Measure the length, width and depth that will actually be filled with gravel. Enter the pipe outside diameter and the aggregate supplier’s bulk density to estimate the space for gravel and its weight.

Formula: net gravel ft³ = length × (width × filled depth − π × (pipe outside diameter in ÷ 24)²); yd³ = net ft³ ÷ 27; short tons = yd³ × entered short tons/yd³

Worked example

A 50 ft run with a 1 × 1 ft gravel envelope and a 4 in outside-diameter pipe leaves about 45.64 ft³, or 1.690 yd³, for aggregate. At an entered 1.4 US short tons/yd³, that is about 2.366 short tons.

Measure only the gravel envelope

The filled depth excludes any soil cap. The model assumes one straight centered pipe throughout a constant rectangular section; fabric, fittings, bends and voids around irregular rock are not separately measured. NDS describes the pipe-and-gravel arrangement of a traditional French drain.

Pipe diameter is outside diameter

Subtract the space occupied by the pipe body, not its hollow bore. The pipe must fit inside the measured gravel rectangle. A zero diameter is allowed when you are estimating a gravel-only trench.

Density is a purchase input

The tonnage follows the supplier’s bulk density for the specified aggregate and placement state; do not use solid-rock mineral density. This page estimates quantity, not drainage capacity, grading, outlet design or installation suitability.

Explore the related calculation

For a different starting point, try the gravel coverage calculator or return to the main calculator. Browse the complete tool directory for this site.

Reference and corrections

The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.

Found a problem? Email contact@cubicyardstotons.org.

French drain gravel calculator

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