Slope Conversion: Percent, Ratio, and Inch-Per-Foot in Plain English
Introduction: Who Uses Slope on a Job Site?
Slope is a critical, real-world measurement that nearly every trade on a construction site relies on. Because different professions use different tools and work on different parts of a project, they have evolved their own unique “languages” to describe the exact same incline.
When these trades collaborate on the same job, a lack of communication can cause major headaches. Learning how to translate between these vocational languages ensures everyone stays on the same page.
If you are new to the trades or managing a DIY project, here are the primary vocations you will encounter and the specific slope languages they speak:
Landscapers & Civil Engineers
These pros live in the world of site drainage, grading, and outdoor earthworks. They talk about slope in percentages (like a 2% slope) to make sure a patio or yard channels rainwater safely away from a building’s foundation.
Plumbers & Drainlayers
Tasked with gravity-fed waste lines, these trades think in ratios (written as “1 in X,” like 1 in 50). Getting this exact gradient right ensures that wastewater moves at the perfect speed to carry waste away without leaving solids behind.
Carpenters & Tilers
When building a deck frame or prepping a indoor shower floor, these craftspeople measure slope as a physical rise-per-run drop (such as a quarter-inch per foot). They speak this language because it matches what they can easily mark on a board with a pencil and tape measure.
Roofers
Roofers speak their own distinct dialect called pitch, which is always expressed as a ratio out of 12 (like 6:12). They use specialized protractor squares to translate the pitch on a blueprint directly into a physical angle cut on a timber rafter.
The Tradesperson’s Guide to Slope Conversions
A plumber writes “1 in 80” on a drawing. The landscaper reading it calls the same line “1.25%.” The carpenter building the deck beside them calls it “a hair under 3/16 of an inch per foot.” Three trades, three numbers, one slope.
The confusion is not academic. It’s how a waste pipe ends up running flat, or a patio sheds water back toward the house instead of away from it.
Slope is just rise over run, how far something drops across how far it travels. Every trade measures the same thing. They just write it three different ways. This guide is the slope conversion reference that lets you read all three.
The three ways to express slope
Percent. Rise divided by run, times 100. A 2% slope drops 2 units for every 100 units of length. Civil work, site drainage, and landscaping live in percent.
Ratio, or “1 in X.” One unit of fall for every X units of length. “1 in 50” is the same 2%. Plumbers and drainlayers think in ratios because that’s how pipe gradients are specified.
Rise per run. A measured drop over a fixed length, inches per foot in the US, millimeters per meter most everywhere else. Carpenters and tilers work here because it’s what you actually mark on a board: “quarter inch per foot.”
Slope conversion table
They’re interchangeable. Here’s the same handful of slopes in all three systems, plus the angle in degrees:
| Percent | Ratio (1 in X) | Inch per foot | mm per meter | Degrees |
| 1% | 1 in 100 | ≈ 1/8″ | 10 mm | 0.6° |
| 1.25% | 1 in 80 | ≈ 5/32″ | 12.5 mm | 0.7° |
| 2% | 1 in 50 | ≈ 1/4″ | 20 mm | 1.1° |
| 2.08% | 1 in 48 | 1/4″ (exact) | 20.8 mm | 1.2° |
| 8.33% | 1 in 12 | 1″ | 83 mm | 4.8° |
The math behind it, if you need a value that isn’t on the chart:
- Inch-per-foot to percent: divide the rise by 12 and multiply by 100. A 1/4″ drop over a foot is 0.25 ÷ 12 × 100 = 2.08%.
- Millimeters-per-meter to percent: just divide by 10. 20 mm/m ÷ 10 = 2%. To go back, multiply percent by 10 — 2% is 20 mm/m. (Metric makes this one easy: percent and mm/m are the same number times ten.)
- Percent to ratio: flip the decimal. 2.08% is 0.0208, and 1 ÷ 0.0208 ≈ 48, so 1 in 48.
- Percent to degrees: take the arctangent of the decimal. 0.0208 → 1.2°. (Not the same as percent. More on that below.)
Standard slopes worth memorizing
- Waste and drain pipe: roughly 1/4″ per foot — about 2%, 1 in 48, or 20 mm per meter — for pipes up to 3″. Larger lines can run 1/8″ per foot (1%, ~10 mm/m). Too little and solids sit; too much and water outruns the solids and leaves them behind.
- Site and patio drainage: about 2% (20 mm per meter) away from any building, so surface water clears the foundation.
- Shower floor: 1/4″ per foot, or 20 mm per meter, toward the drain is the standard target.
- Accessible ramps: 8.33% maximum — that’s 1 in 12, 1″ of rise per foot, or 83 mm per meter. Steeper isn’t compliant for new construction, per the U.S. Access Board ADA ramp standards.
How to set a slope on site
Knowing the number is half of it. On site you have to build it.
The low-tech method still works: rest a level on the run, then shim one end until the bubble centers. The shim thickness over the level’s length is your rise — do the division and you have your slope. For drainage you can skip the math entirely with a level that has fixed slope vials; the 105 TOPGRADE reads dead-level plus 1% and 2% directly, so a drain run is just “center the 2% bubble.”
Roof work speaks its own dialect — pitch, written as rise-in-12, like 6:12. That’s an angle, and the cleanest way to set or check it is to read the angle itself. A protractor square like the 449 gives you a direct degree reading, coarse and fine, with common rafter measurements alongside it, so you can move between the pitch on the plan and the cut on the timber without guessing.
Percent vs degrees: the mistake to avoid
Degrees and percent are not the same scale, and treating them as if they are will wreck a job. A 45° slope is 100%, not 45% — because at 45° the rise equals the run. Percent climbs toward infinity as the slope approaches vertical; degrees stop at 90. For the shallow slopes most trades deal with they track closely (2% is about 1.1°), but never assume “2 percent” and “2 degrees” are interchangeable. They’re nearly double.
Slope conversion FAQ
What is 2% slope in inches per foot?
About 1/4 inch per foot (0.24″) or 20 mm per meter in metric, which is also a 1-in-50 ratio and roughly 1.1°.
What slope is 1 in 80?
1.25% a common minimum fall for drainage. That’s about 5/32″ per foot, or 12.5 mm per meter.
How do I convert mm per meter to percent?
Divide by 10. So 20 mm/m is 2%, and 15 mm/m is 1.5%. To go the other way, multiply percent by 10.
How do I convert percent slope to degrees?
Divide the percent by 100, then take the arctangent. So 8.33% → 0.0833 → 4.8°. They are not the same number: 100% equals 45°.
What is the minimum slope for a drain pipe?
Commonly 1/4″ per foot, about 2% or 20 mm per meter, for pipes up to 3″; larger pipes are often run at 1/8″ per foot (10 mm/m). Always check your local plumbing code.
Slope is one of the few things every trade on a site shares and almost nobody writes the same way. Learn to move between the three and you stop being the person on the job who has to ask what “1 in 80” means. For more on choosing the right tool for layout and leveling, see the Kapro Knowledge Center.
Download the free printable slope conversion cheat sheet now — percent, ratio, inch-per-foot, mm-per-meter, and degrees, all on one page you can keep in your tool bag on site.
