Undersized bag filtration shows up as short bag life, pressure spikes, and bags blowing through; oversized costs you housing money you didn’t need to spend. Sizing is simple if you work through it in order: flow, micron, media, then bag size.
What Size Filter Bag Do I Need?
The short answer: match the bag to your housing first — a size 1 housing takes a size 1 bag, full stop. If you’re specifying a new system, pick the smallest standard size whose clean-water flow rating comfortably exceeds your flow: size 2 (7″ × 32″) covers most industrial duties up to ~180 GPM, size 1 (7″ × 16.5″) up to ~90 GPM, and the 4″-diameter sizes 3 and 4 handle point-of-use flows. The chart below has the numbers; the steps after it cover micron, media, and change-out.
1. Start with Flow Rate — the Filter Bag Size Chart
| Bag size | Dimensions | Surface area | Max flow, clean water* |
|---|---|---|---|
| Size 3 | 4″ × 8″ | 0.5 ft² | ~25 GPM |
| Size 4 | 4″ × 14″ | 1.0 ft² | ~45 GPM |
| Size 1 | 7″ × 16.5″ | 2.2 ft² | ~90 GPM |
| Size 2 | 7″ × 32″ | 4.4 ft² | ~180 GPM |
*Rules of thumb at low viscosity. Halve them for tight micron ratings (≤5 µm) or viscous fluids; halve again for heavy solids. Multi-bag housings scale linearly.
2. Set the Micron Rating
Pick the largest micron rating that still protects the downstream process — tighter is not better, it’s just shorter bag life. See Micron Ratings Explained.
3. Choose the Media
- Felt for high dirt load and sub-35-µm ratings (felt bags)
- Mesh for defined cutoff, reusability, product recovery (mesh bags) — see the felt vs. mesh guide
- Welded when stitch-hole bypass matters (welded bags)
- Oil-absorbing when hydrocarbons are the contaminant (Oil Magnet)
4. Change-Out Pressure
Change bags at roughly 15 PSID across the housing. Running far past that risks bag rupture and drives particles through the media. If bags load up too fast at the right micron rating, go up a bag size (more area), add housings in parallel, or step down solids upstream with a coarser bag in series.
Identifying the Bag You Already Have
Measure diameter across the ring and length ring-to-bottom, note the ring material and top style, and check for a stamped part number. Or skip all that: mail us the tag or a photo via the quote form — we identify competitor bags every day.
Want us to size it for you? Call 800-554-8555 with flow rate, fluid, and target clarity.