How to calculate cubic yards from square feet and inches of depth

The formula, and why 324 works

Volume in cubic feet is length times width times depth, all in feet. Divide by 27, the cubic feet in a cubic yard, and you have cubic yards. Depth is nearly always measured in inches, so the inch value goes through one more step first. For example, 4 inches is 4 ÷ 12 = 0.333 ft. Put the two steps together and the shortcut appears: square feet times depth in inches, divided by 324, because 27 × 12 = 324. Both routes give the same answer; the shortcut just saves a decimal.

The Mississippi State extension example runs it longhand. A 100 sq ft bed mulched 4 inches deep is 100 × 0.333 = 33.3 cubic feet. Then 33.3 ÷ 27 = 1.23 cubic yards. By the shortcut, 100 × 4 ÷ 324 = 1.23.

Measure in feet and inches, then convert

Tape measures read in feet and inches, and the inches must become decimals before anything is multiplied. Divide the inch part by 12 and add it to the feet. So 12 ft 5 in is 12 + 5 ÷ 12 = 12.42 ft, and 9 ft 10 in is 9.83 ft. Multiplying 12.42 by 9.83 gives 122.1 square feet. Multiplying 12 by 9 and ignoring the inches gives 108, a shortfall of more than a tenth before the depth is even considered. The square footage calculator takes feet and inches as two boxes so the conversion cannot be skipped.

Eight jobs, worked

Job	Area	Depth	Cubic feet	Cubic yards	With 10% extra
10 × 10 ft patio slab	100 sq ft	4 in	33.3	1.23	1.36
12 × 12 ft patio slab	144 sq ft	4 in	48.0	1.78	1.96
20 × 24 ft garage floor	480 sq ft	4 in	160.0	5.93	6.52
10 × 40 ft driveway, gravel surface	400 sq ft	4 in	133.3	4.94	5.43
8 × 12 ft bed, mulch	96 sq ft	3 in	24.0	0.89	0.98
50 × 50 ft lawn, new topsoil	2,500 sq ft	6 in	1,250.0	46.3	50.9
50 × 50 ft lawn, top-dressing	2,500 sq ft	1/2 in	104.2	3.86	4.24
Raised beds, 300 sq ft in all	300 sq ft	10 in	250.0	9.26	10.19

The last column is an allowance, not a rule. Concrete is ordered 4 to 10 percent over the plan quantity on the ready-mix association's advice, and loose materials get whatever settlement the material needs. The top-dressing row shows how a small depth over a big area adds up. Half an inch over a 50 ft square is nearly four yards.

Odd shapes: split, measure, add, subtract

A shape that is not a rectangle becomes one or more that are. An L-shaped patio is two rectangles added. A bed with a curved edge is a rectangle plus a half circle, or a run of narrow rectangles measured every few feet. A circle is 3.1416 times the radius squared. A round bed 8 ft across has a 4 ft radius, so 3.1416 × 16 = 50.3 square feet. At 4 inches that is 50.3 × 4 ÷ 324 = 0.62 cubic yards.

A triangle is base times height divided by 2. A trapezoid bed 12 ft along the fence, 8 ft along the walk and 6 ft between them averages the two long sides. That is (12 + 8) ÷ 2 × 6 = 60 square feet, or 0.56 cubic yards at 3 inches. A ring of rock around a tree is the big circle minus the small one.

Depth can vary too. A bed that is 2 inches deep at the edge and 4 inches at the middle averages 3 inches. A sloped drive that needs 6 inches of base at the low end and 4 at the high end is figured at 5. Cut-outs come off the area before the depth goes on. A patio with a 4 by 4 ft planter in it is 100 minus 16, not 100.

Round to the way it is sold

Cubic yards are a volume. The order is a product with its own steps. Ready-mix concrete is sold by the cubic yard and delivered in truck mixers that hold 8 to 12 yards. Plants set minimum loads. A 2025 Tennessee city bid listed 4, 6 and 4 yards. An Oklahoma county bid opened in December 2025, for January to June 2026, listed 3, 3 and 4 yards. A New York county award listed 4 and 7 yards, with flat short-load fees of $200 to $300 under the minimum. A 2020 Missouri county sheet billed a 2 yard minimum. Its fees stepped from $125 for a 2 yard load to $25 for a 5 yard load. A 1.36 yard patio pour is a short load everywhere, so the real choice is the fee against 62 bags of mix. The association's advice is to avoid a second short load to finish a placement: order the full amount once.

Bulk gravel, mulch and topsoil sell by the cubic yard or by the ton, in whole or half yards from most suppliers. The ton quote needs the material's weight per yard to compare. Bags sell by the cubic foot and round up to the next bag. Whatever the unit, the finished layer is not the order for anything that compacts. A crushed-stone base placed 6 inches loose ends at about 4.5 inches after rolling, by Penn State's road aggregate handbook. So a base that must finish at 4 inches is ordered at 5.3. The gravel calculator keeps that allowance separate from waste so both show on the list.