Where the factors come from
Every conversion on this site starts from three exact lengths, the international inch, foot and yard defined in 1959 and used by NIST:
- 1 inch = 0.0254 m exactly (2.54 cm).
- 1 foot = 12 inches = 0.3048 m exactly.
- 1 yard = 3 feet = 0.9144 m exactly.
An area is a length times a length, so its factor is the length factor squared. A volume is three lengths, so its factor is cubed. That is the whole reason a square yard is 9 square feet and a cubic yard is 27 cubic feet: 3 × 3 and 3 × 3 × 3. Liquid measures such as gallons and liters are a different family, explained with paint in paint coverage and coats.
Square feet, square yards and square meters
| Area | In square feet or yards | In square meters |
|---|---|---|
| 1 sq ft | 144 sq in | 0.09290304 m² (exact) |
| 1 sq yd | 9 sq ft | 0.83612736 m² (exact) |
| 1 m² | 10.764 sq ft, or 1.196 sq yd | 1 m² |
Example: an invented floor of 15 × 12 ft is 180 sq ft. In square yards that is 180 ÷ 9 = 20 sq yd, and in square meters 180 × 0.09290304 = 16.72 m².
The room planners show areas in square feet when you measure in feet and inches, and in square meters when you measure in meters. If a price or a quantity is given in square yards, multiply it by 9 to compare it with square feet: 20 sq yd is the 180 sq ft above.
Cubic feet, cubic yards and cubic meters
| Volume | In cubic feet or yards | In cubic meters |
|---|---|---|
| 1 cu ft | 1,728 cu in | 0.028316846592 m³ (exact) |
| 1 cu yd | 27 cu ft | 0.764554857984 m³ (exact) |
| 1 m³ | 35.315 cu ft, or 1.308 cu yd | 1 m³ |
How many bags make a cubic yard follows from the 27: bags of 2 cu ft need 27 ÷ 2 = 13.5, so 14 bags to cover a full cubic yard; bags of 1.5 cu ft need 18, bags of 3 cu ft need 9. These bag sizes are examples: use the volume printed on the bag.
A depth in inches to cubic yards
Beds, borders and paths are measured in feet but filled to a depth in inches, and bulk material is often sold by the cubic yard. The steps:
- Area in square feet: length × width, both in feet.
- Depth in feet: the depth in inches ÷ 12. Three inches is 0.25 ft, not 0.3 ft.
- Volume in cubic feet: area × depth in feet.
- Volume in cubic yards: cubic feet ÷ 27. In one step: square feet × inches ÷ 324, because 12 × 27 = 324.
- Round up to whole bags or delivery units, never down. The leftover is what you buy minus what you need.
Worked examples, both invented:
- A bed 24 × 6 ft, 2 in deep: 144 sq ft × 2 ÷ 12 = 24 cu ft, and 24 ÷ 27 = 0.889 cu yd (or 144 × 2 ÷ 324). One cubic yard covers it with 3 cu ft left over; in bags of 2 cu ft it is 12 bags.
- A gravel path 30 × 3 ft, 4 in deep: 90 sq ft × 4 ÷ 12 = 30 cu ft, which is 1.111 cu yd. Sold by the whole cubic yard, that is 2 cu yd, with 24 cu ft left over, because 1 cu yd would be short. If the supplier sells half yards, enter 0.5 yd³ as the delivery unit: 1.111 ÷ 0.5 = 2.22, so 3 half yards (1.5 cu yd), with 10.5 cu ft left over. In bags of 2 cu ft it is 15 bags.
The mulch, soil and gravel calculator does exactly this. With US (ft, in, ft³, yd³) units, lengths are in feet and the depth in inches, and the result shows cubic feet and cubic yards. The bulk option takes the volume of your delivery unit, such as 1 yd³ or 0.5 yd³, and rounds up on its own, as how pack rounding works explains for every package.
Typing feet and inches
In the room planners, choose Feet and inches as the length unit to type lengths the way a tape measure shows them. These all mean 8 feet 6 inches, 8.5 ft or 2.5908 m: 8’ 6”, 8 ft 6 in and 8 6. A dot makes a decimal: 8.6 is 8.6 feet, which is 8 ft 7.2 in (2.62128 m), 1.2 inches more than 8’ 6”. The same mix-up appears in the measuring mistakes of how to measure a room.
- 12 1/2 is 12 feet and half an inch, because the second number is inches. For twelve and a half feet, type 12.5 or 12 6.
- 12 15 is refused: after the feet, the inches must be under 12, so it is not read as 13 ft 3 in.
- 30” with the inch mark is 30 inches alone.
On a whole room the difference adds up: a floor of 12’ 6” × 10’ 6” is 131.25 sq ft, but typed as 12.6 × 10.6 it becomes 133.56 sq ft.
The mulch, soil and gravel calculator works in decimal feet for lengths and inches for depths. There, 8’ 6” with the marks is still read as 8.5 ft, but 8 6 cannot be read and the field asks for a number. A depth in inches takes a fraction, such as 2 1/2.
Common mistakes
- Dividing square feet by 3 for square yards. 180 sq ft ÷ 3 = 60, three times the real 20 sq yd. Divide by 9.
- Dividing cubic feet by 3 or 9 for cubic yards. A cubic yard is 27 cubic feet.
- Writing inches as tenths of a foot. A 24 × 6 ft bed at 3 in is 36 cu ft (1.333 cu yd). With the depth typed as 0.3 ft, which is 3.6 in, it becomes 43.2 cu ft (1.6 cu yd): 22 bags of 2 cu ft instead of 18.
- Rounding a volume down. 1.111 cu yd is not “about a yard”: 1 cu yd leaves the path short.
- Mixing feet and inches in one product. Multiply only lengths in the same unit: 24 ft × 6 ft × 2 in gives 288, which is neither cubic feet nor cubic yards until the 2 in becomes 2 ÷ 12 ft.