Scissor jack lift length is proportional to the weight of the scissor support weight and integrity of the steel cross sections more than the length. For a stable scissor jack platform, the weight of the scissor structure and length are corollaries of one another; it is the former that determines the latter. An extreme example might be a length of 20-foot featherweight scissor lift lengths made of styrofoam that weighs next to nothing. It won't support much and it certainly won't support much at much of a height. Both the American National Steel Institute (ANSI) and the Scaffold Industry Association use weight of the scaffolding beams as the measure of support to comply with both Occupational Safety and Health Administration regulations and those of ANSI specifications addressed under A92.6.
Determine how high you need to support a platform that will support both the weight of the platform and the weight of the objects and occupants of the platform.
Determine the number of cross-sections of scissor folding support sections that will be needed at full extension to reach the required height within the weight support parameters. The last thing you want is to send a few workers up on a platform with materials on a platform supported by an unstable structure that is prone to tipping.
Ascertain the weight of each scissor cross-section. This is crucial to its stability and determining the length and weight of the scissor lift you will need.
Determine the angle between the scissor arms when fully extended measured against the horizontal or ground level.
Plug in all the variables into the formula that will tell you the force needed to sustain the scissor arms, the weight load and length of the lift platform to create a stable platform. The formula is: F = (W + (WA/2))/tangent angle between the scissor arms and the horizontal. F equals the force needed to hold the scissor lift, W equals the weight of the payload and platform and WA equals the combined weight of the two scissor arms. The equation will determine the force needed to support the platform and load weight on the platform for a single scissor mechanism. Multiply the result by the number of scissor mechanisms needed to attain a desired height.
If you're good at math, you'll know how to isolate any of the variables to determine the scissor lift height, weight and length. In other words, if you already know what force you're capable of providing but don't know the tangent of the scissor arms and horizontal, isolate the tangent and plug in all the known figures and you can determine the tangent angle to horizontal. The same isolation of the unknown quantity applies to any single part of the equation.