Construction Unit Conversions

Building work has an awkward habit of measuring in one unit, specifying in a second, and buying in a third. You measure a driveway base in square feet, specify it in inches of depth, calculate it in cubic yards, and buy it by the ton. This is a reference for the conversions that come up most often, and for the ones where a single number cannot be given honestly.

Volume

These are fixed conversions, geometry rather than material properties, so they hold regardless of what you are measuring.

From	To	Multiply by
Cubic yards	Cubic feet	27
Cubic feet	Cubic yards	0.037 (÷ 27)
Cubic yards	Cubic meters	0.765
Cubic meters	Cubic yards	1.308
Cubic feet	Gallons (US)	7.48
Cubic yards	Gallons (US)	202
Cubic yards	Liters	765

The one worth memorising is 27 cubic feet to the cubic yard, because it is buried inside almost every bulk material formula. The familiar shortcut for slabs and beds is length in feet times width in feet times depth in inches, divided by 324. That divisor is just 12 (inches to feet) times 27 (cubic feet to cubic yards) combined into one.

Area and coverage

Situation	Conversion
Square feet to square yards	÷ 9
Square feet to squares (roofing)	÷ 100
1 cubic yard spread 1 inch deep	324 sq ft
1 cubic yard spread 2 inches deep	162 sq ft
1 cubic yard spread 3 inches deep	108 sq ft
1 cubic yard spread 4 inches deep	81 sq ft
1 cubic yard spread 6 inches deep	54 sq ft

The coverage figures are the ones people actually need on site, and they all come from the same 324 divisor. A cubic yard covering 324 square feet at one inch means it covers half that at two inches, a third at three, and so on.

Weight: the conversions that need a material

This is where published conversion tables become misleading. There is no general cubic-yards-to-tons conversion. It depends entirely on what is in the yard.

Material	Approx. Tons per Cubic Yard	Approx. Cubic Yards per Ton
Crushed stone / gravel	1.4	0.71
Sand	1.3	0.77
Topsoil (average moisture)	1.1	0.91
Screened dry topsoil	1.0	1.00
Wet topsoil	1.2	0.83
Wood mulch	0.4–0.5	2.0–2.5
Asphalt (compacted)	1.9–2.0	0.50–0.53

A ton of mulch is roughly five times the volume of a ton of asphalt. Anyone who tells you “a cubic yard is about a ton and a half” is describing crushed stone and nothing else.

Moisture moves these figures more than most people expect. Topsoil after a wet week can be 20% heavier than the same soil dry, which is a real problem when you have ordered by weight and are expecting a volume. If you are buying by the ton to fill a space you measured by volume, order against the wetter figure and accept a small surplus. The gravel calculator and topsoil calculator both expose the density assumption so you can match it to what your supplier actually quotes.

Trade units that are not really units

Several figures in construction look like measurements but are conventions for ordering.

Roofing square. 100 square feet of roof surface. Three bundles of standard architectural shingles cover one square, though heavier or specialty products may run four or five bundles. The critical detail is that squares apply to the roof surface, not the building footprint. A 2,000 sq ft footprint under a 6/12 roof is about 2,236 sq ft of surface, which is 22.4 squares, not 20. Convert with the slope factor from the roof pitch calculator before ordering.

Board foot. 144 cubic inches of lumber: nominally 1 inch by 12 inches by 12 inches. Because it uses nominal dimensions, a 2x4 is calculated as 2 by 4 even though it actually measures 1.5 by 3.5 after dressing. Board feet are how rough and hardwood lumber is priced; dimensional framing lumber is usually sold by the piece instead. The board foot calculator works in nominal dimensions for this reason.

Square (siding). Also 100 square feet, same as roofing. Vinyl siding is typically boxed in two-square units.

Yard (concrete). Always a cubic yard, never a linear or square one. Ready-mix trucks carry 8 to 10 cubic yards, and most suppliers apply a short-load fee below about 3 or 4 yards, often enough to make a slightly larger order cheaper than a smaller one.

Bag counts

Bagged material is sold by volume or by weight depending on the product, which makes cross-comparison harder than it should be.

Product	Bag Size	Bags per Cubic Yard
Topsoil / mulch	0.75 cu ft	36
Topsoil / mulch	1 cu ft	27
Topsoil / mulch	1.5 cu ft	18
Topsoil / mulch	2 cu ft	14
Concrete mix	40 lb (0.30 cu ft)	90
Concrete mix	50 lb (0.375 cu ft)	72
Concrete mix	60 lb (0.45 cu ft)	60
Concrete mix	80 lb (0.60 cu ft)	45

Concrete bag yields are the ones worth checking against the bag itself, since they vary slightly between manufacturers and between standard and high-strength mixes. The concrete calculator uses the common yields above and lets you switch bag size to compare.

The crossover point between bagged and bulk is worth calculating rather than guessing. Forty-five 80 lb bags to a cubic yard means a modest 2-yard pour is 90 bags, around two tons of material to move by hand, and almost always more expensive than a short-load delivery once you price the labour.

Length and depth

From	To	Multiply by
Inches	Feet	0.0833 (÷ 12)
Feet	Inches	12
Feet	Meters	0.3048
Meters	Feet	3.281
Inches	Millimeters	25.4

Depth is the most common place for a units error to cause a real problem, because depth is almost always specified in inches while length and width are in feet. Multiplying three numbers that are not in the same unit produces an answer that is wrong by a factor of 12, and a base that is one twelfth of the specified depth will fail in its first winter.

A note on rounding

Convert first, round last. Rounding at each intermediate step compounds the error, and on a large order the accumulated difference can be a meaningful fraction of a delivery.

The exception is the final number, which should be rounded up to whatever unit the supplier actually sells: whole bags, whole squares, whole pallets, or the supplier’s minimum increment for bulk. See our waste factors guide for why that final rounding matters more than the precision of the arithmetic that preceded it.