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Foundation Sheet Drain Drainage Rate Calculator

Calculate the total drainage flow capacity of a dimpled sheet drain across a foundation wall's width.

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  • Every formula cited
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SettingsSettings for this calculationUS
Market
Imperial · sales tax
The drainage product's rated flow capacity per unit width, from the manufacturer's data sheet.

Dimpled sheet drain products are rated for flow capacity along a linear meter (or foot) of drain width, typically at a stated compressive load condition. This field wants the PER METER figure even when the width box below is set to imperial — it does not follow the unit toggle. US product data states the same rating in US gallons per minute per foot of width, and that unit differs from L/s per m in the volume and the time as well as the width, so the two stand 4.83 apart rather than the 3.28 the meter-to-foot factor alone suggests: 1 gallon per minute per foot is 0.207 L/s per m. Divide a gallons-per-minute-per-foot rating by 4.83 before entering it here. Typed straight in, a rating of 15 or under sits inside this field's 1-15 range and is accepted in silence at 4.83x the true capacity; anything larger — the 5 L/s per m default here is 24.2 in those units — is clamped to the 15 maximum instead, which still triples the answer. Both mistakes overstate the drain.

The total width of foundation wall covered by the sheet drain.

Measure the horizontal run of foundation wall being drained, along the base of the wall.

Total drainage capacity

213 CFM

Medium confidence

Rated flow capacity varies by product and compressive load condition (capacity typically decreases under soil/backfill load) — use your specific sheet drain product's rated capacity under expected field loading, not an uncompressed lab value.

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • Total drainage capacity = the drain product's rated flow capacity per unit width × total drainage width, the standard method for scaling a manufacturer's per-unit-width flow rating to a full installation

Inputs used

Rated Flow Capacity per Meter (L/s per m)
5
Total Drainage Width
66 ft
Final result213.13 CFM

Confidence note: Rated flow capacity varies by product and compressive load condition (capacity typically decreases under soil/backfill load) — use your specific sheet drain product's rated capacity under expected field loading, not an uncompressed lab value.

What this calculation does not cover

  • This is the supply side only. A capacity means nothing until it is set against the flow that actually arrives at the wall, and that comes from the catchment above and around the excavation, the permeability of the backfill and the natural soil behind it, and the design storm — none of which appear on this page. A drain rated at three times the flow it will ever see and one rated at a tenth of it return the same confident number here.
  • Rated flow assumes the drain stays open, and what keeps it open is the filter fabric. Fines migrating out of the backfill blind the geotextile over years, and once it blinds, the core behind it is dry and useless whatever the lab rating says. The fabric's opening size has to be matched to the actual backfill gradation — a silty or fine sand backfill against the wrong fabric is the usual way a drainage plane stops draining.
  • Capacity assumes an unbroken path in and out. The sheet has to start at the footing drain and run continuously up to near grade, with the top terminated so surface water and soil cannot get in behind the fabric, and every joint lapped in the direction of flow. A plane stopped short of the footing, or one torn during backfill, delivers a fraction of this figure regardless of what the product is rated at.

Add the equipment this sizes

This result is a specification — 213 CFM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

66 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1
  1. Total drainage capacity = the drain product's rated flow capacity per unit width × total drainage width, the standard method for scaling a manufacturer's per-unit-width flow rating to a full installation
Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

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    Filling is the one operation that can destroy a membrane without leaving a mark: board it, place it in lifts, and prove the drain takes what arrives.

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    Below the table a wall has to hold head, not shed it: set the design water level, work out what the ground delivers, then confine the bentonite.

Called something else where you work? Soakaway and infiltration structures — the term in each market, how close the equivalence really is, and the standard that governs it.

Still deciding? Sheet Drain vs Protection Board — the factors that actually differ, with no invented prices.

How to calculate foundation sheet drain drainage rate in 3 steps

  1. Rated Flow Capacity per Meter (L/s per m)The drainage product's rated flow capacity per unit width, from the manufacturer's data sheet.
  2. Total Drainage WidthThe total width of foundation wall covered by the sheet drain.
  3. Total drainage capacityThe tool computes the total drainage capacity from those figures and shows the formula, its sources, and a confidence rating alongside it.

Total drainage capacity by total drainage width

Page defaults, not your figures above.

Total Drainage WidthTotal drainage capacity (CFM)
40 ft129
60 ft194
80 ft258
100 ft323
120 ft388

Frequently asked questions

Why does total drainage capacity scale directly with width?
A dimpled sheet drain's rated flow capacity is expressed per unit width because the dimpled core channels water vertically down the drainage plane — laying more width side by side simply adds parallel drainage channels, so total capacity scales linearly with the total width installed.
Why does the rated capacity depend on compressive load?
Backfill and soil pressure compress the dimpled core, reducing the open channel height available for water to flow through. Manufacturers typically publish flow capacity at one or more compressive load conditions — always use the figure that matches your expected backfill depth and soil load, not an unloaded lab rating.
Does this account for the drain's connection to a footing drain or sump?
No — this only estimates the sheet drain's own flow capacity across the given width. The downstream footing drain, collector pipe, or sump pump must also be sized to handle this flow rate without becoming the bottleneck.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.