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The mass of bentonite clay per unit area of the waterproofing panel.
This is typically printed on the panel manufacturer's product data sheet.
The unhydrated (dry) bulk density of the bentonite clay.
As delivered, before it has met any water. Bentonite's entire usefulness is that these two states are different materials in behaviour, so the dry density is a starting point rather than a property of the finished seal. The swell it develops also depends on being CONFINED — unconfined, it expands loosely and stays soft instead of sealing, which is why a mat or a plug that is not held down does not do the job it was bought for.
How many times larger the bentonite's volume becomes once fully hydrated.
Sodium bentonite commonly cited swell factors run roughly 10-15x by volume, depending on grade and confinement condition.
Hydrated (swollen) gel thickness
2.46 in
Swell factor varies by bentonite grade and confinement condition — this is a general reference figure. Bentonite panels require adequate confining pressure (backfill or structural confinement) to develop their full sealing performance; unconfined swell can behave differently.
- Dry (unhydrated) thickness
- 0.25 in
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Bentonite Waterproofing Swell Thickness Calculator: 2.46 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Dry bentonite layer thickness = bentonite content per area ÷ dry density; hydrated bentonite clay swells dramatically when wetted (commonly cited swell factors of roughly 10-15x by volume) to form a self-healing, low-permeability gel that seals small punctures and cracks
Inputs used
- Bentonite Content
- 1.02 lb/sq ft
- Bentonite Dry Density
- 49.94 pcf
- Volumetric Swell Factor
- 10
Intermediate steps
- Dry (unhydrated) thickness
- 0.25 in
Confidence note: Swell factor varies by bentonite grade and confinement condition — this is a general reference figure. Bentonite panels require adequate confining pressure (backfill or structural confinement) to develop their full sealing performance; unconfined swell can behave differently.
What this calculation does not cover
- The swell factor on a data sheet is measured in clean water. Sodium bentonite gives up much of that swell where the groundwater is saline, hard or rich in calcium, because calcium displaces sodium in the clay and the hydrated volume collapses toward a fraction of the laboratory figure — on brackish or contaminated ground the panel has to be a contaminant-resistant grade, and the number above stops describing it.
- A gel thickness is not a seal. Bentonite waterproofing leaks at panel laps, at pipe penetrations, at the footing-to-wall joint and at the top termination, not through the middle of a panel, and every one of those needs its own lap width, granular bentonite or detail strip — none of which follows from the thickness computed here.
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This result is a specification — 2.46 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
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 · v1.1.1
Regulatory standards & verification citations1
- Dry bentonite layer thickness = bentonite content per area ÷ dry density; hydrated bentonite clay swells dramatically when wetted (commonly cited swell factors of roughly 10-15x by volume) to form a self-healing, low-permeability gel that seals small punctures and cracks
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