Bentonite
Calculator
Estimate how much granular sodium bentonite your project needs. Rates follow USDA NRCS Conservation Practice Standard 520 and are applied per 6-inch compacted lift, with interior side-slope area included.
Project Inputs
Estimated Requirement
- Total sealed area—
- Pond-bottom area—
- Side-slope area included—
- Application rate—
- Compacted lifts—
- Weight in pounds—
Estimate only. Enter the area at the design waterline; the bottom and the side-slope band are derived inside that footprint, so flattening the slopes trades bottom area for slope face instead of enlarging the pond. Geometry is a circular-equivalent approximation and will differ from a surveyed quantity. Application rates should be confirmed against soil classification and, where required, laboratory permeability testing.
Need a measured number instead of an approximation? Run a Pond Soil Analysis — trace your pond on aerial imagery for a true geodesic surface area, add depth soundings, and get a treatment plan built on your actual shape rather than a circular equivalent.
How the Calculation
Works
Sodium bentonite application rates for a compacted soil liner are governed by USDA NRCS Conservation Practice Standard 520 — Pond Sealing or Lining (Compacted Soil Treatment). The rate is expressed in pounds per square foot per 6-inch compacted lift, and it is driven by the pore structure of the subgrade soil: the coarser and more open the soil, the more clay is needed to fill and bridge those pores.
| Soil Classification | Rate (lb/ft² per 6" lift) | Why |
|---|---|---|
| Silts and clayey silts | 2 – 3 | Fine-grained native soil already provides partial sealing |
| Silty sands | 3 – 5 | Moderate permeability; the most common pond-site condition |
| Clean sands and gravelly soils | 5 – 6+ | Large, well-connected pores require the most bentonite |
Step 1 — Total the area that must be sealed
The liner has to be continuous, so every surface below the design water line needs treatment — not just the flat bottom. One acre is 43,560 square feet. This calculator estimates the interior side-slope band from your design depth and slope ratio and adds it to the bottom area. On a shallow, broad pond the slope contribution is small; on a deep pond with 3H:1V slopes it can be a significant fraction of the total.
Step 2 — Classify the subgrade soil
Use the soil classification of the material that will actually be amended and compacted, not the topsoil that gets stripped. If the site has been characterized by a geotechnical report, use that classification. Where a project warrants it, laboratory permeability testing on soil-bentonite mixtures at trial rates will produce a more defensible number than any table.
Step 3 — Scale for liner thickness
This is the most common and most expensive mistake in bentonite estimating. CPS 520 caps compacted lift thickness at 6 inches, and the published rates are defined per lift. A 12-inch design liner is two 6-inch lifts and requires twice the bentonite. Under no circumstances should bentonite be applied assuming cumulative thickness without a proportional increase in application rate.
Step 4 — Add contingency
Add 10 to 15 percent for detail work: trenching and hand-mixing around concrete structures, drain pipes, spillway penetrations, and anti-seep collars, plus repair of areas disturbed by construction traffic. These are the locations where ponds actually leak, and they justify generous dosing.
Step 5 — Convert to packaging and freight
PondLock ships in 25 lb and 45 lb bags for small repairs and rate testing, and in 2,000 lb (28 ft³) and 3,000 lb (42 ft³) bulk sacks for full-scale work. Bulk sacks move by LTL freight or full truckload flatbed direct from Wyoming. For anything above a few tons, call for a freight-inclusive quote before you finalize a budget.
One-Acre Pond
in Silty Sand
A pond with a 1-acre water surface in silty sand, 8 feet deep at the deepest point, with 3H:1V interior side slopes and a single 6-inch compacted lift. Note that the bottom is smaller than the surface, because the slopes are cut inside the footprint:
| Quantity | Value |
|---|---|
| Water-surface area (entered) | 43,560 ft² (1 acre) |
| Pond-bottom area (derived) | ≈ 27,600 ft² |
| Interior side-slope band area | ≈ 16,800 ft² |
| Total sealed area | ≈ 44,400 ft² |
| Application rate (silty sand) | 3 – 5 lb/ft² per 6" lift |
| Subtotal | ≈ 66.6 – 111.1 tons (133,300 – 222,100 lb) |
| With 10% contingency | ≈ 73 – 122 tons (146,600 – 244,300 lb) |
| Equivalent 2,000 lb bulk sacks | 74 – 123 sacks |
Measure the surface, not the bottom. On this example the slope band is about 16,800 ft², roughly 61% on top of the 27,600 ft² bottom, because 740 feet of shoreline each carry a 25-foot slanted slope face. Two mistakes are common and they pull in opposite directions: pricing the flat bottom alone leaves the slope band out and comes in short, while entering the bottom area and letting the slopes grow outward from it inflates the footprint and overstates the order — by about 45% on this example.
The range matters too. The spread between the low and high rate for a single soil class is a factor of about 1.7 — on this example, about 49 tons of material. Narrowing it takes a soil classification you trust and, on larger projects, laboratory permeability testing on soil-bentonite mixtures at trial rates. Our team reviews soil data and application rates at no cost; that review is usually worth more than the calculator.
Application Rate
Questions
How much bentonite do I need per square foot of pond?
Under NRCS CPS 520 guidance the minimum rates per 6-inch compacted lift are 2 to 3 lb/ft2 for silts and clayey silts, 3 to 5 lb/ft2 for silty sands, and 5 to 6+ lb/ft2 for clean sands and gravelly soils. Multiply the rate if the design liner thickness exceeds 6 inches.
How much bentonite does it take to seal a 1-acre pond?
Work from the water-surface area, because that is the number you can actually measure. One acre at the waterline is 43,560 square feet; on an 8-foot-deep pond with 3H:1V interior side slopes the bottom and the slope band together come to about 44,400 square feet. At 4 lb/ft2 for silty sand across a single 6-inch compacted lift that is roughly 177,700 lb, about 89 tons. Do not enter the bottom area and let the slopes grow outward from it — that inflates the footprint and, on this example, overstates the quantity by about 45 percent. Interior side slopes must be added on top of that. For a pond 8 feet deep with 3H:1V slopes the slope band adds about 20,625 square feet, bringing the total to roughly 64,185 square feet and 106 to 177 tons at 3 to 5 lb/ft2 with a 10 percent contingency.
Do I include the side slopes in the sealed area?
Yes. The liner must be continuous, so every surface below the design water line needs treatment, including interior side slopes. This calculator estimates side-slope band area from your design depth and slope ratio and adds it to the bottom area. Interior side slopes should be no steeper than 3H:1V so compaction equipment can operate effectively.
Does a thicker liner need a higher application rate?
Yes, proportionally. NRCS CPS 520 defines application rates per 6-inch compacted lift and limits lift thickness to 6 inches. A 12-inch liner is two 6-inch lifts and requires twice the total bentonite. Bentonite must never be applied assuming cumulative thickness without a proportional rate increase.
How many pounds are in a bulk sack of bentonite?
PondLock bulk sacks are available in 2,000 lb at 28 cubic feet per sack and 3,000 lb at 42 cubic feet per sack. Bags are available in 25 lb and 45 lb sizes for small repairs and rate testing.