1995
Obsidian mulch chart
Put your bed area, obsidian mulch depth and waste allowance in: 7800 sq ft at 3 inches with 10% waste needs 8,580.00 sq ft, 79.44 cubic yards exact — order 80 cubic yards.
★ CONSTRUCTION · MULCH · OBSIDIAN MULCH CHART
Obsidian Mulch Chart: 7800 Sq Ft, 3 Inches, Order 80
On the defaults — a 7800 square foot bed at 3 inches deep with a 10% waste allowance — you need 8,580.00 square feet of cover, 79.44 cubic yards exact, so order 80 cubic yards. This is the OBSIDIAN version — the mulch chart for obsidian, a hard igneous stone in red and purple tones with visible crystals, laid for beds, paths and paving, that never breaks down.
What does 79.44 cubic yards of obsidian mulch mean?
It means a 7800 square foot bed, once the 10% waste allowance is added, needs 8,580.00 square feet of cover at 3 inches deep — 2,145.00 cubic feet, which is 79.44 cubic yards. Because mulch is sold by the whole yard, you order 80 cubic yards: the exact figure sits 0.56 yards below the next whole yard, so the order carries 0.56 yards of headroom for spillage and uneven ground. At 3 inches, one cubic yard covers 108.0 square feet, so the same bed at 2 inches would need only 52.96 yards, and at 4 inches it would need 105.93 yards.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Area needed | area × (1 + waste ÷ 100) | 8,580.00 sq ft — 7800 × 1.10. |
| Volume in cubic feet | area needed × depth ÷ 12 | 2,145.00 cubic feet — 8,580.00 × 3 ÷ 12. |
| Exact volume | area needed × depth ÷ 324 | 79.44 cubic yards — 8,580.00 × 3 ÷ 324. |
| Order quantity | round exact volume up to a whole yard | 80 cubic yards — the next whole yard above 79.44. |
| Coverage per yard | 324 ÷ depth | 108.0 sq ft per yard — 324 ÷ 3. |
| Area behind a target volume | target volume × 324 ÷ depth ÷ (1 + waste ÷ 100) | 7800 sq ft — 79.44 × 324 ÷ 3 ÷ 1.10, rounded. |
| Depth behind a target volume | area needed × depth ÷ 324, solved for depth | 3 inches — the depth that turns 8,580.00 sq ft into 79.44 yards. |
| Waste behind a target volume | target volume × 324 ÷ depth ÷ area − 1, as a percent | 10% — the allowance that turns 7800 sq ft into 8,580.00 sq ft. |
Three things to check before you trust the number
First, measure the bed honestly: length times width for each bed, added together, with L-shaped beds split into rectangles. A guessed area moves the order by the same percentage as the guess. Second, keep the depth honest: obsidian does not break down, so the layer you lay is the layer that stays — and the pieces are angular, so they lock together and hold their depth on slopes better than round stones do, though loose pieces at the edges still wander. Third, do not skip the waste allowance on obsidian: even tightly packing angular pieces leave gaps, and spillage at the edges and uneven ground still eat into the load, so the 10% allowance is the difference between finishing the bed and running short with the truck already gone. The headline order of 80 yards carries 0.56 yards of headroom, which covers ordinary spillage on a bed like this one.
Worked example
Area 7800 sq ft, depth 3 inches, waste 10%. Area needed: 7800 × 1.10 = 8,580.00 sq ft. Exact volume: 8,580.00 × 3 ÷ 324 = 79.44 cubic yards. Order: round 79.44 up to 80 cubic yards. Drop the waste to 0% and the exact volume falls to 72.22 yards, order 73; raise the waste to 5% and it is 75.83 yards, order 76; raise it to 15% and it is 83.06 yards, order 84. Drop the depth to 2 inches and it falls to 52.96 yards, order 53; raise it to 4 inches and it climbs to 105.93 yards, order 106. Depth moves the answer faster than waste does, because every inch adds a full layer across the whole bed.
Obsidian mulch lookup table
Every row below applies 10% waste to the area first, then divides by 324: yards = area needed × depth ÷ 324, rounded up to the next whole yard for the order. Find your bed size and depth, read across, and order the last column.
