Drain Pipe Slope Calculator (According to International Plumbing Code)

This drain pipe slope calculator will calculate the slope and total drop (fall) of a drainage pipe over a specified length of pipe.

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Drain Pipe Anatomy

The following diagram lays out the different terms that are used in the calculator:

drain pipe slope diagram

How to Use the Calculator

First, figure out the diameter of the pipe you are working with. For household sinks, the drain pipe diameter is often either 1.5″ or 2″. Toilet waste lines are often 3″ or 4″. All the fixtures in your house will connect to a main drain line, which is usually 4″ in diameter.

If you need to know the thickness, outside diameter or inside diameter of a PVC pipe, check out this calculator.

Next, measure the length of the pipe you are working with. This will allow the total pipe drop (or fall) to be calculated. If you just want to see the required slope per foot, skip this step.

Hit calculate to see the results.

If you want to make things really easy when getting the proper pitch, get yourself a digital level like this one:

It will automatically calculate the angle of any pipe you put it on top of.

International Plumbing Code

The slopes given in the calculator vary based on the diameter of the pipe. The International Plumbing Code states what these slopes should be.

According to the International Code Council, 35 states follow the International Plumbing Code. If you live in one of the following states that does not follow the IPC, please check with your local state code for the required drainage slopes:

Alaska—California—Hawaii—Idaho—Kentucky—Louisiana—Main—Massachussets—Minnesota—Montana—New Jersey—North Dakota—Oregon—South Dakota—Wisconsin

Tips on Calculating Drain Pipe Slopes

    • Use the longest level possible for your application for the best accuracy
    • When the bubble on a level is about 1/4 of the way past the line it is about a 1/4″ slope. Same logic with 1/8″ slope, 1/16″ slope, etc.

Indoor Plumbing

    • Dry fit PVC/ABS components first to ensure there is enough space to properly slope the entire pipe
    • Check out the pros and cons of ABS vs PVC pipe here
    • A slight rotation of the end fitting (elbow or tee) may be necessary to accommodate a sloped pipe

French Drains

    • To calculate gravel for a French drain check out the French drain calculator
    • After calculating total pipe drop, ensure the end of the French drain will drain to daylight and not end underground
    • Make the trench deep enough to bury the whole pipe (except for the end)
    • Weigh down pipe before back-filling to prevent pipe from lifting

When installing a drain pipe, always make sure it has the proper slope throughout the length of the pipe to prevent backups and overflows. Ensuring this will keep your plumbing system healthy and keep you happy.