RO Membrane Element Size Reference: 1812 / 4040 / 8040
Standard dimensions for spiral-wound RO membrane elements — nominal diameter, length and how to read the model number — so you can set up production and match components to the right size.
e.g. “4040” = 4.0 inch diameter × 40 inch length
Key Takeaways
- The model number = diameter (inches) + length (inches).
- A 4040 is ~100 mm × 1016 mm; an 8040 is ~200 mm × 1016 mm.
- Dimensions are nominal — confirm finished diameter with the calculator.
- Match central tube, brine seal and end cap to the element size.
Reading RO Membrane Element Model Numbers
Spiral-wound RO membrane elements use a four-digit model number. The first two digits are the nominal outer diameter in inches and the last two are the nominal length in inches. So a 4040 element is about 4.0″ in diameter and 40″ long; an 8040 is about 8.0″ in diameter and 40″ long.
These are nominal figures. The exact finished diameter depends on your leaf count and material thicknesses — calculate it with the Roll Diameter Calculator before setting machine tooling. For full production parameters by size, use the production setup guide.
RO Membrane Element Size Reference Chart
Nominal dimensions for the most common spiral-wound element sizes. Use them to plan production tooling, housings and component matching.
| Model | Nominal diameter | Nominal length | Typical application |
|---|---|---|---|
| 1812 | ~46 mm (1.8″) | ~305 mm (12″) | Residential / under-sink |
| 2521 | ~64 mm (2.5″) | ~533 mm (21″) | Light commercial |
| 2540 | ~64 mm (2.5″) | ~1016 mm (40″) | Light commercial |
| 3013 | ~76 mm (3.0″) | ~330 mm (13″) | Residential / small commercial |
| 4040 | ~100 mm (4.0″) | ~1016 mm (40″) | Commercial / industrial |
| 8040 | ~200 mm (8.0″) | ~1016 mm (40″) | Industrial / large systems |
Nominal sizes for production reference. Confirm exact finished diameter with the Roll Diameter Calculator and your raw-material thicknesses; central tube, brine seal and end cap dimensions vary by series — see the linked data sheets below.
Match Components to the Element Size
Element size sets more than the outer dimensions. The central tube, brine seal and end cap and connector all change with diameter and must be specified for each size you produce.
If you are new to these parts, start with the basics: what the central tube does and why elements need a brine seal. Then quantify the full set for any size with the BOM Calculator.
Changes with element size
- Central tube diameter & length
- Brine seal dimensions
- End cap / ATD & connector
- Membrane width & leaf length
- Housing and machine tooling
Getting the Size Wrong Cascades Through Production
Element size is the reference point for tooling, components and testing. A mismatch at this stage shows up everywhere downstream.
Housing Fit
An out-of-tolerance diameter or length will not seat or seal correctly in the pressure vessel.
Component Mismatch
Wrong tube, seal or end cap for the size means rework or scrapped elements.
Tooling Limits
A machine not set for the size cannot hold tension or diameter reliably.
Test Failures
Dimensional errors surface late as leak or fit failures at final testing.
Setting up production for a new element size?
Tell us which sizes you plan to produce — we’ll match the equipment, central tubes, seals and end caps to each one.
RO Membrane Element Size FAQ
What do RO membrane element model numbers mean?
The four-digit model number gives the nominal size: the first two digits are the diameter in inches and the last two are the length in inches. For example, 4040 is about 4.0 inches in diameter and 40 inches long, and 8040 is about 8.0 inches in diameter and 40 inches long.
What are the dimensions of a 4040 RO membrane element?
A 4040 element is approximately 100 mm (4.0 inches) in nominal diameter and 1016 mm (40 inches) in length. The exact finished diameter depends on leaf count and material thicknesses, so confirm it with the Roll Diameter Calculator.
What is the size of an 8040 RO membrane element?
An 8040 element is approximately 200 mm (8.0 inches) in nominal diameter and 1016 mm (40 inches) in length. It is the standard size for industrial and large commercial systems.
How do I match components to a given element size?
The central tube, brine seal and end cap all change with element diameter. Use this size reference to pick the size, then confirm each component against its data sheet and quantify the full set with the BOM Calculator.