You can measure a curtain's length by measuring the fabric or by measuring the space along which the curtain hangs. Measuring the wall provides stability. Use the curtain rod as a stabilizer for your measuring tape. Rely on the solid wall to run the tape along the length of the curtain.
Hold the bottom outside corner of the curtain tightly from the rod to the bottom of the curtain. Make a light mark on the wall where the bottom of the curtain hits. Keep the curtain pulled back until you finish measurements.
Push the outer top of the curtain inward to expose the curtain rod. Hook the end of the measuring tape over the top of the curtain rod. Pull the tape down to the pencil mark.
Verify that the measuring tape is not at an angle as it runs from the curtain rod to the pencil mark. Adjust the tape to straighten it if necessary. Record the measurement. Repeat on the other end of the curtain rod to double-check your measurements.