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# i want to make a spiral stair but i need a computation for the dimension.So i need a formula for making a spiral stair? Answered

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I am in Afghanistan and I am building a Steel spiral staircase for our shop. to add on a loft with limited space. I am just trying to figure out how to get the dementions for how deep each rise should be. I have not built one before.

A spiral stair has the same rise and run need as a straight stair.

Take the top to bottom measurement and divide by the rise (height of each step) and you get the required number of steps. The rise can be between 5.5 inches and 7.5 inches. This should be measured at 2/3 the width of the step on circular stairs. Different people or situations require different rises. You will have to decide.

A 4' diameter circular stair is very useable, but not legal in the US.

A 5' diameter works fine.

Remember the rule is that for comfortable steps the rise puls the run is 16 to 20 inches
The best way to design a circular stair is to draw it in sketchup- but you have to learn google sketchup.

There are a variety of ways to build circular stairs. I only have built wood and concrete circular stairs. Here are some circular stair pics from my mexico houaw
https://www.me.com/gallery/#100075

Best bet is to build from purchased plans or a kit for your first spiral. There are some that require you only to know the height of the staircase (the rest is adjustable). What material are you building them with?