🔧 Drain Slope Calculator
Find the right fall for a sink drain line — the total drop and percent grade from a run and slope, or the slope from a measured drop — so your pipe drains cleanly and meets code.
🔧 Run & Slope
What is a Drain Slope Calculator?
It turns a drain run into the fall it needs to flow properly. Enter the horizontal length and the slope per foot and it returns the total vertical drop and the grade as a percent; enter a measured drop over a run instead and it returns the effective slope, so you can check an existing line.
Use it to lay out a new sink or branch drain to the 1/4-inch-per-foot standard, to set a laser or level to the right pitch, or to diagnose a slow drain that may simply be too flat. The figures are for planning — always confirm against your local plumbing code.
❓ Frequently Asked Questions
How much slope does a sink drain need?
The plumbing-code standard is a fall of 1/4 inch per foot — 0.25 inches for every foot of horizontal run — for drain pipe 3 inches and smaller, which covers sink, lavatory, and most branch lines. For larger pipe over 3 inches, 1/8 inch per foot is allowed. This calculator defaults to 1/4 inch per foot and lets you enter your own.
What happens if the slope is too shallow or too steep?
Too little slope lets water move slowly and leaves solids and grease behind, which builds into clogs. Too much slope can drain the water faster than it can carry the solids with it, again stranding waste in the pipe. The 1/4-inch-per-foot target keeps the flow fast enough to self-scour without outrunning the solids.
How do I turn slope into a percent grade?
Divide the fall per foot in inches by 12 (inches in a foot) and multiply by 100. A 1/4-inch-per-foot slope is 0.25 divided by 12, times 100, which is about 2.08% grade. This tool shows the percent grade alongside the total drop so you can set a level or laser to the right pitch.
Can I check the slope of a drain that's already installed?
Yes — switch the calculator to work backward. Measure the total vertical drop between the two ends of the run and the horizontal length in feet, and it returns the effective slope per foot and percent grade. That tells you whether an existing line meets the 1/4-inch-per-foot standard or is running too flat.