Plumbing Math, Sizing Calculations, and Plan Reading
Conversions worth memorizing
- 1 cubic foot = 7.48 gallons; 1 gallon of water weighs 8.34 pounds.
- 1 psi = 2.31 feet of head; 1 psi is about 27.7 inches of water column.
- Grade: ¼ inch per foot is about 2.08 percent; ⅛ inch per foot is about 1.04 percent.
- 45-degree offsets: travel = 1.414 × offset, and the horizontal advance equals the offset.
- Pipe volume: area × length, with area = πr². Convert diameter to feet first.
Plan reading
Check the stated scale and verify it against a known dimension before scaling anything. Read the floor plan for locations and the riser diagram for pipe sizes, fixture units, and venting. Invert elevations give you fall; subtract inverts to verify slope. The fixture schedule and general notes carry code items that are easy to miss when you only look at the drawing.
Worked example
How many gallons in 100 feet of 2-inch pipe? Radius = 1 inch = 0.0833 ft. Area = π(0.0833)² = 0.0218 ft². Volume = 0.0218 × 100 = 2.18 ft³. Gallons = 2.18 × 7.48 = 16.3 gallons. A second example: an 8-inch offset at 45 degrees needs 1.414 × 8 = 11.3 inches of travel, and the run advances 8 inches.
Traps
- Using diameter in place of radius.
- Mixing inches and feet in the same formula.
- Scaling a reduced print without confirming the stated scale.
- Reading the shortest gas run or the wrong column in a sizing table.
- Forgetting that fall along a trap arm is limited to one pipe diameter.