Horizontal or vertical — decide before you read any chart
A sling angle is a single number that can be written two ways, and the two ways are complements of each other. Sixty degrees from horizontal is thirty degrees from vertical, and it is also a 60 degree included angle between two opposite legs. Three descriptions, one geometry, and a chart that uses one of them will give a badly wrong answer if you feed it another.
This page measures from horizontal throughout, and says so on the field label, in the results and in the table. It also prints the same angle from vertical and the included angle at the hook, so that whatever your tag or chart uses, you can line them up before you trust anything.
Where the numbers on the defaults come from
A 2,400 lb load with 60 lb of slings and shackles hanging under the hook is 2,460 lb on the hook. On two legs at 60 degrees from horizontal, the vertical share is 1,230 lb per leg, the load angle factor is 1.155, and the tension in each leg is 1,420 lb. Each pick point also gets pulled sideways by 710 lb, toward the other one.
Flatten those same two legs to 30 degrees and the factor doubles to 2.000. Each leg now carries 2,460 lb — the entire weight on the hook, in each of the two legs — and the sideways pull at each pick point goes to 2,130 lb, three times what it was. Nothing was added to the load.
The sideways force does not go into the sling
The leg tension is the number everyone looks up. The horizontal component is the number that quietly breaks things, because it is not carried by the rigging at all — it is carried by the load. Two lugs on top of a skid, pulled toward each other by 710 lb each, are being bent rather than pulled, and a lug that was drawn for a vertical pull is not the same lug in that condition.
This is why spreader beams exist. A spreader takes the horizontal component into itself as compression and lets the legs come down to the pick points vertically. It also adds its own weight below the hook, which is why there is a field for that.
What "legs that carry" actually means
A four leg bridle on a rigid load is a statically indeterminate problem: four legs, three equations, and the answer depends on which leg is a quarter inch shorter than the others. It is common practice to assume fewer legs carry than are fitted, and the number to assume is a judgement about the load and the sling set, not a constant. The field is there because pretending four legs always share four ways is the single most optimistic thing a rigging calculation can do.
What this page will not tell you
It will not tell you whether your sling is up to the tension it just calculated. The working load limit lives on the tag, it depends on the hitch, the angle, the fitting and the condition of the sling, and comparing a computed force against a rated one is the part of the job that belongs to a qualified person looking at the actual gear. This page stops at the force.
Questions people ask
Is the sling angle measured from the horizontal or from the vertical?
This page measures from horizontal: flat along the top of the load is 0 degrees, straight up is 90. It prints the same angle from vertical and the included angle at the hook alongside the result, because tags and charts use all three and mixing them up flips the answer. Sixty degrees from horizontal is the same geometry as thirty from vertical.
What is the load angle factor?
One divided by the sine of the angle from horizontal. Multiply a leg's vertical share by it and you get the tension in that leg. At 90 degrees it is 1.000, at 60 degrees 1.155, at 45 degrees 1.414, at 30 degrees 2.000. Below 30 it climbs fast, which is why a small error in a shallow angle moves the answer more than the same error in a steep one.
Why is each leg carrying the whole load at 30 degrees?
Because the vertical share on two legs is half the load, and the factor at 30 degrees is 2.000. Half times two is the whole thing. With the page defaults that is 2,460 lb in each of two legs holding a 2,460 lb total — twice what the same two legs carry hanging vertically.
What is the horizontal force at the pick points for?
It is the sideways part of the leg tension, and it is pushed into the load rather than into the sling. Two pick points get squeezed toward each other by it. Lugs, eye bolts and welded ears are much weaker sideways than they are straight down, so it is worth knowing the number even though it never shows up on a sling tag. A spreader beam exists to take that force instead of the load.
Should I assume all four legs of a bridle carry?
That is the judgement the page hands back to you rather than making for you. Four legs on a rigid load is an indeterminate problem, and small differences in leg length decide which ones take the weight. Assuming fewer legs carry than are fitted is the conservative direction, and how many is a decision for the person who can see the load and the sling set.
Can this tell me if my sling is strong enough?
No, and it is built not to. It gives you a force. The working load limit is stamped on the tag of the sling you are actually holding, it changes with the hitch and the angle and the condition of the sling, and putting the two numbers side by side is work for a qualified person with the gear in front of them.