I can't believe no one came up with the answer that was asked for by the poster. She said she does not want to cut a strip first. I have done this numerous times.
I lay a corner of the desired fabric on my cutting mat. Place a ruler over the corner so that the desired size block is under the ruler (check all sides several times, watch your selvedge, cut just a wee bit larger). Cut up one side just to the corner. It works best to turn the cutting blade for the last bit instead of pushing it here.
Fold the excess fabric to the top of the cut line and a little more so that the fold is going up from the corner.
Cut the other side.
Now you have a square (or whatever piece you wanted). Flip that around and cut the other two sides exactly.
This way you do not have large cuts in your remaining fabric for the next time you want to use it.
I have lots of pieces of fabric in my stash that have a corner cut out. LOL But the cuts don't extend into the fabric any more than necessary.