Drain Slope Calculator
Drop, run, and slope — for plumbing drains, gutters, and shower pans.
How to use
Type any two of (slope, run, drop) and the third updates. Slope can be a standard value (1/4″, 1/8″) or a custom percentage.
- Enter the run in feet — the horizontal length of the pipe.
- Pick a standard slope (1/4″ per foot, 1/8″ per foot, 1/16″ per foot) or enter a custom percent.
- The drop is the vertical fall over that run. Read it in inches and millimetres.
- Plumbing codes typically require 1/4″ per foot for 1.5″–3″ pipe and 1/8″ per foot for 4″ and larger.
Reviewed 3 June 2026 · methodology cited
About this calculator
Plumbing drains carry waste downhill by gravity. The slope — how much the pipe drops over its horizontal run — needs to be enough that solids move along with the water but not so much that the water races ahead, leaving solids behind. The Goldilocks slope, codified in the International Plumbing Code (IPC), the Uniform Plumbing Code (UPC), and Canada's National Plumbing Code (NPC), is 1/4 inch per foot for smaller pipes and 1/8 inch per foot for larger ones.
This calculator gives you the drop across any run at the standard slopes, plus a custom-percentage input for gutters, shower pans, and non-code applications. It does not size the drain, calculate fixture units, or check vent requirements — that is a plumber's job, not a calculator's.
The math
Drop = run × slope. With slope expressed as inches per foot, drop in inches = run (ft) × slope (in/ft). With slope as percent (rise/run × 100), drop in inches = run (in) × percent / 100. In SI: drop (mm) = run (mm) × slope ratio.
The standard slopes convert to percentages: 1/4″ per foot = 2.083 %; 1/8″ per foot = 1.042 %; 1/16″ per foot = 0.521 %. Going the other way: 1 % = 1/8.4″ per foot; 2 % = 1/4.2″ per foot.
A worked example: a 30-foot run at 1/4″ per foot drops 7.5″ (190.5 mm). The same run at 1/8″ per foot drops 3.75″ (95.25 mm). For a kitchen gutter at 1/200 slope (0.5 %), a 20-foot run drops 1.2 inches.
Code-required slopes
| Pipe size | Required slope | Common use |
|---|---|---|
| 1-1/4″ to 2-1/2″ | 1/4″ per ft (2.08%) | Lav, sink, shower drain |
| 3″ | 1/4″ per ft (2.08%) | Toilet branch, kitchen sink |
| 4″ | 1/8″ per ft (1.04%) | Building drain, main sewer |
| 6″ | 1/16″ per ft (0.52%) | Larger building sewers (code permitting) |
| 8″ and larger | 1/16″ per ft (0.52%) | Municipal and large commercial |
| Gutter (residential) | 1/16″ per ft (0.52%) | To downspout |
| Shower pan (per IPC) | 1/4″ per ft (2.08%) | To drain |
Notes for plumbing and gutters
Plumbing slope is direction-sensitive: drains slope away from the fixture toward the main, vents slope toward the drain. Below 1/8″ per foot on a 4″ pipe, solids tend to stick; above 1/2″ per foot on any pipe, the water races ahead and leaves them behind. The code minimums exist for fluid-mechanical reasons, not arbitrary tradition.
Gutters and shower pans use different slopes. A roof gutter typically slopes 1/16″ per foot (0.52 %) to a downspout. A shower pan slopes 1/4″ per foot toward the drain — the same as small plumbing drains. A wet-room or curbless shower may use a linear-drain detail with the entire floor sloped 1/8″ per foot or less, but those installations need a careful waterproofing membrane and proper substrate prep. Always confirm the actual slope against the manufacturer's installation instructions for the drain or fixture you are installing — and once again: real plumbing installation belongs to a licensed plumber.
Frequently asked questions
Why does drain slope matter?
Slope determines whether solids move along with the water. Too shallow and solids stick (causing blockages). Too steep and water races ahead, leaving solids behind to dry out and clog. Plumbing codes settle on 1/4″ per foot for smaller pipes and 1/8″ per foot for larger pipes as the slopes that reliably scour solids without dewatering them.
Can my drain be steeper than the code minimum?
The code says "not less than" — so steeper is permitted, up to a maximum. The IPC allows up to a 45-degree slope on a drain (about 1 ft drop per 1 ft run) — anything steeper is treated as a vertical riser, not a horizontal drain. Most practical installations stay between 1/8″ and 1/2″ per foot.
How do I express 1/4″ per foot as a percent?
1/4 inch ÷ 12 inches × 100 = 2.083 %. Or expressed as a ratio: 1:48. Engineers and code documents often use the percent form; plumbers more often use the inches-per-foot form.
What slope for a shower pan?
IPC and UPC both require 1/4″ per foot minimum for shower pans. Some manufacturers recommend up to 1/2″ per foot for steeper drainage. For curbless or barrier-free showers using a linear drain, the slope drops to 1/8″–3/16″ per foot, but the installation requires a waterproofing membrane behind the tile or finished surface.
What about backwater from a vent?
This calculator only computes slope and drop. Drain venting, p-trap protection, and backwater valves are separate code requirements — vent stack heights, dry-vent distance limits, and combination drain-and-vent arrangements all live in the plumbing code and require a licensed plumber to design and inspect.
Can I use this for a gutter?
Yes — pick "Custom" and type the slope you want. A typical residential rain gutter slopes 1/16″ per foot (about 0.5 %) — slow enough that the gutter still drains without water sloshing past the outlet. Larger commercial gutters with internal scuppers use steeper slopes or staged drainage with multiple downspouts.