Landscape Rock Calculator

Loose-fill densities run from about 50 lb per cu ft for lava rock at the light end to roughly 170 lb per cu ft for boulders at the heavy end. The middle is where most jobs live: pea gravel 100, river rock 105, decomposed granite and crushed stone 110, marble chips 120, flagstone 160. A cubic yard (27 cu ft) therefore weighs 1,350 lb in lava rock, 2,700 lb in pea gravel, 2,835 lb in river rock, 2,970 lb in granite or crushed stone, 3,240 lb in marble chips, 4,320 lb in flagstone, and 4,590 lb in boulders. Pick the rock type in the calculator and the density swaps automatically, or punch in a custom number if your supplier publishes one.

Decorative ground cover around plantings and borders works at 2 to 3 in deep, enough to hide the soil and weed barrier without burying the stone's color. Pathways and walking areas need 3 to 4 in so feet do not push down to the dirt. French drains, downspout splash zones, and dry creek beds need 4 to 6 in to actually carry water and stay in place during heavy runoff. Driveways need 4 to 6 in over a compacted crushed-stone base, and never use pea gravel or marble chips for a driveway because the round, smooth shapes migrate under tire loads. The calculator lets you sweep depth from 1 to 6 in to compare cost and weight before you order.

The break-even point is 1 cubic yard. Under 1 cu yd, the standard 50 lb (0.5 cu ft) bags from a home center usually win on price once you factor in the flat delivery fee (typically $50 to $150 in US metro areas). At or above 1 cu yd, bulk wins decisively: most landscape yards charge $35 to $80 per cu yd for common rock types, while the same volume in bags runs three to five times that price. The calculator flips its recommended delivery mode at the 1 cu yd line and tells you the exact tonnage to ask the yard for, so you do not over- or under-order. If you have no way to receive a dump truck, bags stay the only option regardless of volume.

Because pea gravel is small and uniform (3/8 in), the void space between stones is consistent and relatively high, so loose-fill density runs about 100 lb per cu ft. River rock cobbles are larger (1 to 5 in) and more varied in size, so smaller stones settle into the gaps between larger ones during loading and transport. That denser packing pushes the loose-fill number up to about 105 lb per cu ft, roughly 5 percent heavier than pea gravel for the same volume. Crushed angular stone packs even tighter (110 lb per cu ft) because the broken faces interlock instead of rolling apart.

Yes for almost every job. Rock laid directly on soil sinks into the dirt within a season, especially in beds you water or in walking areas. A woven landscape fabric (4 to 6 oz weight) under 2 to 4 in of rock keeps stones on top, blocks most weeds, and lets water through. For dry creek beds and drainage, use a heavier non-woven geotextile rated for sub-base use. Skip the cheap perforated black plastic, which traps water and rots roots. Add 10 percent to your fabric square footage for overlap at seams, and pin every 12 to 18 in along the edges so wind and runoff do not lift the corners before the rock goes down. Flagstone and boulders set into a compacted base are the exception: they get bedded directly so they do not shift over time.