| Area (sq ft) | Depth (inches) | Area needed (sq ft, 10% waste) | Exact (cubic yards) | Order |
|---|---|---|---|---|
| 300 | 2 | 330.00 | 2.04 | 3 cubic yards |
| 300 | 3 | 330.00 | 3.06 | 4 cubic yards |
| 600 | 2 | 660.00 | 4.07 | 5 cubic yards |
| 600 | 3 | 660.00 | 6.11 | 7 cubic yards |
| 800 | 3 | 880.00 | 8.15 | 9 cubic yards |
| 800 | 4 | 880.00 | 10.86 | 11 cubic yards |
| 1100 | 2 | 1,210.00 | 7.47 | 8 cubic yards |
| 1100 | 3 | 1,210.00 | 11.20 | 12 cubic yards |
| 1300 | 2 | 1,430.00 | 8.83 | 9 cubic yards |
| 1500 | 3 | 1,650.00 | 15.28 | 16 cubic yards |
| 1600 | 3 | 1,760.00 | 16.30 | 17 cubic yards |
| 1600 | 4 | 1,760.00 | 21.73 | 22 cubic yards |
| 1800 | 2 | 1,980.00 | 12.22 | 13 cubic yards |
| 2200 | 3 | 2,420.00 | 22.41 | 23 cubic yards |
| 2600 | 3 | 2,860.00 | 26.48 | 27 cubic yards |
| 3200 | 2 | 3,520.00 | 21.73 | 22 cubic yards |
Every branch, checked
A 7800 square foot bed at 3-inch depth with 0% waste gives 72.22 cubic yards exact, order 73 cubic yards. The same bed at 2-inch depth with 10% waste gives 52.96 cubic yards exact, order 53 cubic yards. A 7500 square foot bed at 3-inch depth with 10% waste gives 76.39 cubic yards exact, order 77 cubic yards. A 8100 square foot bed at 3-inch depth with 10% waste gives 82.50 cubic yards exact, order 83 cubic yards. Each branch changes exactly one input, so the difference from 79.44 isolates that input.
The reverses, worked backwards
The area behind 79.44 cubic yards is 7800 sq ft: at 7500 sq ft the figures are 76.39 cubic yards exact, order 77 cubic yards, and at 8100 sq ft they are 82.50 cubic yards exact, order 83 cubic yards. The depth behind 79.44 cubic yards is 3 inches: at 2 inches the figures are 52.96 cubic yards exact, order 53 cubic yards, and at 4 inches they are 105.93 cubic yards exact, order 106 cubic yards — one inch either way moves the order by about 26.5 yards. The waste behind 79.44 cubic yards is 10%: at 5% the figures are 75.83 cubic yards exact, order 76 cubic yards, and at 15% they are 83.06 cubic yards exact, order 84 cubic yards.
What the defaults assume
The defaults assume a flat 7800 square foot bed, a uniform 3-inch layer, and a 10% waste allowance. Real obsidian beds are rarely flat: paths and tree trunks eat into the area, and loose pieces at the edges wander into the lawn, so measure the planted area only and keep the waste allowance on sloped ground. Zero square feet is the honest edge: 0.00 cubic yards exact, order 0. A depth of zero or below, a negative area, or a negative waste allowance is rejected, because none of them describes a real bed.
Who uses this
A gardener dressing beds and paths with obsidian: the 7800 square foot headline is a large garden bed or a long drive, and the table covers everything from a 300 square foot border to a 3200 square foot yard. A landscaper quoting a obsidian job: the waste column protects the quote, and the order column is the number given to the supplier. A homeowner laying obsidian on a path: the 3-inch rows are the bed depths, and the 2-inch rows are the path depths. A community garden coordinator splitting a bulk delivery: each plot reads its own row, and the orders add up to the truckload.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is in the inputs. A guessed area, a depth measured before the pieces settle, or a skipped waste allowance will each move the answer by exactly the amount of the error, every load. The model does not include pieces wandering at the edges, delivery minimums, or the gaps between angular pieces — a supplier may sell only in whole yards or charge extra below a minimum, so confirm the order quantity with the supplier. This page is a calculation tool, not landscaping advice.
Cite this calculator
Obsidian mulch 7800 sq ft at 3-inch depth with 10% waste needs 8,580.00 sq ft, 79.44 cubic yards exact, order 80 yards. Free calculator, plus 16-row table. Source: Bright Side Kit, https://brightoolbox.com/obsidian-mulch-chart/, accessed 31 October 2026 (2026-10-31).
Frequently asked questions
Why does the table apply waste to the area first?
Because waste is lost cover: spillage, scattering and uneven ground mean you need more covered area than the bed measures. Adding 10% to the area, then converting to yards, keeps the waste in the same units as the bed. Applying it after the conversion gives the same number here, but the area-first order keeps the table readable.
How deep should obsidian mulch really be?
Three inches is the standard bed layer on this page, and 2 inches suit paths and top-ups. Obsidian does not break down, so a 3-inch spread stays 3 inches; if pieces scatter, set them back or top up the thin spots.
Does obsidian mulch introduce weed seeds?
Obsidian is mineral, so it carries no weed seeds at all; weeds can still root in the dust and debris that collects on top of it, so use fabric underneath and clear the debris. This calculator prices volume only and does not judge the gardening choice.
What if my bed is not a rectangle?
Split it into rectangles, measure each, and add the areas before entering the total. A circle uses radius times radius times 3.14. Guessing one number for an irregular bed is the most common reason orders come up short.
Why order 80 yards when the exact figure is 79.44?
Because mulch is sold by the whole yard, and 79.44 rounds up to 78 — the order carries 0.56 yards of headroom for spillage and uneven ground. That headroom is the buffer the exact figure does not have.
Can I use this chart for other mulch types?
The volume math is the same for any loose mulch — area, depth and waste convert the same way — but settling and coverage differ by material. Obsidian does not settle like bark, so use the obsidian rows for obsidian and the matching chart for bark or wood chips.