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
Efficiency values are typical figures. Real knots lose more or less depending on rope material and construction, so use the maker's ratings for anything safety critical.
Higher-efficiency alternatives
| Knot | Efficiency | Resulting strength |
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Learn more: how much a knot weakens rope
The efficiency percentage
Rope is rated for its straight, unknotted breaking strength, and tying a knot lowers it. The calculator multiplies the rope's rating by a percentage for the knot you pick. Its table has a retraced figure-eight at 80%, a bowline at 70% and a reef knot at 45%. A 20 kN rope with a bowline comes out at 14 kN.
These are fixed, typical figures, not test results for your rope. Alpinesavvy's article on knots and cord strength calls a loss of about 50% "a conservative rule of thumb". Its own tests of a figure-eight kept 57.6% of strength in VT-X tech cord and 75.6% in 6 mm nylon cord.
The article adds that efficiency depends on the material, with tech cord losing a larger share than stretchy nylon. A flat 80% for a figure-eight is therefore a starting point. For a stiff or slippery cord or sling, plan on a larger loss.
The safe working load figures
The tool divides the knotted strength by 5, 10 and 15 to show three safe working loads. Its labels are 5:1 for overhead lifting and general rigging, 10:1 for life safety and climbing, and 15:1 for arboriculture and critical safety. These are the tool's own presets, not a published standard, and the right factor for your job comes from your standards body, employer or the equipment maker.
For the 14 kN bowline above, 10:1 gives 1.4 kN. A rope's real strength is far higher than that, but the point of the factor is to leave a margin for wear, shock loads and knots that tie less well than the table assumes.
FAQ
Which knot in the tool keeps the most strength?
The retraced figure-eight, at 80% in the tool's table. Alpinesavvy's tests found it kept between about 58% and 76% in the cords they tried, so treat 80% as a best case.
Why does the same knot lose different amounts in different ropes?
Alpinesavvy notes that knot efficiency varies with material. Static cords such as tech cord lost more than stretchy nylon in their tests, so a knot that holds up in one rope can cost more in another.
Can I use this to rate gear for climbing or lifting?
No. It is a quick estimate from typical figures. For anything where a failure could hurt someone, use the manufacturer's ratings and the standard that applies to your work.