What Is Pool Filter Sand? - The #20 Silica Sand Explained
Pool filter sand is a specific grade of silica sand with 0.45-0.55mm grains designed to trap pool debris without clogging or passing through the filter laterals. This guide explains what it is, why the grade matters, and how it differs from every other type of sand.
HTH #20 Pool Filter Sand - 50 lb Bag
#20 grade silica sand with consistent 0.45-0.55mm grains. The correct media for any residential sand filter and widely available at pool stores and hardware retailers.
What Pool Filter Sand Is
Pool filter sand is #20 grade silica sand - a precisely manufactured filtration media with a consistent grain size of 0.45-0.55mm.
It is not beach sand, play sand, or any other general-purpose sand. It is specifically graded and washed for use as a filter medium in residential and commercial pool sand filters.
The most common form sold at pool supply stores and online is 50 lb bags labeled “pool filter sand” or “#20 silica pool sand.”
Why the Grain Size Matters
A sand filter works by trapping debris in the spaces between sand grains. The grain size has to be precisely right - for two reasons.
Too fine: Grains smaller than ~0.3mm pass through the slotted laterals at the bottom of the filter tank and return to the pool. Fine particles also pack tightly, restricting water flow and driving up filter pressure.
Too coarse: Larger grains leave bigger gaps between them. Debris slips through without being captured, and water clarity suffers.
#20 grade (0.45-0.55mm) hits the correct middle ground. Grains are large enough to stay above the laterals, sized to create small enough inter-grain gaps to capture debris, and consistent enough that the whole bed behaves uniformly.
How It Filters Water
When the pump runs on FILTER mode, water enters the filter tank at the top, flows downward through the sand bed, and exits through the laterals at the bottom.
As water passes through the packed sand:
- Particles larger than the inter-grain gaps get physically trapped
- Smaller particles bind to the sand grains through adhesion
- Very fine particles (including some algae) build up a filter cake on the top layer of sand, which improves filtration over the first few days of operation
The filtered water exits through the laterals, flows back to the pump, and returns to the pool through the return jets.
What #20 Grade Means
Sand is graded by mesh screen size. #20 grade means the sand passes through a mesh with 20 openings per inch but is retained by a finer mesh below.
The practical result: a consistent grain size between 0.45mm and 0.55mm with very little variation. This consistency is what separates pool filter sand from regular hardware store silica, which may have the right average grain size but wide variation around that average.
How Long Pool Filter Sand Lasts
Under normal residential use, pool filter sand lasts 5-7 years before it needs replacing.
Over time, the mechanical action of backwashing slowly wears the grains smooth. Sharp, angular grains trap debris well. Worn, smooth grains do not.
You will know the sand is worn out when:
- Water clarity is poor despite correct chemistry and regular backwashing
- Filter pressure rises faster than it used to after backwashing
- Backwashing returns dark or heavily soiled water even on a regular schedule
Full guide: How Long Does Pool Filter Sand Last?
Alternatives to Pool Filter Sand
Two alternative filter media work in the same filter tank as sand and outperform it:
ZeoSand (zeolite) - filters particles down to 2-3 microns vs. sand’s 20-40 microns. Use at 50% of the weight of sand your filter requires. Costs more upfront but can extend cleaning intervals.
Filter glass (recycled glass media) - filters down to 5 microns. Also reduces chemical usage. Use at a 1:1 weight ratio with sand. More expensive than sand.
Neither alternative is required - standard #20 silica sand is perfectly effective for the large majority of residential pools.
Where to Buy Pool Filter Sand
- Pool supply stores (In The Swim, Leslie’s)
- Hardware stores (Home Depot, Lowe’s) - seasonal section, typically spring and summer
- Amazon and online retailers - often cheaper per bag but check shipping weight costs
Pool filter sand on Amazon - order in multiples of 50 lb bags based on your tank size.
Related Guides
- What Type of Sand for Pool Filter? - comparing #20 silica, ZeoSand, and filter glass
- How Much Sand for Pool Filter? - sizing table by filter model
- How to Change Pool Filter Sand - complete replacement walkthrough
- Can You Use Play Sand in a Pool Filter? - why it does not work
- How Long Does Pool Filter Sand Last? - when to replace
Frequently Asked Questions
What is pool filter sand made of?
Is pool filter sand the same as regular sand?
What does pool filter sand do?
How much pool filter sand do I need?
When should pool filter sand be replaced?
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