Working load limit against breaking strength
A ratchet strap label typically carries two numbers. The breaking strength is roughly where the assembly is expected to come apart in a test. The working load limit is a fraction of that, assigned by the manufacturer, and it is the only one that has anything to do with securing a load. For synthetic webbing the relationship is often around one to three, so a strap with a 3,300 lb breaking strength is commonly marked at 1,100 lb WLL.
The consequence of mixing them up is direct. Someone reading the larger number off four straps concludes they have 13,200 lb of securement when they have 4,400. That is not a conservative error in the safe direction — it is a threefold overestimate of the only figure that matters, made by a person who is about to drive onto a highway.
The gap between the two numbers is not padding to be spent. It absorbs shock loading from potholes and hard braking, the reduction that comes from the angle the strap actually pulls at, wear that has already happened to the webbing, and the fact that a load in motion generates forces well above its static weight. It is doing work.
Where the aggregate figure comes from
The common starting figure is that the tie-downs on a load should have an aggregate working load limit of at least half the weight of the load. That is the number this page defaults to, and it is a reasonable place to begin thinking. It is not a rule stated here, because the rules that actually govern securement are federal for commercial carriers, differ by state for other use, differ again by commodity, and change. What counts as a tie-down, how much of a given tie-down counts toward the aggregate, how many are required by article length and weight, and what a particular commodity requires are all questions with real answers in a written rule, and the rule is short enough to read. If you haul for a living, for an employer, or for hire, that reading is not optional and your employer will have a position on it.
The half-of-each-tie-down option in the selector exists because how much of a tie-down counts depends on how it is attached — whether it runs from an anchor point over the load and back to another anchor, whether it attaches directly to the article, and whether the article is prevented from moving forward by something else. Rather than pick a rule for you, this calculator asks. Default to the conservative reading if you are not sure, and then go and find out which applies.
The things that break securement that have nothing to do with the count
| Problem | Why the number does not catch it |
|---|---|
| Webbing over a sharp edge | A strap cut halfway through has no known rating. Corner protectors exist for this. |
| UV-degraded webbing | Faded, stiff, fuzzy webbing has lost strength that cannot be assessed by looking |
| Knots in a strap | A knot concentrates stress and takes out a substantial and unpredictable share of the rating |
| A shock-loaded assembly | Once something has been jerked hard, its history is unknown. Retire it. |
| Anchor points | The strap rating is irrelevant if it is hooked to a stake pocket, a rail or a rusted D-ring that is weaker |
| A load that can slide | Aggregate WLL assumes the tie-downs are resisting movement, not arresting an already-moving mass |
| Chain and hooks | Grade matters, the hook and the binder have their own ratings, and a stretched or gouged link is finished |
Every one of these is a real failure that has happened repeatedly, and none of them shows up in a division. The number tells you whether the plan is in the right order of magnitude. Inspection before every trip is what tells you whether the equipment can deliver it.
Before you drive
Load shift at speed is not something a driver recovers from, and the consequence is not confined to your own vehicle. A load that leaves a trailer on a highway becomes an obstacle at 65 mph for people who had no part in how it was secured. That asymmetry is the whole reason to be conservative here: the cost of an extra strap is a few dollars and two minutes, and the cost of being wrong is borne by strangers.
Stop and re-check the tie-downs after the first few miles, and again at every stop. Loads settle, straps that felt tight take up slack, and ratchets back off. This is also the moment to check tire pressures and hub temperatures, which the trailer axle load calculator gives you the context for. If the load itself is still being positioned, the tongue weight and placement calculator should come first, because securement is easier to plan once you know where things are going to sit. For a long haul, road trip planning covers the stops, and the maintenance schedule guide covers what should have been checked before you left.
Questions people ask
What is the difference between working load limit and breaking strength?
Breaking strength is the force at which the assembly is expected to fail in a controlled test. Working load limit is the manufacturer-assigned maximum force it should be subjected to in use, and it is a fraction of the breaking strength — often around a third for synthetic webbing, though the factor differs by material and by product. Only the working load limit is used for securement arithmetic. The margin between the two is consumed by dynamic loading during braking and over bumps, by the geometry of how the strap actually pulls, by wear the strap has already suffered, and by the ordinary variation between one manufactured item and the next. Treating breaking strength as usable capacity removes all of that at once. If a tie-down is not marked with a working load limit by its manufacturer, it does not have one you can rely on and it should not be counted.
Does a strap that runs over the load and back count once or twice?
It depends on how it is attached and which rule governs your load, which is exactly why this calculator asks rather than deciding. A tie-down that runs from an anchor on one side, over the article, and down to an anchor on the other side is doing something different from one that attaches directly to the article, and the written rules treat those cases differently. The conservative reading — counting half of the marked working load limit per tie-down — will never leave you short, and it is the default here for that reason. The correct answer for your situation is in the rule that applies to you: federal motor carrier regulations if you are hauling commercially, and your state DMV otherwise. Do not take it from a forum, and if you haul for an employer, ask the employer, because they are responsible for it too.
Are four straps enough for a car on a trailer?
Vehicle transport is one of the categories with its own specific requirements, and a general weight-fraction calculation is not the right tool for it. The considerations include where on the vehicle the tie-downs may attach, whether the suspension is compressed, whether the wheels are secured rather than the body, and what happens as the suspension moves during the trip. Those are covered explicitly in the securement rules for vehicles, and they are more specific than anything a percentage of weight will give you. Use this page to get a sense of the scale of the forces, then go and read what applies to vehicles, and if you are hauling cars regularly get the method shown to you by somebody who does it properly.
My strap has no label left on it. Can I estimate its rating?
No. The label is the rating, and a strap without one is a strap with an unknown working load limit and an unknown history. Webbing degrades from ultraviolet exposure, chemicals, abrasion and heat in ways that are not reliably visible, and a strap old enough to have lost its label has been exposed to all of them. The same reasoning applies to chain without a tag, since grade cannot be judged by eye and grade is most of the rating. Estimating from the width of the webbing or from what a similar strap is rated at is guessing about the one number in the whole exercise that has to be right. Retire it and buy a marked replacement; the cost is trivial against what it is holding.
Do I need to stop and check the straps during the trip?
Yes, and the first check should be early rather than at the end. Loads settle in the first few miles as vibration works everything into place, webbing that was tensioned over an irregular surface finds a new path, and ratchets can give up a little tension without anyone touching them. A strap that was correct in the driveway may be loose ten miles later, and a loose strap is doing nothing at all. Build the check into fuel and rest stops, and use the same stop to put a hand on the trailer hubs and check that nothing has shifted position. There are also required inspection intervals for commercial hauling that are more specific than this, and those are in the federal rules rather than here.