A secondary containment calculator needs three inputs: the volume of the largest single container, the space taken up by everything else inside the berm, and enough freeboard for rain. Get those three right and the wall height falls out of simple division. Skip one and you end up with a berm that holds a spill on a dry day but overtops in a storm.
Below are the federal rule’s actual wording, the unit conversions, three worked examples for oilfield tanks, and the mistakes that eat into capacity after the berm is built. Work these examples with your own numbers. The engineer who certifies your SPCC Plan makes the final call.
What the SPCC rule actually requires
For tank batteries, separation and treating installations at onshore production facilities, 40 CFR 112.9(c)(2) requires “a secondary means of containment for the entire capacity of the largest single container and sufficient freeboard to contain precipitation.” The wording for bulk storage at other onshore facilities, 112.8(c)(2), is the same. The rule doesn’t say 110 percent and doesn’t name a storm.
EPA’s inspector guidance fills in two details that matter for the math:
- Manifolded tanks count as one. If tanks are permanently manifolded so they work as a single storage unit, their combined capacity is the “largest single container.”
- Freeboard is an engineering call. EPA considers the precipitation from a 25-year, 24-hour storm the proper standard for “sufficient freeboard.” It deliberately did not make that enforceable, and it leaves the method to good engineering practice. Whatever method you use should be documented in the Plan.
EPA also “strongly recommends” keeping the containment calculations with the Plan so an inspector can see them. Appendix H of the guidance has blank worksheets for common layouts.
Where the 110 percent number comes from
“110 percent of the largest tank” is a rule of thumb, and many state aboveground tank rules use it. EPA’s guidance says it may be acceptable in many situations but warns that in some places it “may not provide enough volume to contain precipitation from storm events.” In EPA’s own example, a dike holding 112 percent of the largest tank had room for only 1.8 inches of rain. Treat 110 percent as a starting point to check against local rainfall, not as the answer.
Conversions you’ll need
| From | To | Multiply by |
|---|---|---|
| Barrels (bbl) | Gallons | 42 |
| Gallons | Cubic feet | ÷ 7.48 |
| Barrels | Cubic feet | 5.615 |
| Inches of rain | Feet | ÷ 12 |
| Vertical tank footprint | ft² | 3.1416 × diameter² ÷ 4 |
Rain figure used below: NOAA Atlas 14 gives a 25-year, 24-hour rainfall of about 5.0 inches at Midland International Airport (point estimate, retrieved October 2026). Pull the figure for your own site from the NOAA precipitation frequency server. It varies across the Permian.
Example 1: one 500-barrel tank in a square berm
Setup: one vertical 500-bbl tank inside a 30 ft × 30 ft lined berm (900 ft²).
- Tank volume: 500 × 5.615 = 2,808 ft³.
- Height to hold the tank: 2,808 ÷ 900 = 3.12 ft.
- Rain: 5.0 in ÷ 12 = 0.42 ft over the whole berm.
- Required wall height: 3.12 + 0.42 = 3.54 ft, or about 42.5 inches.
For comparison, sizing to 110 percent gives 3,089 ft³ ÷ 900 = 3.43 ft. That’s about 3.7 inches of freeboard, which is short of the 5-inch storm. The tank that fails isn’t subtracted as displacement, because the oil below the wall height stays inside its shell. EPA’s examples follow the same approach.
Example 2: a four-tank battery with displacement
Setup: four vertical 400-bbl tanks, each 12 ft in diameter, not manifolded, inside a 50 ft × 40 ft berm (2,000 ft²).
- Largest single container: 400 × 5.615 = 2,246 ft³.
- Footprint of each tank: 3.1416 × 12² ÷ 4 = 113.1 ft².
- Displacement: when one tank fails, the other three still stand in the berm. Their footprint is 3 × 113.1 = 339.3 ft².
- Effective floor area: 2,000 − 339.3 = 1,660.7 ft².
- Rain on the whole berm: 2,000 × 0.42 = 833 ft³.
- Required wall height: (2,246 + 833) ÷ 1,660.7 = 1.85 ft, or about 22 inches.
Without rain the same berm needs about 16 inches. If the four tanks were permanently manifolded, the largest container would be all 1,600 barrels (8,984 ft³). The berm would need to be far taller or far larger, or the tanks would need to be isolated with valves.
Example 3: checking a berm that’s already built
This is the calculation an inspector runs, laid out the same way as EPA’s Figure 4-6.
Setup: the same four tanks, inside an existing 50 ft × 40 ft berm with 20-inch (1.67 ft) walls.
- Gross capacity: 2,000 × 1.67 = 3,333 ft³.
- Displacement of the three tanks that didn’t fail, up to wall height: 339.3 × 1.67 = 566 ft³.
- Net capacity: 3,333 − 566 = 2,767 ft³.
- Freeboard left after the largest tank: 2,767 − 2,246 = 521 ft³.
- Rain it can hold: 521 ÷ 2,000 = 0.26 ft, or about 3.1 inches.
That berm holds the tank, but not a 5-inch storm on top of it. Raising the wall a few inches or extending the footprint fixes it. Whether 3.1 inches is “sufficient” for the site is the Plan certifier’s decision, and it should be written down either way.
What eats containment capacity after it’s built
The numbers above assume an empty, clean berm. In the field, capacity disappears in ordinary ways:
- Standing rainwater left in the berm takes up the freeboard you designed in.
- Silt and sand blown or washed in raise the floor. A few inches of sediment across a 2,000 ft² berm is hundreds of cubic feet.
- New equipment such as a pump skid, a separator or a chemical tote placed inside adds displacement nobody calculated.
- Walls pushed down by vehicles, or liners that slumped at the corners.
- Tanks swapped for larger ones without redoing the math.
EPA’s guidance tells inspectors to compare “available capacity versus design capacity.” That’s the question to ask at every inspection.
A quick sizing checklist
- Identify the largest single container, and check whether any tanks are permanently manifolded.
- Measure the berm inside the walls, not the outside.
- Subtract the footprint of every other tank or object up to wall height.
- Add rain for your site’s design storm over the full berm area.
- Compare against the actual wall height at its lowest point.
- Write the calculation into the SPCC Plan or keep it with the Plan.
- Recheck whenever a tank, a skid or the layout changes.
Let someone else run the numbers
Site Safe Solutions designs and installs containment systems for well pads and production sites in the Permian Basin, with layouts designed around the site. That includes primary pads, modular drive-over containment and high-wall systems for large volumes or tight footprints. For trucks and short transfers, Fast Mats cover the smaller jobs. If you’re not sure whether the site needs containment in the first place, start with does my site need secondary containment. Have the tank sizes and a rough site layout ready when you get in touch.
