A french drain order usually goes wrong in the quantity sheet, not on the truck. One wrong dimension gets multiplied along the whole run, and one skipped allowance is enough to leave the trench half-filled or pay for a load that comes back unused.
Why do french drain orders come up short or long?
The arithmetic is short, so the errors are short too: measuring the design trench instead of the dug trench, subtracting the pipe with a nominal diameter, shrinking the stone order by the void percentage, forgetting the handling allowance, converting tons with a generic density, ordering fabric with no overlap, and planning the spoil haul on bank volume. Fixing each one takes one correction on the worksheet.
Use the Drainage Gravel Calculator for the arithmetic once the measurements are right. This checklist fixes the measurements that go into it. For the full measurement sequence, see how to measure a french drain system, and for the order sequence, see planning haulage and delivery for a french drain system.
When does this checklist apply?
This checklist applies to a new subsurface collection drain installed in an excavated trench, where the estimator measures and records the envelope, pipe, fabric, and spoil quantities before ordering. It covers one straight collection run and its discharge to a planned outlet. Selecting the drain's hydraulic capacity is design work; this page works the quantity arithmetic only. Material and soil decisions are covered in the french drain material and soil guide.
The worked fixture behind every mistake
Each mistake below uses one stated fixture so every number can be checked. The collection run is 60 ft long. The trench is 12 in wide and 18 in deep. The pipe is 4-in nominal. The net stone fixture from the measurement guide is 54.1722 ft³, which is 2.0064 yd³. The handling allowance fixture is 10%. The selling increment is 0.5 yd³. The swell fixture is 30%, and the truck takes 2 yd³ of usable loose volume.
Net stone = 54.1722 ft³ ÷ 27 = 2.0064 yd³
Envelope per inch of trench width = 60 × (1 ÷ 12) × 1.5 = 7.5 ft³ = 0.2778 yd³
The run length, trench dimensions, and allowance and swell percentages are stated fixture assumptions for this article, not recommendations for any project.
The ten mistakes, and the correction for each
1. Ordering against design dimensions instead of the measured trench
Trenches rarely come out at design width. A bucket-cut trench runs wide in places, and every extra inch along the full 60 ft run adds 0.2778 yd³ of envelope. Ordering against 12 in when the trench averages 13.5 in leaves the stone order short by 0.4167 yd³.
Correction: measure the as-dug width at several points along the run, average the readings, and recompute the envelope from those numbers. Record the stations where you measured so a wide section can be traced later.
2. Shrinking the stone order by the void percentage
Washed angular stone fills roughly 60% of the envelope space; the rest is voids. Some order sheets multiply the envelope by 0.6 and order the solid rock volume. That leaves the trench about 40% short of stone, because the voids are part of the space the stone must fill. Water occupies those voids once the drain is working; they were never empty space to delete from the order.
Correction: order the gross envelope volume with the pipe displacement subtracted, as the measurement guide does. The void percentage never enters the order sheet.
3. Forgetting the handling allowance
The net stone figure is what the trench holds, not what the supplier delivers. Spillage, uneven trench walls, and placement losses take a share of every load.
Order volume = 2.0064 × 1.10 = 2.2070 yd³, rounded up to 2.5 yd³ on the 0.5 yd³ increment
Correction: multiply the net volume by 1 plus the stated handling allowance, then round up to the supplier's selling increment. Name what the allowance covers on the record, because a bare 10% cannot be checked or revised later.
4. Converting cubic yards to tons with a generic density
Bulk density changes with moisture, grain size, and the lot in the pile. A density copied from an online table can move the tonnage 10% or more against the number the weighbridge prints. With the labelled example-only density of 1.45 tons/yd³, the fixture order weighs 3.2002 tons; a different supplier's record would give a different number.
Correction: use the supplier's loose bulk density for the actual material from the product record or the weighbridge ticket, and record the density, the date, and the moisture condition beside the tonnage.
5. Subtracting the pipe with a nominal diameter
A 4-in nominal pipe is not 4 in on the outside. The SDR 35 specification record puts the average outside diameter at 4.215 in. Over the 60 ft run, the nominal diameter subtracts 5.2360 ft³ of displacement while the actual OD subtracts 5.8144 ft³: a 0.5784 ft³ gap, which is 0.0214 yd³. Small on one run, and a systematic error repeated on every drain the sheet prices.
Correction: read the actual outside diameter from the supplier's spec record for the lot and use it in the displacement. Name the product, the standard, and the OD on the record.
6. Ordering fabric with no overlap or fold-over
The fabric area is not the trench-wall area. Seams need overlap, and the excess on both sides folds over the top of the stone to close the wrap. An order cut to the bare wall area runs out before the last overlap.
Correction: compute the wrap perimeter, add the fold-over allowance on both sides, add overlap per seam, and order the roll width and length that cover that area with the least waste. Record the roll width on the order so a narrower roll does not force an extra seam.
7. Ordering the stone by nickname instead of specification
"Washed #57" means something different from one yard to the next, and a nickname carries no fines record. Stone with fines blinds the envelope voids and compacts, which is the failure the fabric is meant to prevent. The gravel size and filter checks guide shows how the size and fines record are verified.
