Knot Breaking Strength & Safe Working Load Calculator
Enter your rope breaking strength, select a knot, and instantly see how much strength is lost and what safe working load you have at different safety factors. Nothing uploaded.
Knot strength
Safe working loads
Higher-efficiency alternatives
| Knot | Efficiency | Resulting strength |
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Learn more: knot efficiency and safe working load
Every knot reduces rope strength - by how much matters for safety
A 20kN rated rope with a bowline at 70% efficiency has a system breaking strength of 14kN. Divide by your safety factor (10:1 for life safety) and your safe working load is 1.4kN - approximately 143kg. For a tree surgeon rigging a limb, or a climber building an anchor, these numbers matter. Getting them wrong is a life safety issue. The calculator flags knots inappropriate for certain applications - a reef knot has only 45% efficiency and is not a load-bearing joining knot.
How to calculate safe working load in three steps
First, know your rope breaking strength in kN or lbf - this is on the rope tag or in the manufacturer data sheet. Second, select your knot from 16 options with published efficiency percentages for climbing, sailing, rigging, and arboriculture. Third, multiply the rope's breaking strength by the knot's efficiency percentage to get the resulting breaking strength, then divide by your safety factor (5:1 for general overhead lifting, 10:1 for life safety, 15:1 for arboriculture) to get your safe working load - the maximum load you should place on the knot in normal use.
Understanding knot efficiency and bend radius - why some knots are stronger than others
Knot efficiency is expressed as a percentage of the rope's full breaking strength retained when the knot is tied. A figure-eight retains 80% because the knot's internal bend radius is gradual. A bowline retains only 70% because it creates a sharper bend in the load-bearing strand. The tighter the bend radius, the greater the stress concentration at the knot, and the greater the strength reduction. A reef knot (used only for joining two ropes of equal diameter) loses 55%, making it unsuitable for critical load-bearing applications.
FAQ
What if I need a higher efficiency knot?
The calculator shows the top 3 alternative knots with higher efficiency than your selection. If your current knot is too weak for your intended load, the alternatives suggest stronger knots suitable for the same application.
Does rope material affect the actual strength of a knot?
Material selection does not change the knot's efficiency percentage, but it affects real-world strength degradation. Nylon and polyester absorb water and lose strength when wet; Dyneema and UHMWPE resist water absorption; manila loses more strength when wet than synthetics.
Why use a 15:1 safety factor for arboriculture?
Tree work is dynamic - shock loads from sudden branch failure can exceed static load calculations. A 15:1 factor accounts for impact and uncertainty in field conditions that climbing and rigging may not face.