Drain Slope Calculator
IPC and UPC plumbing codes require drain pipes to slope 1/4" per foot for 3" diameter and smaller, and 1/8" per foot for 4" and larger. Enter pipe length and slope to get total drop, or enter drop to get slope percentage.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
What is the minimum drain slope required by code?
The International Plumbing Code (IPC) requires a minimum slope of 1/4" per foot (2%) for drain pipes 3" diameter and smaller, and 1/8" per foot for 4" and larger pipes. Slopes steeper than 1/2" per foot can cause solids to separate from liquids, causing blockages.
How do I measure drain slope in the field?
Use a 4-foot level with a ruler: place the level on the pipe, raise one end until it reads level, then measure the gap at the low end. A 1" gap over 4 feet = 1/4" per foot slope. A digital level/inclinometer reads slope percentage directly.