Rope Strengths

One area of misunderstanding that needs to be brought to the surface is the proper interpretation of rope strength, appropriate usage and care. Let’s start by defining two important terms: “tensile strength” and “working load“. Tensile strength is the average strength of new rope under laboratory conditions. This is determined by wrapping the rope around two large diameter capstans and slowly tensioning the line until it breaks. The manufacturer’s recommended working load is determined by taking the tensile strength and dividing it by a factor that more accurately reflects the maximum load that should be applied to a given rope to assure a comfortable safety margin and longevity of the line. Of course that factor varies with the type of fiber and the weaving construction. There are however always exceptions, most notably the fact that rope is susceptible to degradation and damage in numerous ways that are not controllable by the manufacturer. It may surprise you to find out that the working load for most kinds of rope is between 15% and 25% of the tensile strength. Now consider the fact that any time you tie a knot in a rope you effectively cut the tensile strength in half. The knot when tensioned cuts the line. While certain kinds of knots damage the line less than others, the 50% loss of tensile strength is a good general rule to live by. Research has shown that the figure 8 knot reduces the tensile strength by approximately 35% instead of 50% for other common knots tested.
https://www.ropeinc.com/ropetensilestrength.html

Tensile Strength in Pounds

Diameter Circumference Manila Polypropylene Blend Nylon Polyester
3/16” 5/8” 405 800 1000 1000
1/4″ 3/4” 540 1250 1098 1650 1650
5/16” 1” 900 1900 1728 2550 2550
3/8” 1-1/8” 1215 2700 2650 3700 3700
7/16” 1-1/4″ 1575 3500 3600 5000 5000
1/2″ 1- 1/2″ 2385 4200 4500 6400 6400
 9/16″ 1-3/4″ 3105 5100 5450 8000 8000
 5/8″  2″ 3960 6200 6400 10400 10000
 3/4″ 2-1/4″  4860 8500 8400 14200 12500
 13/16″  2-1/2″  5858 9900 10200 17000 15500
 7/8″  2-3/4″  6930 11500 12000 20000 18000
 1″  3″  8100 14000 15000 25000 22000
 1-1/16″  3-1/4″  9450 16000 17100 28800 25500
 1-1/8″  3-1/2″  10800 18300 19300 33000 29500
 1-1/4″  3-3/4″  12150 21000 22000 37500 33200
 1-5/16″  4″  13500 23500 25000 43000 37500
 1-1/2″  4-1/2″  16650 29700 31300 53000 46800
 1-5/8″  5″  20250 36000 38300 65000 57000
 1-3/4″  5-1/2″  23850 43000 46500 78000 67800