Strapping should be selected based on the load requirements, not just by looking at the cargo and choosing a wider strap. Heavy cargo may need stronger strapping, but the correct size depends on several factors, including the cargo weight, the number of straps used, and the strength of the complete strapping system.
Another common mistake is to treat break strength as the amount of force the strap can safely handle during normal use. Break strength only tells you when the strap will fail.
In this guide, we explain the main strength numbers, show the basic calculation with a practical example, and cover the two common mistakes to avoid when choosing strapping for a load.
Understanding Break Strength vs. System Strength
Three numbers get used interchangeably. They shouldn’t be.
- Break strength — the force a single length of strap withstands before it snaps. Measured in kg or daN. This is the marketing number.
- System strength — the force needed to break the whole assembly: a loop of strap plus the buckle. This is the real number.
- Working load limit (WLL) — what you’re allowed to actually apply, after the safety factor. This is the number you design to.
The gap between the first and the second is called joint efficiency, and it’s where the strength goes. A buckle grips the strap by clamping and friction, and the strap always fails at or near the joint before it fails mid-span.
The formula the industry uses:
System Strength = (Break Strength × 2) × Joint Efficiency
You double the break strength because a closed loop has two legs sharing the force. Then you knock off the joint loss. With a good wire buckle and correct threading you keep most of it. With the wrong buckle-to-strap pairing you can lose a third — and you will never know, because it looks identical.
That’s the single most important sentence in this article. Strap and buckle are tested as a pair. Mixing a 32 mm strap with a buckle sized for a different profile doesn’t give you 32 mm performance. It gives you whatever the joint gives you.
What Is a Safety Factor?
Safety factor exists because the real world isn’t a test bench. UV exposure, edge abrasion, knot points, temperature, and the difference between a static pull in a lab and a shock load at 3 a.m. on a wet highway.
For cargo securing with strapping, design to a factor of 2 to 3 against system strength. Lifting is a different discipline with much higher factors — never borrow a strapping figure for a lifting application, and never the reverse.
So: Working Load Limit ≈ System Strength ÷ 2.5
How to Calculate Strapping Requirements
Here’s the whole thing.
Step 1 — Get the true gross weight. Cargo plus pallet plus packaging. Weighed, not estimated.
Step 2 — Pick your acceleration coefficient. From the CTU Code, based on the worst leg of the journey:
- Road: 0.8 g forward, 0.5 g sideways
- Rail (combined): 0.5 g, up to 1.0 g in abnormal conditions
- Sea, Area A (waves ≤ 8 m): 0.5 g transverse
- Sea, Area C (waves > 12 m): 0.8 g transverse
Step 3 — Subtract the friction you already have. Friction is free securing force. Steel on bare wood gives roughly 0.2–0.3. Rubber anti-slip mat gives about 0.6. The force your straps must actually resist is:
Required force = (acceleration coefficient − friction factor) × load weight
Step 4 — Divide by the number of straps, then apply the safety factor.
Required system strength per strap = (Required force ÷ number of straps) × safety factor
Example Calculation for Strapping
A 2,000 kg machinery skid. Sea freight, Area A, plus a road leg to the port. Worst case is the road leg at 0.8 g forward. Steel skid on wooden container floor, no mats: friction ≈ 0.25.
- Required force = (0.8 − 0.25) × 2,000 = 1,100 kg
- Using 4 straps: 1,100 ÷ 4 = 275 kg per strap
- Safety factor 2.5: 275 × 2.5 = 687.5 kg system strength required per strap
Look that up: 16 mm cord strap sits at 650–700 kg. Cutting it fine. Move to 19 mm at 900–1,100 kg and you have real margin.
Now add anti-slip mats and rerun it. Friction goes to 0.6:
- Required force = (0.8 − 0.6) × 2,000 = 400 kg
- Per strap over 4 straps: 100 kg → × 2.5 = 250 kg system strength
13 mm strap now covers it. Same cargo, same route, less than a third of the strapping requirement — because of a rubber mat. This is the most underused lever in cargo securing and it costs almost nothing.
Strapping Strength Reference Table
| Width | System strength | Typical application |
| 13 mm | 485–500 kg | Light palletised goods |
| 16 mm | 650–700 kg | Medium industrial bundling |
| 19 mm | 900–1,100 kg | Construction materials, machinery |
| 25 mm | 1,400–1,650 kg | Heavy equipment, steel coils |
| 32 mm | 2,000–2,300+ kg | Extreme heavy-duty, container lashing |
Use system strength for design work. If a supplier quotes you only break strength, ask which buckle it was tested with. If they can’t answer, that tells you something.
Should You Use More Straps or a Wider Strap?
Both work mathematically. They don’t work equally in practice.
Go wider when:
- The load has hard edges that concentrate stress
- You want fewer application points and faster packing
- The load is a single rigid mass — machinery, a coil, a casting
Go with more straps when:
- The load is long and could hinge or sag in the middle — pipe, timber, profiles
- You need to distribute pressure across soft or crushable packaging
- Access is limited and a wide strap won’t route cleanly
One general rule from the floor: for bundles longer than about 2 metres, never rely on two straps. Three minimum, and space them so no unsupported overhang exceeds a quarter of the total length. Long loads don’t fail at the strap — they fail between the straps.
Common Strapping Mistakes
- Designing to break strength instead of system strength. Straight 30–40% overestimate. Most common error there is.
- Mixing strap and buckle brands or profiles. Untested pairing, unknown joint efficiency.
- Ignoring the road leg. 0.8 g on a highway beats most sea conditions longitudinally.
- Assuming friction you don’t have. Painted steel on a wet, oily container floor is not 0.3.
- Using nominal cargo weight. The drawing weight, minus what was actually machined off, plus a crate nobody weighed.
- Over-tensioning to compensate. If the strap is undersized, tension won’t fix it — it just moves the failure earlier.
Frequently Asked Questions
What’s the difference between break strength and system strength again? Break strength is one length of strap on a test bench. System strength is a closed loop with a buckle — the way you actually use it. System strength = break strength × 2 × joint efficiency. Design to system strength, always.
Which safety factor should I use for export cargo? 2 to 3 against system strength for general securing, and use 3 if the cargo is high-value, the route is rough, or the surface is slippery. Some buyers and P&I clubs specify their own minimum — check the contract before you calculate.
Do I need to redo the calculation if the route changes? If the transport mode or sea area changes, yes. A load calculated for Area A coastal shipping is under-secured for an Area C ocean crossing at 0.8 g transverse. Same cargo, different number.
How do I account for stacked loads? Calculate each tier’s mass against the surface it sits on, not the total. The top tier’s friction is against the tier below it — often a much slipperier pair than the container floor. Stacked loads are where friction assumptions go wrong most often.
Can I just use the biggest strap available and skip the maths? You can, and plenty do. It costs more per pallet, it doesn’t help if the buckle is mismatched, and it doesn’t produce the documentation a surveyor asks for. Do the calculation once per cargo configuration and reuse it.
Why You Should Calculate Strapping Strength
Weight, coefficient, friction, straps, factor. Five inputs, one answer, and then you never guess again for that cargo type. Do it once for each configuration you ship, write it on the packing instruction, and hand it to the person who actually does the strapping.
Send us your load weight, route and surface pair — we’ll run the calculation and tell you the width.
Amass Strapping Solutions manufactures composite cord strapping, container lashing strips and matched wire buckles from Rajkot, Gujarat. Our strap and buckle systems are supplied and rated as pairs, from 13 mm to 32 mm, so the system strength you calculate is the system strength you get.
Request a sample roll and a free strapping calculation for your load → amass-strap.com