Slope / Fall Calculator for Drain Pipes

Gravity drainage is a bet on geometry: enough fall per foot keeps water and solids moving without puddling, and too much fall only makes the water outrun the solids. The slope is the quiet spec nobody checks until a line plugs.

Slope / Fall Calculator converts between inches-per-foot, percent and ratio, and works out the total fall for any run.

• Slope to fall: Total drop for your run.
• Fall to slope: Back out the grade you achieved.
• Three units: Inches per ft, percent, ratio.
• Per-10-ft reference: Quick field check.
• Min-slope guide: What small and large pipe require.
• Export: Copy or download the report.

1. Enter the run length in feet.
2. Pick a mode: give a slope to find fall, or give a fall to find the slope.
3. Read the conversions and the total drop.
4. Check it against the min-slope guide for your pipe size.

Example - 40 ft of 4 in sewer. At the 1/8 in/ft minimum, the far end of the line sits 5 in lower than the start. Raise it to 1/4 in/ft and the drop becomes 10 in - the invert elevation changes by that much at the wall cleanout.

• Layout numbers fast: invert drops for your run.
• Field and office: the per-10-ft check is handy on site.
• Any unit: percent for specs, ratio for drawings, inches for the level.
• Free and private: no uploads, no logging.