Uprise42
Use the same place where the old flight of stairs used to be: the house’s structure and floor statics were calculated to have stairs there. You don’t want to mess with a house’s structure without the help of a statics engineer!
A little bit of “Stair maths” to start. Sorry for metric units, you might have to convert them if you’re in the US.
The ideal stair has an angle of 30°, a rise of 17 centimetres up, from step to step, with steps 29 cm deep, from front to back. Two rises plus one depth should be as close as possible to 63 cm because of the length of a human’s step.
You won’t get this ideal in most cases, because the distance between the upper and liwer floor will rarely be an exact mulitiple of 17 cm.
1: measure this distance, finished upper floor to finished lower floor. Divide by 17 cm. Round up or down to get the number of steps you need.
2: Divide the distance between the floors by the number of steps from above
3: Use the “2 rises plus 1 depth = 63 cm” to determine the ideal depth. Stay as close to that as possible to make the stairs easy, safe and comfortable to walk on. It’s a good idea to make a drawing to scale at this point, to see how the stairs fit in the floorplan.
4: Now you can calculate the length of the stairs using good old Pythagoras (a^2 + b^2 = c^2, “a” being the distance between the floors, “b” is the depth of one step multiplied by the number - from above, “c” is the length of the stair - and the boards (“stringers”) on either side as well as the handrails).
Now you can calculate the material you need. Two stringer boards, the required number of steps of the correct length, plus brackets and screws on either sude of each, plus one or two handrails plus balusters.