How Much Filter Media Fits in Your FRP Tank? (With Easy Calculation Examples)

Getting this right is not just about filling the tank - it's about ensuring proper filtration performance, safe backwashing, and efficient use of media. In this article, you'll learn exactly how to calculate the amount of media for any FRP tank model, with simple formulas and real examples (including the popular 3072 FRP tank).

FRP (Fiberglass Reinforced Plastic) tanks are critical components in water treatment systems, and their ability to hold filter media depends primarily on two dimensions: diameter and height. The volume of a fiberglass tank is directly determined by its diameter and height.

Most fiberglass tank models on the market are named according to this principle. For example, 1665 refers to a FRP tank with a diameter of 16 inches and a height of 65 inches.

Here are some common models:

However, the full geometric volume of the tank is not all usable. 

To facilitate backwashing and prevent filter media loss, and to allow space for accessories such as connectors, water pipes, and water distributors, the filter media is generally filled to about 80-85% of the total volume of the fiberglass tank.

The basic formula to calculate a cylindrical tank's volume is:

V=πr²h

Where:

• r = tank radius (in meters)
• h = height (in meters)

Then multiply by 1,000 to convert cubic meters to liters.

Let's take model 3072 as an example for calculation:

Example: 30 × 72 inch FRP tank

• Diameter = 30 in = 0.762 m
• Height = 72 in = 1.83 m

V=3.1416×(0.762/2)²×1.83≈0.834 m³=834L

As mentioned in Part 1, the actual usable fill material is about 80-85% of the total volume, which is about 667L-700L.

This will serve as the base number for calculating the amount of filter media you need.

Different filter media have very different densities.
Below is a practical reference table:

💡 Tip: Always confirm the density from your media supplier.
Wet, dry, or backwashed media can differ slightly in weight.

Let's take the 3072 FRP tank (one of the most common industrial sizes) and calculate the media requirement for different materials.

Step 1 - Determine usable volume

Use 700 L as the working capacity (after deducting internal space).

💡 Tip: It depends on the specific design of the water treatment system.

Step 2 - Multiply by bulk density

So, if a customer asks, "How many bags of Activated Carbon (25 kg each) do I need for a 30×72 FRP tank?"
👉 The answer is around 13 bags for a full fill, or 12 bags if leaving space for backwash expansion.

FRP tanks should never be filled to 100% with filter media.
You must reserve a freeboard space (10–15%) at the top to allow media expansion during backwash.

Typical design guidelines:

• Freeboard height: 8–12 inches (20–30 cm)
• Support gravel layer: ~15–20% of total media volume
• Filter media layer: ~70–75% of total volume
• Headspace for backwash: ~10%

Example layering structure:

• Bottom layer: Gravel or garnet (3–5 mm)
• Middle layer: Quartz sand or glass media
• Top layer: Activated carbon or anthracite

This structure ensures good flow distribution and prevents fine media from clogging the underdrain.

*Values based on glass media at 1.35 kg/L density.
If using heavier sand, multiply by ~1.2–1.3.

Overfilling the tank
→ Causes poor backwash and media loss.

Ignoring freeboard allowance
→ Media can escape through the upper distributor.

Wrong density assumptions
→ Each supplier may define bulk density differently (wet or dry).

Skipping support layer
→ Leads to underdrain clogging or uneven flow.

No check after hydration (for resin)
→ Resin expands after soaking; always verify level after 24 hours.

Knowing your FRP tank's correct media capacity saves both cost and operational headaches.
✅ You avoid buying too much media.
✅ You maintain optimal bed depth for filtration efficiency.
✅ You ensure proper backwash performance and longer tank lifespan.

For engineers and resellers, providing accurate sizing also builds trust and credibility with clients - especially when you can show the calculation behind your recommendation.

Need a quick reference for your project?
You can request a free FRP Tank Media Capacity Chart (PDF) that includes common models (1035 to 3672) with corresponding media weights for sand, carbon, resin, and glass.

👉 Contact our technical team to get the chart to make your filter design easier.

References：

Ahmad, Shehbaz; Mac-Allister Cedraz, Fernanda; Borne, Nicholas; Albert, Julie N. L.; Aw, Tiong Gim. "Characterization and evaluation of recycled glass sand as water filtration media." Journal of Water and Health, Vol 23, Issue 6, 2025. DOI:10.2166/wh.2025.023.

Dalwadi, Mohit P.; Griffiths, Ian M.; Bruna, Maria. "Understanding how porosity gradients can make a better filter using homogenization theory." arXiv preprint, July 2015.

"Technical Standard of Quartz Sand Filter Media for Water Treatment." PoolKing website article.

"Filtration Process and Alternative Filter Media Material in Water Treatment." MDPI Water, Vol 12, Issue 12, 2020.

"Multi-Media Filtration, Quartz Sand, Activated Carbon, Manganese Sand." CDFS Filter product detail page.