CALCULATOR

Knot Strength Calculator

Apply a knot's typical strength retention to a line's breaking strength, so the weak point of the system is the number you actually plan with.

What this gives you

Inputs: Line breaking strength, knot type, load direction, line condition. Outputs: Strength at the knot, retention, safe working load, suggested knots. The knot is the weak point in almost every system. A line rated at 3,600 kg with a bowline in it is a 2,340 kg line — and that is before any allowance for age or wear.

Line breaking strength (kg)

Knot or termination

Tucks (splices only)

Load direction

Line condition

Strength at the knot

—

Retention—

Safe working load—

Strength lost—

Stronger options—

strength_at_knot = MBL × retention × condition × direction
SWL = strength_at_knot ÷ 5
splice retention scales with tuck count

∞ RELATED

Put the number to work

Free tool

Rope Strength Calculator

Fibre→Breaking strength

Breaking strength, working load and stretch for a given fibre, diameter and duty.

Calculator

Free tool

Rigging Load Calculator

Load weight→Tension per leg

Sling tension against angle — and why a wide angle multiplies the load so fast.

Calculator

INTERACTIVE

3D Studio

See the same numbers as geometry — a rope under load, a splice worked tuck by tuck, force vectors on a rig.

3D Studio

These calculators use simplified models and typical catalogue values. They are for planning and teaching, not for certifying a lift. Real capacity depends on the specific product, its condition, the load path and the environment. Where a manufacturer’s rating or a regulation gives a different figure, that figure governs.