If you are researching commercial fuel storage or reading through government environmental guidelines, you will immediately encounter the phrase “Secondary Containment.” Shortly after, you will inevitably run into the industry’s golden standard: The 110% Rule.
Whether you are governed by the EPA’s SPCC regulations in the United States or OFTEC and the Environment Agency in the UK, the “bunded tank 110 rule” is the foundational law of liquid storage. Failing to understand the math behind this rule can result in voided insurance policies, failed inspections, and massive environmental fines.
Here is the definitive guide to understanding exactly what the 110% rule is, why it exists, and how to calculate it for both single and multiple-tank installations.
1. What is the Bunded Tank 110% Rule?
By definition, a bunded tank features a primary inner tank (which holds the fuel) surrounded by an outer secondary containment barrier (the bund).
The 110% Rule states that this outer secondary containment system must be legally capable of holding at least 110% of the maximum capacity of the inner tank.
- The Basic Calculation: If your primary inner tank holds 5,000 liters of diesel, the outer bund must have a watertight capacity of exactly 5,500 liters (5,000 x 1.10).
Modern “integrally bunded” tanks (where the manufacturer builds the outer plastic or steel wall directly around the inner tank) come pre-engineered to meet this 110% capacity. However, if you are building an external concrete bund around a single-skin tank, you must perform these calculations yourself before pouring the concrete.
2. Why 110%? (Understanding the 10% Buffer)
A common question from facility managers is: “If my tank holds 5,000 liters, why does the bund need to hold 5,500? Where does the extra fuel come from?”
The extra 10% acts as a critical safety buffer—known as freeboard—designed to account for three specific real-world variables:
- Rainwater Accumulation: If your tank is situated outdoors, rainwater will inevitably pool inside an open concrete bund. The extra 10% capacity ensures that even if a heavy storm fills the bottom of the bund with water, there is still enough room left to catch 100% of the fuel if the tank ruptures.
- Dynamic Rupture Surges: If a tank suffers a catastrophic structural failure, the fuel does not leak out slowly; it surges out in a tidal wave. This violent movement causes the liquid to splash up the sides of the bund walls. The extra height (the 10%) prevents the fuel from sloshing over the edge.
- Volume Displacement: The inner tank, its support legs, and the internal pipework all take up physical space (displacement) inside the bund. The 110% calculation ensures there is enough actual empty volume to catch the liquid.
3. The “Multiple Tank” Calculation: The 110% vs. 25% Rule
The math becomes slightly more complicated if you place multiple tanks inside the same shared containment area (for example, a large concrete berm housing three separate 10,000-liter tanks).
According to both UK and US environmental regulations, if multiple tanks share a single bund, the containment system must be able to hold whichever of the following two numbers is GREATER:
- Option A: 110% of the largest single tank’s capacity.
- Option B: 25% of the total combined capacity of all the tanks in the bund.
Example Scenario:
You have three tanks in a shared concrete bund.
- Tank 1 = 10,000 Liters (The Largest)
- Tank 2 = 5,000 Liters
- Tank 3 = 5,000 Liters
- Total Volume = 20,000 Liters
Let’s do the math:
- Option A (110% of Largest): 10,000L x 1.10 = 11,000 Liters.
- Option B (25% of Total): 20,000L x 0.25 = 5,000 Liters.
Because Option A is the greater number, your concrete bund must have a minimum watertight capacity of 11,000 liters.
4. How to Ensure Compliance in 2026
If you are constructing your own external masonry bunds, you must ensure there are no drainage valves at the bottom (as leaving a valve open defeats the entire purpose of the containment). You must also coat the concrete in an epoxy sealant, as raw concrete is porous and will eventually absorb diesel fuel.
The Easiest Solution: To eliminate the headache of concrete displacement math, hiring structural engineers, and worrying about rainwater management, modern businesses rely on integrally bunded tanks. Because the outer steel or plastic skin is fully enclosed, rainwater cannot get in, and the manufacturer guarantees the 110% compliance out of the box.
Need a tank that is guaranteed to pass your next environmental inspection? [Click Here to Use the Bunded.com Smart Quote Engine] (Enter your required fuel type and capacity, and we will supply you with a fully certified, 110%-compliant tank solution).
5. Frequently Asked Questions
What is the bunded tank 110 rule? The bunded tank 110 rule is an environmental regulation stating that a secondary containment system (a bund) must be capable of holding at least 110% of the maximum volume of the primary fuel tank it is protecting.
Why does a bund need to hold 110%? The extra 10% capacity provides a safety buffer. It accounts for rainwater that may have accumulated inside the bund, the physical space taken up by the tank’s support legs (displacement), and prevents splashing over the sides in the event of a sudden, catastrophic tank rupture.
How do you calculate bund capacity for multiple tanks? When multiple tanks share a single bund, the regulations state the bund must hold whichever is the greater amount: either 110% of the capacity of the largest single tank, or 25% of the total combined capacity of all the tanks sitting within that bund.
Do double-skinned tanks count as 110% bunded? Not necessarily. A “double-skinned” tank simply has two layers of material tightly wrapped together for extra strength. An “integrally bunded” tank has a deliberate gap between the inner and outer walls specifically engineered to hold 110% of the inner tank’s volume. Always verify that a tank is explicitly labeled as “bunded” rather than just double-skinned.
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