12 square yards to square feet

Twelve square yards is 108 square feet

Twelve square yards is 108 square feet. Each yard of length gives three feet, so a square yard is a frame three feet by three feet holding 9 square feet, and twelve of those frames hold 108.

The same surface is 15,552 square inches and 10.03352832 square meters. NIST Handbook 44, Appendix C, 2026 edition lists one square yard as exactly 9 square feet, exactly 1,296 square inches and exactly 0.836 127 36 square meters. Twelve times the last figure is 10.03352832, with nothing rounded away.

For a gardener the useful column is water, because a shallow basin is a container before it is a planting bed. At 6 inches deep, 108 square feet holds 54 cubic feet, which is 403.9 gallons.

Read across any row and it is one basin described three ways. The 20 square yard row is the 180 square foot row, which is 16.7225 square meters and 673.2 gallons at the same depth.

A 6 by 18 foot strip and other 108 square foot shapes

Ground has no preferred outline. A strip 6 feet wide and 18 feet long is 108 square feet, and so are an 8 by 13.5 foot rectangle, a 4 by 27 foot border along a fence, and a square with sides of 10.3923 feet, because 10.3923 x 10.3923 = 108.

That size sits at the small end of what the Wisconsin Department of Natural Resources 2018 homeowner guide, publication PUB-WT-776, calls a typical residential rain garden. The guide puts the usual range at 100 to 300 square feet and the usual depth at three to eight inches, which makes 12 square yards a workable first basin for a single downspout.

Keep the outline simple. A long narrow basin has more edge per square foot than a square one, and edge is where the berm is built and where a mower has to turn. The ponding area in every calculation on this page is the flat bottom of the basin, not the top of the berm.

Worked example: 12 square yards at six inches of ponding

Area times depth gives volume. One hundred and eight square feet at 6 inches is 108 x 6 / 12 = 54 cubic feet, which is exactly 2 cubic yards. Multiply by 7.48052 gallons per cubic foot and the basin holds 403.9 gallons, or 7.34 barrels of 55 gallons.

Depth is the only variable once the outline is fixed. At 3 inches the same surface takes 27 cubic feet, 1 cubic yard and 202 gallons. At 4 inches it is 36 cubic feet, 1.333 cubic yards and 269.3 gallons. At 8 inches, 72 cubic feet, 2.667 cubic yards and 538.6 gallons, which is the deepest the Wisconsin guide recommends before a basin starts to look like a hole in the ground and becomes a trip hazard.

A 12 inch row is in the table as a reference only: 108 cubic feet, 4 cubic yards and 807.9 gallons. That much standing water sits for days in clay soil and turns into a pond with mosquitoes rather than a garden that dries between storms.

The gallons column assumes fresh water at 7.48052 gallons per cubic foot, the figure that follows from the US liquid gallon of 231 cubic inches. Silty water with soil in suspension is slightly denser, which changes nothing you can measure with a watering can.

Worked example: soil mix and the bill for 108 square feet

Excavation is the same volume as the water: 2 cubic yards at 6 inches, and that is the number a soil supplier will ask for. Bagged at 1.5 cubic feet per sack, 54 cubic feet is 36 sacks, and at an example price of $4.25 a sack the bill is $153.00. Two cubic yards of a bulk blend usually beats that, so price both before filling a cart.

When the job is quoted by area instead, the arithmetic runs the other way. At $7.50 per square yard, 12 square yards costs $90.00, which is $0.83 per square foot. A 10 percent contingency takes the area to 13.2 square yards and the bill to $99.00, which is worth adding when the outline curves.

Compost is the line people forget. A planting mix for a basin is not straight topsoil, and the amended layer under the ponding volume is a separate order from the 2 cubic yards of basin volume. Ask for the mix by volume, not by sack count, then convert: 36 sacks is 2 cubic yards only if every sack is 1.5 cubic feet.

Sizing the basin to the roof it drains

The Wisconsin guide works backwards from the roof. Multiply the drainage area by a sizing factor that depends on soil and depth, and the result is the ponding area. Turned around, a 108 square foot basin divided by the factor gives the roof area it can serve.

On silty soil at six to seven inches the factor is 0.30, so 108 / 0.30 = 360 square feet of roof. Sandy soil at the same depth uses 0.13 and serves 831 square feet. Clay at 0.35 serves 309 square feet. A shallow basin three to five inches deep uses 0.17 on sand and serves 635 square feet; the same shallow basin in clay uses 0.44 and serves 245.

The roof area in that table is the area that drains to the basin, not the whole roof. A downspout carrying half the roof makes the effective drainage area half of the roof plan, and a downspout thirty feet from the basin delivers less water than one at the edge, because some of it soaks in on the way.

One inch of rain, counted in gallons

Rainfall converts exactly like ponding depth. One inch over 108 square feet is 108 / 12 = 9 cubic feet, or 67.3 gallons. Half an inch delivers 33.7 gallons and a two inch storm 134.6 gallons.

The same rule scales with area. One inch on 1,000 square feet is 83.33 cubic feet, or 623.4 gallons, which is why roofs and basins are matched by area instead of by eye. The 360 square foot roof from the silty soil case sheds 224.4 gallons for every inch of rain.

Put that beside a rain barrel. The 108 square foot basin holds 403.9 gallons at 6 inches, seven barrels and a third, and it hands that water to the soil over a day or two instead of holding it. A single barrel holds less than one inch of rain from a 360 square foot roof, so the basin is doing the work a barrel cannot.

The US EPA Soak Up the Rain pages on rain gardens describe the same job from the runoff side, and the agency marks that section as no longer updated, so a state guide is the better place to take sizing numbers from.

Five ways a twelve square yard basin goes wrong

Counting the side slopes. The ponding area is the flat bottom. A basin whose top measures 108 square feet stores less water once the sides slope in, and a 6 by 18 foot top with 3 to 1 sides at 6 inches deep loses roughly 3 square feet of flat bottom on each of the four sides.

Measuring to the top of the berm. Depth runs from the soil surface to the overflow, not to the highest point of the mound. Six inches to the overflow and eight inches to the crest means the last two inches spill down the street.

Using the floor plan as the roof area. Roofs are measured on the slope, over the eaves, so a pitched roof plan understates the drainage area it feeds.

Treating the basin as a pond. Standing water for days means the soil is not draining, and the fix is a different site, not a deeper hole.

Ordering sacks by area. Two cubic yards is 36 sacks of 1.5 cubic feet, and a supplier selling 2 cubic foot sacks would need 27. Read the sack before multiplying.