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Anchor Load Calculator

Calculate force on each anchor point based on angle between strands. Based on UIAA geometry.

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For educational reference only. Assumes a perfectly equalised anchor. Real anchors have imperfect load distribution. This tool is not a substitute for qualified instruction.

Peak impact force on the system — use Fall Factor Calculator to estimate

→ Calculate with Fall Factor Calculator

Angle between the two strands at the masterpoint

F = (5.00 kN / 2) / cos(60° / 2) = 2.89 kN per anchor

RESULT

2.89

Low load

per bolt (kN)

×1.15

Load multiplier

per anchor vs total

5.0 kN

System total

unchanged at masterpoint

Angle reference (kN)

AngleMultiplierPer bolt at 5.0 kN
0°×1.002.50 kN
30°×1.042.59 kN
60° ←×1.152.89 kN
90°×1.413.54 kN
120°×2.005.00 kN
150°×3.869.66 kN
170°×11.4728.68 kN

Why angle matters

The 60° rule: keep the angle between anchor strands below 60°. At 60° each bolt receives only 15% more than half the load. At 120° each bolt receives the full impact force. At angles above 120° the force on each bolt exceeds total impact force.

Optimal anchor geometry

  • Equalise anchors so both strands are equal length
  • Aim for angle < 60° between strands
  • Longer slings reduce angle when bolts are far apart
  • In a Y-hang, angle is determined by bolt spacing and sling length
Also useful: Fall Factor Calculator → · Rope Retirement Calculator →

Based on UIAA anchor geometry · F = (F_impact / 2) / cos(θ / 2)

Anchor geometry explained

Why 60° is the critical threshold

The load multiplier — how much force each bolt receives relative to half the total load — increases slowly at low angles and explosively at high angles.

Key angles to remember

  • 0°: strands are parallel. Each bolt receives exactly 50% of impact force. Impossible in practice but the theoretical ideal.
  • 60°: each bolt receives 57.7% — only 15% more than at 0°. This is the standard maximum recommended by UIAA and most instruction.
  • 90°: each bolt receives 70.7% — the load multiplier is ×1.41. Already significantly higher than at 60°.
  • 120°: each bolt receives 100% of the impact force — equivalent to the full load on a single point. This is why 120° is considered the absolute limit.
  • Above 120°: force on each bolt exceeds the total impact force. The geometry is working against you.

The cosine formula is precise, but in field conditions you estimate angles visually. When in doubt, use longer slings to keep the angle safely below 60°.

Frequently asked questions

What angle should I aim for in a climbing anchor?

Aim for less than 60° between anchor strands. At 60° each anchor point receives only 15% more than half the impact force — an acceptable margin. Above 90° load increases rapidly, and at 120° each bolt receives the full impact force with no load-sharing benefit.

Why does force per bolt increase with a wider angle?

With a wider angle, the two strand tensions must point further apart from each other to produce an upward resultant force. This means each strand must be under higher tension individually to produce the same combined upward force. The relationship is governed by the cosine of half the angle — at 60° the cosine factor is 0.866, at 120° it is 0.5, meaning twice the tension in each strand.

Can a force exceed the input impact force on a bolt?

Yes — at angles above 120° the calculated force per bolt exceeds the total impact force. This seems counterintuitive but is mathematically correct. With very wide angles, each strand must be under enormous tension to produce a vertical resultant that matches the falling climber's weight. This is why angles above 120° are considered extremely dangerous and must be avoided.

Does this calculation account for anchor equalization?

The formula assumes a perfectly equalised anchor where both bolts receive equal load. In practice, anchors are never perfectly equalised — one bolt typically takes more load than the other. This makes the real-world case slightly better than the worst-case (non-equalised) scenario but potentially worse than the ideal. Always build redundant anchors and keep angles low.

What is the impact force I should use as input?

Use the peak impact force from a fall event. You can estimate this with the Fall Factor Calculator on this site, which takes into account rope type, fall length, and rope length. For most sport climbing falls with a modern single rope and good belay technique, impact forces are 5–8 kN. The UIAA test limit for single ropes is 12 kN.

How do I reduce anchor angle when bolts are far apart?

Use longer slings or a cordelette. The masterpoint sits lower with longer material, reducing the angle between strands. A long sling of 120–240 cm or a 6mm cordelette of 5–6 m can bring the angle below 60° even on bolt spacings of 1–1.5 m. Always confirm the reduced angle visually before loading the anchor.