It would depend on two parameters:
1. The sidewalk width.
2. The gravel type, i.e. how many pounds per sq. feet per 1 inch.
Follow the link below for the gravel calculator:
http://www.aqua-calc.com/calculate/gravel-rectangular-solidThe calculations below was performed for:
1. The sidewalk width: 6 feet or 72 inches
2. The regular gravel, 7 lbs per square feet per 1 inch depth:
3. The sidewalk length 400 feet or 4800 inches
The aqua-calc.com calculation result follows:
33600.00 lbs or 15245.79 kg or 15.3 tons.
Make sure to find out the exact characteristics of the gravel you are planning to use first.
