How to Calculate Drainage Slope: Complete Guide with Calculator
Learn how to calculate proper drainage slope for sewer lines and drains. Step-by-step guide with drainage slope calculator, code requirements, and installation tips.
Step-by-Step Guide
1. Understand Slope Requirements
Minimum slopes per IPC/UPC: 1-1/2" to 3" pipe = 1/4" per foot (2% slope), 4" pipe = 1/8" per foot (1% slope), 6"+ pipe = 1/16" per foot (0.5% slope). These are minimums for self-cleaning velocity.
2. Measure Pipe Length
Measure the total horizontal length of the drain line. For sloped runs, use horizontal distance, not the sloped length. Include all sections of the drain line.
3. Determine Pipe Size
Identify the pipe diameter. Common sizes: 1-1/2" for sinks, 2" for showers/tubs, 3" for toilets, 4" for main drains, 6"+ for sewer lines. Larger pipes require less slope.
4. Calculate Required Drop
Drop = Length × Minimum Slope. For 20 feet of 2" pipe: 20 × 1/4" = 5 inches total drop. For 4" pipe: 20 × 1/8" = 2.5 inches drop. Use our Drainage Slope Calculator for exact calculations.
5. Verify Velocity
Check that slope provides minimum 2 ft/s velocity for self-cleaning. Higher slopes (up to 1/2" per foot) acceptable for short runs. Avoid slopes steeper than 1/2" per foot.
Frequently Asked Questions
Code Compliance
Drainage slope must comply with IPC Chapter 7 or UPC Chapter 7. Minimum slopes ensure self-cleaning velocity. Local codes may have additional requirements for specific applications.