riprap newbie

I am new to all of this. We just started a business. One of my customers asked me for a price on doing riprap in his drainage ditch. I am not sure how many ton I am suppose to order. I get different numbers everywhere I look. Could use a little help with this please. The measurements are 170 ft x 10 ft. I am looking at 3 to 5" riprap.
How do I figure out how many ton I need delivered? Thank you so much in advance!!