Bentonite Calculator finds the sloped bottom and bank area of a pond, then multiplies it by a sodium bentonite rate for your soil to give the pounds, tons, 50 lb bags and the cost.
Measure at the waterline. Sloped sides are added to the bottom area.
Side Slope and Rate Guide
Bags, Waste and Cost
Compare Rate Guides
| Guide | Soil | Rate | Total | Bags |
|---|---|---|---|---|
| Texas Bentonite | Clay Soil | 1.25 lb/sq ft | 7,864 lb | 158 |
| Texas Bentonite | Sandy Silt, 20% Clay | 2.25 lb/sq ft | 14,154 lb | 284 |
| Texas Bentonite | Silty Sand | 3.25 lb/sq ft | 20,445 lb | 409 |
| Texas Bentonite | Clean Sand | 4.25 lb/sq ft | 26,736 lb | 535 |
| Texas Bentonite | Rock or Gravel | 5.50 lb/sq ft | 34,599 lb | 692 |
| Natural Waterscapes | Fair Clay | 2.00 lb/sq ft | 12,582 lb | 252 |
| Natural Waterscapes | Poor Clay | 3.00 lb/sq ft | 18,872 lb | 378 |
| Natural Waterscapes | Silty Loam | 3.50 lb/sq ft | 22,018 lb | 441 |
| Natural Waterscapes | Sandy Loam | 4.50 lb/sq ft | 28,309 lb | 567 |
| Natural Waterscapes | Sand and Gravel | 6.00 lb/sq ft | 37,745 lb | 755 |
| Natural Waterscapes | Gravel, Fractured Rock | 7.00 lb/sq ft | 44,036 lb | 881 |
| MU Extension | Soil With Some Clay | 1.00 lb/sq ft | 6,291 lb | 126 |
| MU Extension | Mixed or Unknown Soil | 2.00 lb/sq ft | 12,582 lb | 252 |
| MU Extension | Sand-Gravel | 3.00 lb/sq ft | 18,872 lb | 378 |
Sealing a Leaking Pond With the Bentonite Calculator
The Bentonite Calculator works out how much sodium bentonite it takes to seal an earthen pond. It measures the real bottom and bank area from the pond’s size and side slope, then applies a rate for your soil from three published charts.
Farm and ranch pond owners use it before ordering clay by the pallet. Pond contractors use it to compare rate guides on a quote, and landowners with a slow leak use it to size a spot treatment instead of the whole basin.
Pond Size, Soil Type and Rate Guide
Enter length and width at the waterline, or the diameter for a round pond. An irregular pond takes its surface area instead. Max depth and side slope, from 2:1 to 4:1 run to rise or vertical walls, set how far the banks reach.
The Rate Guide picks the chart behind the soil list, and Point in Range picks the low end, midpoint or high end of that soil’s range. A second panel sets bag size, extra for waste and price per bag. Results come in pounds, US tons and tonnes, or kilograms in metric mode.
Measuring the Area the Bentonite Calculator Seals
Bentonite has to cover the bed and the banks, not just the water surface. Sloped banks pull the bottom in from the waterline by the slope times the depth on every side.
$$A_{bottom} = (L – 2sD)(W – 2sD)$$
Here s is the run per foot of rise and D is the max depth. Each bank is a trapezoid measured along the slope, so its height is the slanted length rather than the depth.
$$A_{sides} = (L + L_{b} + W + W_{b}) \times D\sqrt{1 + s^{2}}$$
A 100 ft by 60 ft pond, 8 ft deep with 3:1 banks, has a 52 ft by 12 ft bottom of 624 sq ft and 5,667 sq ft of banks. That is 6,291 sq ft in all, with the banks running 3.16 times the depth at that slope.
A common slip is ordering by surface acreage. Bulk supplier Raw Source notes the dose should come from the measured bed-and-sides area, and the quick shortcut of adding twice the depth to each waterline dimension gives 8,816 sq ft for the same pond, about 40% more than its sloped surface really covers.
Length and width can run up to 5,000 ft, depth up to 60 ft and an irregular area up to 4.4 million sq ft. If the banks meet before they reach the bottom, the tool asks for a steeper slope, a shallower depth or a bigger pond. Round ponds use a cone frustum for the banks, and irregular ponds are treated as a circle with the same surface area.
