pipe subtraction as the explanation for every difference.
Use the yard-drain measurement record to collect the already-chosen envelope, pipe, and product facts. Select the policy in the French-drain material calculator, then inspect the two displayed volumes. The formula record documents the exact implemented sequence.
This is a quantity comparison for a user-designed ordinary yard drain. It does not select or approve pipe diameter, slope, route, outlet, discharge, capacity, infiltration, or work near foundations and other structures. Before excavation, use the official utility-location process; trenches can collapse, so follow applicable excavation-safety requirements and do not enter an unprotected trench. Confirm the chosen discharge meets local rules and will not harm neighboring property or waterways.