FIBC Safety Factor: What 5:1 and 6:1 Actually Prove

The FIBC safety factor is the most quoted number on a bulk bag and the least understood. It is not a strength rating. It is a ratio between a load one bag out of a production run was tested at, once, in a laboratory, and the load you are allowed to fill every bag with in service.

Get that distinction wrong and two things happen. Buyers ask for 6:1 on jobs that never needed it and pay for it. And nobody checks the number that actually decides whether a pallet of bags survives your warehouse — which is not the safety factor at all.

The four numbers that share one bulk bag label

Every field on the label answers a different question, and only some of them are backed by a test you can demand evidence for.

Label field	What it actually states	Where it comes from	Testable claim?
Safe working load (SWL), in kg	The maximum load the bag is rated to carry in service	Declared by the manufacturer against a type test	Yes, via the certificate of construction
Safety factor, e.g. 5:1, 6:1 or 8:1	The multiple of SWL the design was type-tested at	Type-test convention under ISO 21898	Yes, but it describes the design, not your bag
FIBC type: single-trip, standard-duty reusable, heavy-duty reusable	How many trips the design is qualified for	ISO 21898 classification	Yes
UN code, e.g. 13H followed by a category numeral	Qualification for dangerous goods	49 CFR 178.702 and the UN Model Regulations	Yes, and it is the only field with a regulator behind the multiple
Electrostatic type A, B, C or D	Static behaviour in the filling and discharge environment	IEC 61340-4-4	Yes
Certificate of construction number, month and year	Which type test your bag traces back to	ISO 21898 marking clause	This is the field to ask for

Notice the pattern. The safety factor is a property of a design type, and the certificate of construction number is what connects the bag on your pallet to that design. A label that carries a safety factor but no certificate number is telling you a ratio with nothing behind it.

What the FIBC safety factor is, precisely

An FIBC safety factor is the ratio between the load at which a bag design was type-tested and the safe working load the bag is rated to carry in service. A 6:1 bag is a design proven at six times its SWL in a laboratory. It is not a bag that can lift six times its rating on your dock, and it is not a bag that has itself been tested at all — type testing is done on samples from the design, not on every unit.

That is why the safety factor answers "how much margin did the designer prove" and never "how much can this bag hold." The second question is answered by the SWL, full stop.

The ratio also tracks re-use rather than raw strength. ISO 21898 classifies FIBCs for non-dangerous goods by how many trips they are qualified for — single-trip, standard-duty reusable, and heavy-duty reusable — and the safety factor a buyer specifies is usually a proxy for which of those three they want. The trade's own marking guidance lists the field as "Safety factor, e.g. 5:1, 6:1 or 8:1," which tells you plainly that these are conventional steps, not a continuum you negotiate.

The one multiple that is written into regulation

For non-dangerous goods, the 5:1 and 6:1 split is a standards-and-commerce convention. For dangerous goods it stops being a convention and becomes a design-type test in law.

US hazardous materials regulations set the top lift test for flexible IBCs directly: "Flexible IBC design types must be filled to six times the maximum net mass, the load being evenly distributed." The bag is then lifted in the manner for which it is designed until clear of the floor and held there for five minutes. To pass, there must be "no permanent deformation which renders the IBC, including the base pallets when applicable, unsafe for transportation, and no loss of contents."

So when a UN-marked bag says 6:1, there is a federal test method behind the six. When a non-UN bag says 6:1, there is a standard and a test house behind it — which is worth having, but it is a different kind of claim. If a buyer's specification says "6:1" and means "UN certified," those are not interchangeable, and the fix is to write the UN code on the requirement rather than the ratio.

The code itself is built to a pattern rather than being a product name. In the IBC code designation, the numeral 13 marks a flexible IBC "for solids, discharged by gravity," the letter H means plastic, and a trailing numeral identifies the category of construction where an individual section specifies one. If you need the exact category for a shipment, take it from the certificate of construction and the test house report, not from a catalogue heading.

The number nobody quotes: 1.8

Here is the multiple that decides whether your bags survive storage, and almost nobody puts it in a specification.

The stacking test for IBCs applies a superimposed load of "1.8 times the combined maximum permissible gross mass of the number of similar IBCs that may be stacked on top." A flexible IBC under test is "filled to not less than 95 percent of its capacity and to its maximum net mass," the load is applied for at least five minutes, and for flexible IBCs "there may be no deterioration, which renders the IBC unsafe for transportation, and no loss of contents."

Read that against a 6:1 top-lift claim and the asymmetry is stark. A 6:1 rating says something about lifting a single bag by its loops. It says nothing about how many bags may sit on top of each other in a container for three weeks. Those are separate tests with separate multiples, and stacking is where most real-world deformation and split-seam claims come from.

The practical move: state the permitted stack height on the label and on the product image, and state it as a number of bags at a stated SWL, not as "stackable." A buyer who reads "stackable" will stack four high.

