Anchor Angle Force Multiplier Calculator
Why 120 degrees is the critical angle — the force on each anchor leg, drawn to scale as you change it.
This shows the physics. It does not teach anchor building. Anchors are learned in person from someone qualified, on the ground, with feedback — never from a web page. Use this to understand why the angle matters, not as a substitute for instruction.
Why 120 degrees is the number everyone quotes
The force on each leg is the load divided by twice the cosine of half the angle. That cosine is what does the damage: as the angle opens, it shrinks, and the division blows up. At 120 degrees the cosine of 60 is exactly one half, the twos cancel, and each leg is carrying the entire load.
That is the point worth internalising: at 120 degrees your two-piece anchor has stopped sharing anything. Both pieces are taking what one piece would have taken alone, so you have doubled the gear and gained nothing but redundancy. Past 120 it gets actively worse — each piece is now loaded harder than a single piece would have been.
The fix is nearly always the same and nearly always cheap: lengthen the legs. Extending the anchor downward narrows the angle for the cost of a metre of sling. It is also why the widest-spaced pieces are the ones that need the longest extension, and why a cordelette tied short is a common way to build a bad anchor out of good gear.
Load on each leg by angle
| Angle | Each leg | |
|---|---|---|
| 0° | 50% | Perfect sharing — legs parallel |
| 20° | 51% | |
| 30° | 52% | |
| 45° | 54% | |
| 60° | 58% | The practical target |
| 75° | 63% | |
| 90° | 71% | |
| 100° | 78% | |
| 110° | 87% | |
| 120° | 100% | Critical angle — each leg takes the whole load |
| 130° | 118% | |
| 140° | 146% | |
| 150° | 193% | |
| 160° | 288% | |
| 170° | 574% | Nearly six times the load on each piece |
The American Death Triangle
The notorious version of this mistake: a sling threaded through two anchor points and clipped so the legs form a triangle with the load. It looks tidy and it does two bad things at once — the angle at the load is usually very wide, and the horizontal leg pulls the two pieces toward each other, adding an outward force neither was placed to resist. It earned its name honestly, and it is the standard illustration of why the angle is not a detail.
How to use
- Set the angle between the two anchor legs and the load you expect.
- Read the force on each leg — and watch what happens as the angle opens past 90 degrees.
- Use the table to see the whole curve at a glance.
Frequently asked questions
Why is 120 degrees the critical angle?
Because the force on each leg is the load divided by twice the cosine of half the angle. At 120 degrees that cosine is exactly one half, the twos cancel, and each leg carries the entire load — so a two-piece anchor has stopped sharing anything at all.
What happens past 120 degrees?
It gets actively worse. Each piece is loaded harder than a single piece would have been, so adding the second piece has made the anchor weaker rather than stronger. At 170 degrees each leg carries nearly six times the load.
How do I fix a wide angle?
Lengthen the legs. Extending the anchor downward narrows the angle for the price of a metre of sling, which is why widely spaced pieces need the longest extensions and why a cordelette tied short is a common way to build a bad anchor from good gear.
What is the American Death Triangle?
A sling threaded through two points and clipped so the legs form a triangle with the load. It does two bad things at once — a very wide angle, and a horizontal leg that pulls the pieces toward each other in a direction neither was placed to resist. It earned the name honestly.
Can I learn anchors from this?
No, and that is not a formality. Anchors are learned in person from someone qualified, on the ground, with feedback. This page shows why the angle matters; it does not teach you to build one.
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