Pulley System Calculator
Mechanical advantage on paper is not what you feel at the rope. The calculator derates the ideal ratio for pulley friction, adds rope stretch to the rope you must pull, and rates how hard each pull is.
Result
- Effective advantage
- 2.43 : 1
- Rope pulled per meter raised
- 3.06 m
A carabiner used as a pulley sits near 0.7 efficiency and eats most of a 3:1's gain - a real pulley pays for itself.
How the calculation works
The model derates the ideal ratio and adds stretch to the rope travel:
- Efficiency is per bend: 0.9 is a good pulley, 0.7 is a carabiner doing a pulley's job.
- Working stretch: static ~2%, low-stretch ~4%, dynamic ~7%.
- The verdict rates sustained one-person pulling: up to 25 kgf manageable, 25-40 kgf strenuous, above that build more advantage.
Hauling forces share physics with catching falls: the fall factor calculator covers impact forces on the rope, and the anchor force calculator covers what those loads do to the anchor you rig the system on.
Planning estimate only - not safety advice.
Frequently asked questions
Why is my 3:1 not giving me a third of the load?
Because every bend in the system leaks energy to friction. With real pulleys at about 90% efficiency, a 3:1 actually delivers around 2.4:1 - and a carabiner as the "pulley" (roughly 70% efficient) drops it near 2:1. The calculator derates the ideal ratio with the efficiency you give it.
What is a Z-drag and when do I need one?
A 3:1 arrangement built from one rope, two pulleys and progress-capture prusiks, shaped like a Z. It is the standard crevasse-rescue and boat-hauling system: enough advantage to move a heavy load, few enough parts to rig quickly under pressure.
Why does rope stretch matter for hauling?
Stretch means wasted motion: for every meter the load rises you pull the rope length times the mechanical advantage, and a stretchy rope adds its elongation on top - with a dynamic rope at a 3:1 you pull about 3.2 m of rope per meter gained. Static rope does more of each pull turning into lift.
How hard is too hard for one person?
Sustained pulls land around 20-25 kgf for a fit adult; 40 kgf is a burst that cannot last. If the calculator lands above that, add a leg (a 5:1) or piggyback another system rather than grinding a lost bout.