Drain Slope & Grade Calculator

Determine pipe size and required slope

Application: kitchen sink drain, 20-foot run from sink to main stack. Drain diameter: 2-inch (standard for single kitchen sink). IPC requirement for 2-inch drain: 1/4 inch per foot minimum slope. Required total drop: 20 ft x 0.25 in/ft = 5 inches. If the inlet is at 48.0 inches above finished floor, the outlet must be at 48.0 - 5.0 = 43.0 inches. Verify this outlet elevation is above the main drain or building drain invert -- if not, the run must be shortened or the pipe must be rerouted.

Check available drop against structural constraints

Run location: between floor joists, 9.5-inch joist depth. Available drop within joist bay: 9.5 - 2.375 (2-inch pipe OD) = 7.125 inches maximum from top of joist to bottom of joist. With 5-inch required drop over 20 feet, the pipe starts near the top of the joist bay (clearance adequate) and ends 5 inches lower -- still within the joist depth. No notching or boring required. Slope is achievable.

Verify maximum slope not exceeded

Maximum IPC slope for 2-inch drain: 1/2 inch per foot. At 20-foot run: maximum drop = 10 inches. Actual drop 5 inches -- within maximum. If space were more constrained (e.g., sloped ceiling requiring steeper drain slope), verify the slope does not exceed 1/2 in/ft. At excessive slope, self-scouring velocity is lost as liquid outpaces solids.

Calculate outlet invert for multiple fixture connections

Main drain (4-inch) runs 30 feet from cleanout to building drain. At 1/8 in/ft minimum: required drop = 30 x 0.125 = 3.75 inches. Kitchen drain connection 20 feet from cleanout: outlet invert at that point = cleanout invert - (20 x 0.125) = cleanout invert - 2.5 inches. 2-inch kitchen drain must tie in at or above this elevation to flow by gravity into the 4-inch main.