Understanding your result
Gravel to order adds your percentage allowance after subtracting the pipe’s outside volume. Fabric width wraps the bottom, both sides, and top of the rectangular envelope. The fabric area excludes end caps, joins between rolls, and trimming.
If the supplier sells aggregate by mass, use the Gravel Calculator with the bulk density supplied for your chosen stone.
The formula
Convert the pipe diameter from millimeters to meters before calculating displacement. The whole outer cylinder displaces gravel, including the hollow bore. Multiply net gravel by 1 + allowance/100; divide cubic meters by 0.764554857984 for cubic yards.
Worked example
A 10 m trench with a 0.3 m by 0.5 m gravel envelope and 100 mm pipe needs about 1.421 m³ before allowance. With 10% extra, order about 1.564 m³. A 0.3 m top overlap requires 1.9 m fabric width and 19 m² before ends or roll joins.
How to use this calculator
- Measure only the section that will contain gravel, excluding topsoil cover.
- Enter pipe outside diameter, not nominal bore diameter.
- Set an ordering allowance and fabric overlap, then compare the quantities with supplier pack sizes.
Envelope depth is not always excavation depth
A trench may include soil above the wrapped gravel or bedding below it. Enter only the gravel envelope here. Rectangular dimensions assume consistent sides and depth; for changing sections, calculate each section separately and add the material totals.
Why pipe displacement matters
The basic gravel calculator fills an entire volume. A French drain reserves part of that volume for a pipe, and this tool deducts that space. It does not subtract voids between individual stones: bulk gravel already includes those voids. Do not reduce the result again by a porosity percentage.
Material quantity is separate from drainage design
NDS describes conventional French drains as perforated pipe surrounded by gravel and filter fabric. This quantity model does not determine soil infiltration, outlet elevation, pipe slope, discharge permission, or hydraulic capacity. Proprietary gravel-free bundles require their own product sizing and are not estimated here.
Assumptions & limitations
What this calculation assumes
- One continuous circular pipe occupies the full length of a constant rectangular gravel envelope.
What to keep in mind
- No gravel density or tonnage is assumed. Use the supplier’s bulk density if ordering by mass.
- Fabric ends and multiple-roll seam overlaps must be added to the displayed area.
Common questions
Should I use the pipe’s inside diameter?
No. Gravel is excluded by the outside surface, so use the actual outside diameter from the product dimensions.
Does the extra percentage represent compaction?
It is a user-selected ordering allowance only. It does not predict settlement, excavator overbreak, or the grading of a particular aggregate.