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 **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.*
Do you really need to use the holes drilled in the concrete?

If not, pick another pair to be drilled far enough from the original ones. Make the hoes in the metal sheet first, put the sheet in position, secure it from movement and mark the spots or drill through the holes in the concrete.

Another point to be taken is how much the holes shall be coaxial. If the misfit can be in order of milimeters you can measure the position with a scale. If it must fit perfectly, two extra holes are a good tradeoff.