Question: I have a concrete floor and I made two holes to bolt a 1/4"-thick steel plate. The problem is that the steel plates haven't been drilled yet.

How can I determine where the holes will be drilled in the steel plates so they match the holes in the concrete?
[![enter image description here](https://i.stack.imgur.com/1Kt5n.png)](https://i.stack.imgur.com/1Kt5n.png)

Answer: 
The usual methods are:

1. Careful measurement. Really this doesn't need to be that precise, you are not looking for a press-fit between bolts and clearance holes.

If it goes wrong, just elongate a hole into a slot using whatever tools you have to hand (e.g. a round file, clapped-out old Bridgeport, ...) - Remember: "A grinder *file/filler* and paint make me the welder machinist I ain't".
2. Put pointy or painty things into the holes then press the steel plate down in place to mark it. In the dead-tree-carcass world there are things called "dowel pins" used for this kind of thing.
The **hole spacing** is more important than the location of the holes on the metal plate. Therefore, working off of option 1 from @RedGrittyBrick, this is the approach I would take using a wax pencil or marker and a carpenter's square.

(1) On one edge of the plate mark a (rough) centerline.

(2) Align this edge of the plate to the holes in the concrete, with the centerline mark (roughly) centered between the holes, and **carefully mark the center of each hole on the edge of the plate**. You should now have 3 marks on the edge of the plate.

(3) **Carefully transfer the outer marks** as two lines across the steel plate using a square.

(4) Mark a line perpendicular to these two lines and (roughly) on center on the plate. You should have three lines on the steel plate. **The intersection of the three lines marks the center of the two holes to be drilled.**

Repeat the entire procedure if you have multiple pre-drilled locations. *Don't assume that other holes in other places in the concrete are equally spaced or centered.*