Measure the width of the bed by extending your measuring tape across the bed. Let the tape’s hook catch the other side, and record the width of the tape at the base of the tool.
Measure the depth by placing the tape’s hook between the mattress and box spring. Place your hand on top of the mattress, without pressing down, and measure to your hand.
Use an alternate method to measure the depth if you don't think you can avoid pressing the mattress down as you measure. Place a piece of cardboard on top of the mattress and have it hang over the edge. Place another piece of cardboard between the mattress and box spring and have it stick it. Measure the distance between the two pieces of cardboard to arrive at the gusset.