SWL and safety factor
Two numbers decide whether your bag holds or fails. Here is what they mean, and how to read them on a spec sheet.
What is SWL?
SWL (Safe Working Load) is the load the bag is engineered to carry in normal service, in pounds or kilograms. It is the only number that matters when you fill: exceeding SWL voids every warranty and puts your operators at risk.
Safety factor: 5:1 or 6:1
The safety factor (SF) is the ratio between the tested failure load and the SWL. A 5:1 bag rated at 2,200 lb is proof-tested to 11,000 lb. That is not margin to spend: it is the reserve that absorbs lifting shocks, wear and imperfect filling.
5:1 — single trip
The industry standard for single-trip bags. The bag ships to your customer with the product and is not intended to come back. The vast majority of FIBCs sold in North America.
6:1 — multi-trip
Reinforced construction for bags reused several times. Requires a documented inspection program between each cycle: seams, loops, fabric, cleanliness.
How to size your requirement
Start from your product density and the bag volume. A 27 ft³ bag (36 x 36 x 36 inches) filled with a 50 lb/ft³ product holds roughly 1,350 lb, so you need an SWL of at least 1,350 lb plus margin for density variation. Our volume calculator handles the first half of that math.
The three mistakes we see most often
Confusing SWL with break load
Break load is never a working load. It exists only to validate the safety factor in the lab.
Filling “a little more”
A repeated 10 percent overfill wears the seams and turns your safety reserve into a daily risk.
Reusing a 5:1 bag
A single-trip bag that gets reused carries no warranty. If you reuse, you need 6:1 and an inspection program.
Ignoring the lift
A sudden or single-point lift creates peak loads far above the static weight of the product.
Question about your application?
Our specialists have been sizing FIBCs for three generations. Tell us your product and your constraints.