The edition on your label is probably out of date

ISO 21898 is the international standard for FIBCs intended for non-dangerous goods, covering materials, construction and design, type testing and marking. Its current edition is ISO 21898:2024, published on 16 August 2024, and it replaced the 2004 edition.

Widely reproduced marking checklists — including the ones most supplier documents were copied from — still cite EN ISO 21898:2005, the European adoption of that superseded 2004 text. A label or certificate that references the 2005 or 2004 documents is not automatically non-compliant, but it is a signal worth chasing during vendor qualification: ask which edition the type test was run under and when the certificate of construction was issued. For anyone maintaining spec sheets, this is the cheapest correction available this year.

What the marking has to carry

The trade's own marking guidance, drawn from the ISO 21898 marking clause, expects a durable label or print carrying all of the following. Use it as a receiving check.

Name and address of the supplier or reconditioning company
Type of construction, where it applies to one particular FIBC type
Name and address of the distributor, if there is one
Safe working load (SWL) in kg
Safety factor, stated as a ratio
Indication of the relevant norm, with its edition year
Type of FIBC, for example heavy-duty reusable or standard-duty reusable
Number of the certificate of construction, plus month and year of issue
Name of the test house
Date of production
Pictograph for the recommended handling method
Any special handling indications required by the standard
Where the bag is certified for a specific product, the description of that product

If you supply bulk bags and your label carries six of these thirteen, that gap is what turns a routine pre-shipment inspection into a held container. Reference sizes and typical SWL bands per format are collected on the standard bulk bag dimensions page; the label content above is the part that travels with the goods.

Three claims to stop making

"FDA-approved bulk bag." No such product exists. The US Food and Drug Administration does not issue approvals or certifications of FIBCs or FIBC liners. What is actually regulated is the resin: the FDA food-contact regulation for polypropylene requires it to be 100 percent virgin material. In the EU, food-contact plastics are governed by Regulation 10/2011. Say that instead — it is both true and more useful to a buyer's quality team.

"It's 6:1, so it's reusable." Reuse is a system question, not a ratio. Guidance for the sector is that FIBCs should only be reused within a closed-loop system, where the bag is cleaned, reconditioned and requalified to handle the same product. A 6:1 design type used outside such a loop is a single-trip bag with extra margin, not a multi-trip bag.

"Safety factor is the strength." Load ratings on lifting products are exactly where this confusion is most expensive, and the same trap shows up on slings and lifting accessories — the mechanics of ratio-versus-rated-load labelling are worked through in WLL vs MBL load rating labels.

Put the ratings where a buyer will actually read them

Two thirds of the questions a bulk-bag supplier answers by email are questions the product image could have answered. A buyer scanning a catalogue does not open the label PDF; they look at the photograph, and the photograph usually shows a white bag with no numbers on it.

The version that stops the email thread is a product image with the SWL, the safety factor and its edition-dated norm, the FIBC type, the fill and discharge spout dimensions, and the permitted stack height annotated directly onto the bag, each dimension drawn to the edge it is actually measured from. Tools that do this properly work from the measured geometry of the photo rather than from a generated approximation — the difference matters here, because an image tool that invents a plausible-looking dimension on a load-rated product is a liability, not a shortcut. If you sell across furniture, industrial and building-material catalogues, the pattern for laying these out consistently is set out on the industrial spec diagram page.

FAQ

What does a 5:1 safety factor mean on a bulk bag?

It means the bag's design was type-tested at five times its safe working load — so a design rated 1,000 kg SWL was proven against a 5,000 kg test load. It does not mean the individual bag in front of you has been tested, and it does not mean you may load above the SWL. The ratio describes the design's proven margin; the SWL is the number you fill to.

Is a 6:1 safety factor legally required for dangerous goods?

For flexible IBCs, US hazardous materials regulation requires the top lift design-type test to be run at six times the maximum net mass, held for five minutes, with no loss of contents and no permanent deformation that makes the container unsafe for transport. So a six-times multiple is built into the qualification route for dangerous goods, which is why UN-marked bags and 6:1 are so often conflated. The correct thing to specify is the UN code, not the ratio.

How high can I stack FIBCs?

Not a number you can derive from the safety factor. The stacking test applies 1.8 times the combined maximum permissible gross mass of the bags that may be stacked on top, on a bag filled to at least 95 percent of capacity, for at least five minutes. Ask the supplier for the stack height the type test qualified, stated as a number of bags at a given SWL, and put that figure on the label.

Which standard covers FIBCs for non-dangerous goods?

ISO 21898. The current edition is ISO 21898:2024, published 16 August 2024, replacing the 2004 edition; the European adoption of the older text, EN ISO 21898:2005, is still quoted on a great many labels and certificates.

What does the 13H code on a bulk bag mean?

In the IBC code system, 13 designates a flexible IBC for solids discharged by gravity and H designates plastic; a trailing numeral identifies the construction category where one is specified. The authoritative answer for a specific bag is on its certificate of construction and the test house report, not in a product listing.