Water · Gear Review
The Platypus GravityWorks 4L is our pick for groups and household emergency use. Know the virus gap before you buy — hollow fiber alone does not cover it.
The verdict
A gravity bag filter fills a different role than a countertop stainless system. It is portable, lightweight, and designed for moving water: from a stream to a camp, from collected rainwater to a jug, from an emergency source to a container. If your household water comes from a municipal tap and you have a countertop filter, a gravity bag is your field and go-bag complement — not your primary filter.
Best overall
0.2 micron hollow fiber filter. 1.75 L/min flow rate. Two 4-liter reservoirs (dirty and clean). Made in the USA. 1,500-liter filter life. Rated for bacteria, protozoa, and Giardia. Does not remove viruses. Back-flush at start of each use. 11.5 oz complete system.
$110–$130
Best value / DIY gravity system
The Sawyer Squeeze filter (0.1 micron) paired with compatible large-diameter gravity bags (Platypus, Evernew, or MSR reservoir bags) creates a capable gravity system at lower cost than the GravityWorks. Flow rate at 0.1 micron is slower but backflush more effectively. More flexible: the filter also works inline in a hydration bladder or as a squeeze filter.
$35–$50 filter + $20–$30 bags
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The virus gap
What 0.1–0.2 micron hollow fiber covers
Bacteria
E. coli, Salmonella, Campylobacter, Legionella — all removed at 0.1–0.2 micron
Protozoa and cysts
Giardia lamblia, Cryptosporidium, Entamoeba — all larger than 0.2 micron
Sediment and turbidity
Particulates larger than the filter pore size are physically blocked
What 0.1–0.2 micron hollow fiber does NOT cover
Viruses
Hepatitis A, norovirus, rotavirus — 0.01–0.1 micron, smaller than hollow fiber pores. Not removed.
Chemical contaminants
Heavy metals, PFAS, pesticides, chemical runoff — not removed by mechanical filtration alone
Dissolved solids
Salts, nitrates, fluoride — pass through hollow fiber pores
What to do about viruses in the field
In most North American backcountry and emergency scenarios, viruses in surface water are a low risk — wilderness water sources are rarely contaminated with human fecal viruses unless there is nearby human activity. For international travel, post-flood water treatment, or any water source with known human contamination, add chlorine dioxide tablets after filtering: filter first to remove particulates, then treat for 15 minutes (bacteria/viruses) to 4 hours (Cryptosporidium). The filter + chlorine dioxide combination provides broad-spectrum coverage that no single product delivers at this weight and price point.
How we evaluated
Gravity bag filters are slower than pump filters but require no effort during filtration. The Platypus GravityWorks 4L flows at approximately 1.75 L/min when clean — fast enough to filter 4 liters in about 2.5 minutes unattended. The Sawyer Squeeze at 0.1 micron flows more slowly at first but maintains flow well with regular backflushing. For groups of 2 or more, the GravityWorks 4L system is meaningfully more practical than smaller options.
The dirty bag needs to be filled from a water source repeatedly. The GravityWorks dirty bag has a wide zip-top opening that makes scooping from shallow streams practical. Sawyer’s own dirty bag is narrower and harder to fill; most experienced users replace it with a Platypus or MSR reservoir bag. The clean reservoir should have a closure that prevents air exposure during transport — the GravityWorks clean bag uses a push-pull cap.
A gravity filter that cannot be maintained in the field is a liability. Both GravityWorks and Sawyer support back-flushing to restore flow rate. GravityWorks includes a simple back-flush adapter; Sawyer Squeeze includes a cleaning syringe. Back-flush at the start of each outing. Never let the filter freeze with water in the hollow fibers — ice formation cracks the membranes and permanently damages the filter without visible external indication.
The picks
Best overall — integrated system, fastest setup
The Platypus GravityWorks 4L is the benchmark gravity bag filter for groups, families, and households that want a complete system rather than a DIY assembly. Every component is designed to work together: the dirty bag scoops easily from shallow sources, the quick-connect fitting allows the bags to be connected and disconnected without handling the filter element, and the clean bag doubles as a water carrier. Made in the USA.
The 0.2 micron hollow fiber filter removes bacteria, protozoa, and Giardia. At 1.75 L/min, a full 4-liter fill takes under 3 minutes unattended. Hang the dirty bag from a tree branch, a pack strap, or a tent pole; open the inline shutoff valve; return when the clean bag is full. No pumping, no squeezing, no effort during the filtration cycle itself.
The complete 4L system weighs 11.5 oz and packs into a small stuff sack. Practical considerations: do not let the element freeze. Pre-filter turbid water through a bandana before putting it in the dirty bag to extend filter life. Back-flush at the start of each outing to restore flow rate.
At a glance
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Best value — versatile filter, DIY gravity assembly
The Sawyer Squeeze filter at 0.1 micron provides finer filtration than the GravityWorks’ 0.2 micron, though both effectively remove bacteria and protozoa in practice. The Sawyer’s advantage is versatility: the same filter element works inline in a hydration bladder, screwed onto a water bottle, as a squeeze filter, or as a gravity filter. One filter serves all modes.
To use as a gravity system, replace the Sawyer’s included dirty bag with a Platypus GravityWorks dirty reservoir or an MSR bag — both have wider openings that fill more easily and hold their shape better when hanging. Connect the Sawyer inline between the dirty bag and a clean reservoir. The total system cost runs $55–$80, significantly less than the GravityWorks.
The Sawyer Squeeze requires the backflush syringe to be carried in the field — the GravityWorks’ backflush method is simpler and does not require a separate tool. For solo or small-group use, the Sawyer DIY gravity system is a sound choice. For groups of four or more who want the fastest filter-while-you-do-something-else experience, the GravityWorks 4L is worth the premium.
At a glance
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What not to buy
Amazon carries dozens of gravity bag filter systems at $20–$40 from sellers with no traceable certifications. The hollow fiber elements in these products have no independent verification of pore size accuracy. A filter rated at “0.1 micron” that is actually 1 micron passes bacteria. For a portable filter used in emergency water treatment, the certification and documented pore size matters. Sawyer and Platypus publish independent lab data. Most generic brands do not.
Gravity bag filters are designed for field use. They filter a few liters at a time, require hanging, and are not practical for the 2–3 gallons per day a household of four actually consumes at the counter. For daily home water filtration, a countertop gravity system (ProOne, Culligan MaxClear) is the right tool. A gravity bag belongs in your emergency kit, go-bag, or camping gear — not replacing your kitchen filter.
How this fits
The stationary household filter — ProOne, Culligan MaxClear, Waterdrop. How it complements a portable gravity bag.
See countertop review →
The virus layer that hollow fiber filters cannot provide. Filter first, treat second for complete coverage.
Read the guide →
Individual filters (Sawyer Squeeze, LifeStraw Peak) for single-person on-the-go filtration vs. gravity bags for groups.
See portable filter review →