How to use
- Measure the bed area, leaving out spaces that will stay uncovered.
- Choose a rock type and enter the intended decorative layer depth.
- Replace the preset weight with the density quoted by your landscape yard when available.
How the estimate is calculated
Cubic feet equal bed area times depth in inches divided by 12. Divide by 27 for cubic yards, multiply by the selected tons per cubic yard for weight, and divide cubic feet by 0.5 for bags, rounding the bag count up.
Example: 100 sq ft covered 3 in deep holds 25.0 cu ft, or 0.93 cubic yards. At 1.35 tons per cubic yard, the estimated weight is 1.25 tons; the volume equals 50 bags of 0.5 cu ft.
Why lava rock weighs less for the same bed
| Decorative material | Planning density | Source basis |
|---|---|---|
| River rock | 1.35 short tons per cu yd | Derived from a Quikrete bagged-gravel listing: 50 lb / 0.5 cu ft |
| Red lava rock | 0.45 short tons per cu yd | Hello Gravel product summary: about 900 lb per cu yd |
The lava preset is one-third of the river-rock preset, so its weight estimate is one-third for equal volume. It is a product-specific comparison, not a promise for every lava rock. Ask for the actual bulk weight, including the material’s moisture condition.
Rock, mulch, and the layer beneath
This page measures a decorative mineral cover. Use the mulch calculator for an organic cover and the crushed rock calculator for a compacted structural base. If using landscape fabric, plan its seams and planting openings separately; this rock-volume result does not measure fabric rolls or decide whether fabric suits the bed.
Common mistakes
- Ordering lava rock by a river-rock tonnage without comparing density.
- Reading the 0.5 cu ft bag equivalent as a fixed bag weight.
- Assuming a decorative layer estimate includes compacted base material.
Sources
Quikrete All-Purpose Gravel reseller listing supplies the summary basis for the river-rock planning density. Hello Gravel red lava rock listing supplies the commercial lava-rock example. Both were available only as search excerpts; confirm your chosen product with the supplier.
Related calculators
- Mulch Calculator
- Gravel Calculator
- Pea Gravel Calculator
- Driveway Gravel Calculator
- Crushed Rock Calculator
Frequently asked questions
Do lava rock and river rock need different volumes?
For an identical area and depth, their calculated volumes match. Their weights differ because the density presets differ.
Are the bags assumed to weigh 50 lb?
No. Fifty pounds is the source basis for the gravel density, but the bag count here is a volume equivalent. Lava-rock bags can have a different weight.
Can I calculate an irregular garden bed?
Break it into measurable shapes, add their areas, and enter the total bed area. Subtract areas you do not intend to cover.
Does the tool add extra rock?
It returns the geometric volume with no added allowance. Discuss uneven ground and delivery increments with the yard.
Is this suitable for a compacted driveway base?
Use the driveway gravel or crushed rock calculator instead. Decorative rock density and depth do not describe a layered compacted base.