Three Published Rate Charts, Side by Side
Rates are product and agency guidance, not a code standard, and the charts do not agree. The total is simply the area times the rate, with any waste added on top.
$$W_{total} = A \times R \times \left(1 + \frac{w}{100}\right)$$
The Texas Bentonite chart runs from 1.0 to 1.5 lb/sq ft for clay soil up to 5.0 to 6.0 lb/sq ft for rock or gravel. It is based on water up to 8 ft deep and adds 1 lb/sq ft for each further 8 ft, which the tool spreads evenly, so a 12 ft pond on silty sand rises from 3.25 to 3.75 lb/sq ft at the midpoint.
The Natural Waterscapes chart starts at 2 lb/sq ft for fair clay and runs to 6 to 8 lb/sq ft for gravel and fractured bedrock. On the example pond, its silty loam midpoint gives 22,018 lb against 20,445 lb for Texas Bentonite’s silty sand.
MU Extension gives 1 lb/sq ft for soil with some clay up to 3 lb/sq ft for sand-gravel, but each rate is for one 4-inch lift mixed in and compacted. Two lifts double the rate, which is why the tool asks for the number of lifts when that guide is picked.
MU Extension also says rates should come from a lab test, collected through your local NRCS office. With a test result, switch to My Soil Test Rate and the tool applies that number to the whole area.
Bags, Pallets and Spot Treatment
Bags round up to whole units, and the card shows how much is left over in the last one. The example needs 409 bags of 50 lb, or about 8.5 pallets, since Natural Waterscapes sells Benseal by the pallet of 48 bags, 2,400 lb in all.
The fourth card gives the rate as pounds and bags per 1,000 sq ft, plus the area one bag covers. Natural Waterscapes advises sizing a spot treatment by the square footage of the leak zone, so these figures let you price a cove or a leaking bank without running the whole pond.
Cost is bags times your price, before delivery or tax. For the pond volume and a flexible liner as an alternative, the Pond Calculator sizes the sheet, and the Soil Infiltration Rate Calculator turns a percolation test into a seepage rate.
Rate Mistakes That Leave a Pond Leaking
Guessing the soil type means guessing the rate. Pond guides list skipping the soil test as a top mistake, since clay and clean sand differ by three times or more on the same chart.
Deep ponds need a depth add-on in the Texas Bentonite guide. Leaving it out on a 16 ft pond sets the rate a full 1 lb/sq ft short at the very depth where water pressure is highest.
Cutting the rate to save money rarely works. Lone Star Barite’s guide, as quoted by Lawn Gear Lab, calls under-application the most common avoidable mistake, because too little clay cannot fill the soil gaps and the pond keeps leaking at nearly the same rate.
Questions About Sealing Ponds With Bentonite
Can I seal a pond with bentonite without draining it?
Yes, with the sprinkle method, where granular bentonite is spread over the water and settles to the bottom. Texas Bentonite advises planning on 5 lb/sq ft for that method, much more than its blanket rates.
Natural Waterscapes says the pond must be drained to work the clay into the bottom and sides properly. If you plan to sprinkle instead, enter 5 lb/sq ft as My Soil Test Rate.
Should I use sodium or calcium bentonite?
Use sodium bentonite. MU Extension states that only sodium bentonite should be used for pond or reservoir sealing, and notes that it swells to many times its dry volume when wet.
MU Extension also notes bentonite works best on soils with less than 50% fines. That is why sandy and gravelly bottoms are the classic case for it.
How thick is a bentonite layer at these rates?
Thinner than most people expect. Easley Inc. puts one-tenth of an inch of bentonite at about 0.6 lb/sq ft, so 3 lb/sq ft makes a layer roughly half an inch thick before it swells.
That thin layer is why the clay is mixed into the soil and compacted, then covered. On its own it would wash away or crack where the water level drops.
How do I find my own rate with a bucket test?
Natural Waterscapes describes mixing pond soil with bentonite in a 5 gallon bucket at a trial rate and checking whether it holds water. The bottom of their standard bucket is 0.57 sq ft.
Multiply the pounds of bentonite used in the bucket that sealed by 1.75 to get the rate in lb/sq ft. Enter that number as My Soil Test Rate, and the tool runs it across the whole pond.