The FIBC safety factor, 5:1 and 6:1

A multiplier, not a capacity

Two numbers sit next to each other on every FIBC label. The SWL is the most product you may put in the bag and lift by its loops. The safety factor tells you how far beyond that figure the design was pushed before anyone signed off on it. Multiply one by the other and you get the test load, the weight at which a sample had to hold together in the lab.

That margin exists because real handling is rough. A forklift brakes hard with a full bag swinging from its tines. A crane hook snatches instead of easing up. One loop slips off a fork and the other three take the jolt. Product settles, bridges, then drops all at once inside the body. The extra strength is there to absorb those moments, not to be spent on heavier fills.

So when a shipper asks whether a 5:1 bag can take a little more than its label says, the answer from our desk is no. The multiplier is headroom for shocks you can't plan around. Fill to the SWL, never over it.

Think of it like a rated chain or a lifting sling. Nobody reads the breaking strength on a hoist and loads to it. The working figure is the one you plan with; the larger figure is the cushion that keeps an unlucky day from becoming an accident.

How a design earns its rating

Ratings come from type tests run on samples pulled from production, not from every bag on your pallet. ISO 21898:2024, published 16 August 2024 and replacing the 2004 edition, sets materials, construction, design, type test and marking requirements for FIBCs carrying non-dangerous solids. Check the standard itself and the maker's documents for the exact procedures; we name the tests here only so the label makes sense.

• Cyclic top-lift test: a filled sample is lifted by its loops and set down repeatedly at a load above its SWL, to show it survives the up-and-down of ordinary handling.
• Final top-lift, or destruction, test: the sample is pulled until it reaches the target multiple of SWL. A single-trip bag is tested to 5 x SWL; a multi-trip standard-duty bag must reach 6 x SWL, which is 6,000 kg for a 1,000 kg SWL design.
• Compression test for multi-trip standard-duty bags: the maker's handling guide lists a 4,000 kg load held for 6 hours, which speaks to stacking.
• Further type tests in the standard deal with dropping, toppling, righting and tearing, plus the marking; the summary we cite outlines the full scope.

The arithmetic for common labels

Here is the sum for the SWLs we quote most often. Do it in the unit the label prints. If the tag says 1,000 kg, work in kilograms; converting to pounds and rounding to a ton loses the precision the rating depends on.

The gap between columns widens as the SWL climbs. Stepping from 5:1 to 6:1 adds 1,000 lb of test load on a 1,000 lb bag, but 4,400 lb on a 4,400 lb bag. That is one reason dense, heavy fills such as cement, ore concentrate or damp aggregate deserve an exact rating on the purchase order rather than a guess.

Ratings above 6:1

You may see heavy-duty or 8:1 printed on some listings. Some makers rate bags higher; read the label. If a higher figure matters to your job, ask for the maker's documents that back it, and treat a word in a catalog headline as a lead to verify rather than a spec.

What the ratio leaves out

A safety factor answers one question: how much margin did the design have when new, on the test rig. Plenty of things that decide whether a bag suits your fill line sit outside that number.

• Static behavior. Type A, B, C or D is a separate field. A 6:1 Type A bag still has no static protection.
• Food or pharma suitability. That depends on where and how the bag was made and handled, not on the multiplier.
• Present condition. A bag that sat in sunlight, was dragged across a dock or caught a fork tine is not the bag that passed the test.
• Fit with your equipment. Loop height, loop style and spout size decide whether your forklift, frame or discharger works with it.
• Liner strength. A film liner keeps out moisture or dust; it does not add lifting capacity.
• Permission to reuse. Only a 6:1 multi-trip bag under an inspection routine should see a second fill.

Handling that eats into the margin

The test rig lifts a sample cleanly and vertically, by all of its loops at once. A warehouse rarely treats a bag that gently. Every shortcut below loads the fabric in a way the type test never measured, so the real headroom on your dock is thinner than the ratio suggests.

Keep people out from underneath a raised bag, whatever its rating or age. A multiplier protects the product and the equipment from ordinary shocks; it is no reason to walk beneath a suspended ton of material.

• Picking a four-loop bag by two loops, which puts double duty on each strap.
• Square-edged or burred fork tines that cut into a loop under tension.
• Twisted or crossed loops on the hook, so the webbing pulls at an angle.
• Dragging a full bag across concrete instead of lifting it clear.
• Abrupt starts, stops and sharp turns with a load swinging high on the mast.
• Stacking taller than the bag maker allows, or on a pallet with broken boards.

Putting the rating on the order

When you call in an order, give us the SWL and the safety factor exactly as your current label shows them, plus the top, the bottom and the ZIP of the fill line. We match the spec to that line, then plan bales per pallet and pallets per truck. If the old label is faded or missing, tell us the product and the fill weight and we'll work back to a rating that covers it.

One rule we hold to: a 5:1 bag is a single-trip bag. The maker's handling guide recommends single-trip bags not be re-used, so after it is emptied it goes to a recycler, not round again.