How do I measure for a replacement window?

I have an old window in the back hall, how do I measure for a replacement? I dont want to remove the old window to the rough opening, I want a replacement that slides into the opening.How do I measure for a replacement window?if your old window is a double hung unit you measure the width at the point where the sashes meet the old inside jamb and the height from the top where the sash meets to the inside high spot of the sill, hopefully you can remove the inside trim to install .How do I measure for a replacement window?measure the inside of the window frameHow do I measure for a replacement window?if you have a sash window - pretend the window is out of the frame and measure from jamb to jamb, side to side, top to bottom.How do I measure for a replacement window?Usually if you order a replacement window, they will come measure for you. Call Home Depot or Lowes and talk to a window specialist.