Correction: name the material as the guide chose it: angular washed stone at the specified size with a stated fines content. Check the delivery ticket against that specification on arrival: material, quantity, and lot. A wrong stone size is cheapest to catch while the truck is still on site.
8. Running perforated pipe through the discharge section
The perforations collect water along the run. Past the collection zone, the pipe should carry water away, so a perforated discharge line leaks it back into the ground before it reaches the outlet.
Correction: split the pipe order. Perforated pipe covers the 60 ft collection run; solid pipe covers the conveyance to the outlet. Measure the conveyance length separately and add cutting allowances to each pipe type. The outlet itself is part of the design: it must discharge where water can leave safely, not toward the foundation or a neighbor's property.
9. Never checking the trench grade for a continuous downhill fall
Water follows gravity, not intentions. A run with one low spot between two high points holds standing water in the pipe and the envelope. Using the commonly cited 1% grade as a stated fixture, a 60 ft run falls 7.2 in end to end; a 2 in low spot in the middle still traps water in that pool.
Correction: check the trench bottom with a level or laser as you go, and recompute the excavation and envelope quantities from the as-dug profile, not from a straight prism. Note that 1% is a worked fixture here, not a required grade: the governing standard or project specification controls the accepted grade.
10. Planning the spoil haul on bank volume
Excavated soil swells. The fixture run digs 90 ft³, which is 3.3333 yd³ in the bank. At the stated 30% swell fixture, the loose spoil is 4.3333 yd³. A truck with 2 yd³ of usable loose capacity needs 3 trips. A haul plan built on bank volume books 2 trips and finds one more partial load on the day.
Trips = ceil(4.3333 ÷ 2) = 3
Correction: convert the bank spoil to a loose haul volume with a stated swell record, then divide by the usable loose capacity and round any positive remainder up to a whole trip. Check the payload separately. Body volume sets the space; material mass, moisture, legal payload, axle limits, and route restrictions can set a smaller usable load. The trench spoil balance guide shows how to separate spoil you reuse from spoil you export.
Edge cases that change the answer
- A curved or doglegged run needs per-segment measurements; averaging one straight prism across bends undercounts the envelope at the curves.
- In clay or silt soils, the filter decision controls how long the envelope stays open. Use the AASHTO M288 class decision from the material and soil guide and keep the same overlap math.
- Where the outlet must cross a sidewalk or a drive, the conveyance length and the solid pipe order grow; the collection run arithmetic does not change.
- Root zones add a maintenance question that no order quantity solves. Plan cleanouts at the outlets instead.
Safety and professional scope
Haulage and delivery do not change the excavation safety rules. For U.S. work, OSHA identifies cave-ins as the main trench hazard, and 29 CFR 1926.651 covers underground installations, access and egress, hazardous atmospheres, water accumulation, adjacent structures, loose material, and competent-person inspections. Keep spoil, stone piles, vehicles, and equipment away from the trench edge under the governing safety plan; OSHA's standard uses a 2 ft minimum distance for excavated material in 1926.651(j)(2).
Contact 811 a few business days before digging in the United States and wait for utility marks before breaking ground. Other countries use their own workplace and utility rules.
Sources and scope
- NIST Handbook 44, 2026 Appendix C: cubic-foot, cubic-yard, and metric relationships.
- ASTM D3034 specification record (Charlotte Pipe specification sheet, current sheet retrieved 2026-10-17): 4-in SDR 35 PVC, average outside diameter 4.215 in. Confirm the actual shipped OD with the supplier for the lot.
- AASHTO M294-21 scope (wording verified 2026-10-17 via a retrieved full-text copy): corrugated polyethylene pipe, 12-60 in, for surface and subsurface drainage. The official standard governs.
- AASHTO M288 geotextile reference (reference page retrieved 2026-10-17): subsurface drainage is a covered application, survivability Classes 1-3; the official standard and project spec govern.
- Solmax AASHTO M288-24 selection guide (manufacturer, REV 1225, retrieved 2026-10-17): Class 2 is the commonly used default class for subsurface drainage; the project spec governs.
- USDA Natural Resources Conservation Service, Subsurface Drain (Ft.) (606) conservation practice standard (retrieved 2026-10-17): subsurface drain components: trench, bedding, envelope, filter/geotextile.
- Food and Agriculture Organization, Materials for subsurface land drainage systems, Chapter 3 (retrieved 2026-10-17): granular-envelope functions around drain pipe.
- OSHA Trenching and Excavation and 29 CFR 1926.651: U.S. occupational excavation-safety context.
- 811 Before You Dig: the free U.S. before-you-dig service.
Scope: the checklist corrects order quantities for a new subsurface collection drain: envelope, pipe, fabric, and spoil quantities and the records behind them. The governing drawings, survey and testing methods, geotechnical information, supplier records and tickets, hauler vehicle limits, safety plan, applicable law, and responsible qualified people control the accepted allowance, density, swell, grade, and final project quantity.
Review responsibility: Saleem Sial owns the published calculation, source, and editorial checks. Waseem Sial, External Reviewer and Engineer, remains listed for ongoing external review; no completed external-review date is claimed.
Next, check the material choices that the order quantities depend on, or work the arithmetic in the Drainage Gravel Calculator.
Use the French Drain System Calculator to run the full system record for one trench section.
