To do the math, you can convert the length, width, and heighth of your walkway to feet or to inches. Once you have a 'common' measurement, multiply everything out and divide the 'area covered' by 1 50lb bag into this number and you'll get your total cubic yards. Don't you mean .5 cubic yards??? Just make sure when you do the math that ALL the numbers you use are in feet or in inches. If you don't pay attention, you will see really large or really small results! And be mindful of drainage. Put in PVC for water runoff in low spots 'under' your walkway so the water can get to the otherside (if this is an issue). And lastly, I'd put in 3/4" PVC for low voltage wiring that you may want to run along the far side of the walkway. You may, or may not, be thinking of low voltage lighting but if you aren't now is the time to put in the plumbing to get the wire to the other side of the walkway.
... hope I didn't overwhelm you. Best to ya and hope this helps. Be sure to use a tamper on the excavated gravel bed AND after you lay your gravel.