Secondary Containment Calculator
Calculate secondary containment volume and dimensions per EPA SPCC requirements.
About this calculator
EPA's SPCC rule (40 CFR 112.8(c)(2), and the parallel provision at 112.7) requires secondary containment sized to hold the entire capacity of the largest single container plus sufficient freeboard for precipitation — the regulation itself does not specify a fixed percentage. The "110% of largest container" figure this calculator uses (100% plus a 10% buffer) is a widely used industry engineering convention for meeting that requirement, not literal text from the CFR, and the alternative "10% of total aggregate storage" floor likewise comes from common design practice rather than the SPCC rule's own wording. This calculator takes the greater of those two conventional figures as Base Containment, then adds a separately calculated Precipitation Allowance for outdoor installations using your entered 25-Year Storm figure — a genuine attempt at the "sufficient freeboard for precipitation" language, applied on top of the already-conservative 110%/10% baseline.
Largest Container (gallons) drives Required Containment far more than Total Storage (gallons) does under typical ratios, since the 110%-of-largest rule only yields to the 10%-of-aggregate rule when aggregate storage is roughly ten times the largest single container or more. Because SPCC compliance is facility-specific and typically requires a licensed Professional Engineer's certification, treat every number here as a planning estimate to bring to that review, not a substitute for it.
Inputs
Results
Required Containment (gallons)
353
Containment Area (sq ft)
23.6
Figures current as of 2026. Source: 40 CFR 112.8(c)(2) (Spill Prevention, Control, and Countermeasure rule, bulk storage container provisions)
How to Use This Calculator
- Enter the Largest Container (gallons) in the storage area — EPA SPCC requires containment of at least that volume.
- Enter Total Storage (gallons) for the aggregate inventory; the calculator uses 10% of total or the largest container, whichever is greater.
- Select Containment Method (1=Dike/Berm, 2=Double-Wall Tank, 3=Vault, 4=Drainage System) appropriate for your facility.
- Toggle Outdoor Installation and enter the 25-Year Storm (inches) to add precipitation capacity for outdoor berms.
- Review Required Containment (gallons) and Containment Area (sq ft) to design or verify your dike dimensions.
- Use Estimated Cost and Inspection Interval (days) for budget planning and to establish your inspection schedule.
How the result changes with Largest Container (gallons)
| Largest Container (gallons) | Required Containment (gallons) | Containment Area (sq ft) |
|---|---|---|
| 138 | 233 | 15.6 |
| 206 | 265 | 17.7 |
| 413 | 530 | 35.4 |
| 688 | 883 | 59 |
What each input means
- Largest Container (gallons)
- Volume of the single largest container in the storage area.
- Total Storage (gallons)
- Total aggregate storage volume of all containers.
- Containment Method
- The physical containment structure planned for the storage area.
- Installation Location
- Outdoor containment needs extra capacity for precipitation; indoor containment does not.
- 25-Year Storm (inches)
- 25-year 24-hour precipitation amount for your area.
How this is calculated
Worked example, using the default values
- Identify Input Parameters5 parametersLargest Container (gallons) = 275, Total Storage (gallons) = 2000, Containment Method = 1, Outdoor Installation = 1, 25-Year Storm (inches) = 4 = 5 input(s) provided
- Calculate Required ContainmentRequired Containment353 = 353
- Calculate Containment AreaContainment Area23.6 = 23.6
- Calculate Base ContainmentBase Containment303 = 303
- Calculate Precipitation AllowancePrecipitation Allowance50 = 50
Figures and sources
- SPCC secondary containment: entire capacity of the largest single container plus freeboard for precipitation (2026) — 40 CFR 112.8(c)(2) (Spill Prevention, Control, and Countermeasure rule, bulk storage container provisions)
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does the EPA SPCC rule literally require 110% of the largest container's volume?
No. The regulation (40 CFR 112.8(c)(2)) requires secondary containment sized to hold the full capacity of the largest single container plus sufficient freeboard for precipitation — it does not write a specific percentage into the rule text. "110%" (100% plus a 10% buffer) is a widely used industry engineering convention for meeting that requirement, which this calculator applies as a practical, conservative default.
Why does Total Storage (gallons) often have little effect on Required Containment?
The calculator takes the greater of two figures: 110% of the largest container, or 10% of total aggregate storage. Under typical ratios — where the largest container isn't a small fraction of total storage — the 110% figure comes out larger, so small changes to Total Storage don't move Required Containment at all. The 10% rule only takes over once aggregate storage grows to roughly ten times the largest single container or more.
Why does Required Containment increase when I raise the 25-Year Storm figure?
Precipitation Allowance (gallons) is calculated separately from Base Containment, estimating rainfall collected across the containment footprint during a storm of that magnitude, and added on top for outdoor installations. Raising the 25-Year Storm figure directly increases that allowance and therefore Required Containment — indoor installations skip this term entirely, since they aren't exposed to precipitation.
Does this calculator replace a facility's actual SPCC Plan?
No. SPCC compliance is facility-specific, and a real SPCC Plan for a facility that meets the regulatory storage thresholds must be certified by a licensed Professional Engineer who reviews site-specific conditions, applicable state and local requirements, and the actual containment structure design. Use this calculator's output as a planning estimate to bring to that review, not as a substitute for it.
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