5:1 vs 6:1 Safety Factor: Which FIBC Do You Need?

A 5:1 safety factor is the normal choice for a single-trip FIBC, and a 6:1 safety factor is the choice when the bag will be lifted more than once. The ratio is the test load divided by the rated safe working load, so a 1,000 kg bag at 5:1 is proven at 5,000 kg in a top-lift test, while the same bag at 6:1 is proven at 6,000 kg and also has to survive the cyclic and stacking sequence expected of a reusable bag. If the bag is filled, moved and emptied once, 5:1 is the correct specification. If it is emptied, folded, stored and lifted again, 6:1 is.

What the ratio actually measures

The safety factor is a design type, not a durability score. It fixes the proof load for the top-lift test and, for 6:1, adds a cyclic test and a stacking test that a single-trip bag is not required to pass. It says nothing about how many trips a specific bag will survive, because that depends on the load, the slinging method, the filling equipment and where the bag is stored between trips. Two bags can both be marked 6:1 and behave very differently after ten cycles if one is lifted by a forklift under the loops and the other is dragged by one loop off a wet yard.

How the two bags differ on the line

• Fabric: a 6:1 bag usually moves up one weight step, typically 180 to 220 gsm against 140 to 170 gsm for a comparable single-trip bag.
• Seams: 6:1 work is run with a double-lock or double-row seam pattern on the body and base rather than a single chain stitch.
• Loops: wider webbing, usually 70 to 100 mm, with a reinforced loop-to-body stitch pattern and extra rows at the loop root.
• Base: a full-corner or star base reinforcement where the bag will be dropped or set down repeatedly.

Those changes add material cost and weight, which is why specifying 6:1 on a bag that will never be reused is a real cost with no benefit.

When the market or the customer decides for you

On export orders the choice is often not yours to make. Receiving markets that treat a returned or reused bag as a lifting accessory normally require the 6:1 design type and its marking, and many distributors write the design type into their purchase specification regardless of how the bag will be used. If your specification is silent, ask the receiving site whether bags come back; a reused bag entered as 5:1 creates a liability that the bag cannot carry.

QINFENG’s first-hand experience

We build both design types on the same lines, so the difference shows up in our own records rather than in theory. When a 6:1 bag is cut open after several trips, the wear is almost always in the same two places: the loop attachment where the webbing is stitched into the body, and the base seam where the load concentrates during discharge. That is why reusable work here is quoted with a heavier loop root and a reinforced base by default, and why we keep the proof-load result for the batch against the bag serial. On single-trip work the opposite holds: over-building the loop root makes the bag stiffer to handle on an automatic filling line without improving anything the customer will ever see.

Where the safety factor stops protecting you

The ratio assumes the bag is lifted as designed. It does not cover lifting by one loop, lifting a partly filled bag, dropping a filled bag from height, cutting the fabric with a forklift tine, or long-term UV exposure on an outdoor store. It also does not tell you the safe working load of the bag you already have in the yard; that number is on the bag label and must be read there. Where a site has its own lifting rules or a regulated reuse regime, those rules govern and have to come from your written specification. Third-party testing is arranged on request and quoted separately.