Slope Calculator: Grade, Percent, and Inches per Foot

Three Ways to Say the Same Thing

Slope gets expressed three different ways depending on who you’re talking to. They all describe the same physical thing: how much the ground drops over a horizontal distance.

Percent grade: Rise / Run x 100. A 2% grade drops 2 feet over 100 feet.

Inches per foot: Rise in inches per 1 foot of horizontal run. A 1/4” per foot slope drops 0.25 inches every 12 inches.

Ratio: Rise:Run. A 1:48 slope drops 1 unit for every 48 horizontal units.

The conversions:

Percent	Inches per Foot	Ratio
1%	1/8” per ft	1:100
2%	1/4” per ft	1:50
4%	1/2” per ft	1:25
8.33%	1” per ft	1:12
25%	3” per ft	1:4

The conversion formula: Percent x 0.12 = inches per foot. Or: inches per foot / 0.12 = percent.

Drainage: What the Codes Require

Surface Drainage (Yard Grading)

IRC Section R401.3: the ground around a foundation must slope away at a minimum of 6 inches within the first 10 feet. That’s 5% grade, or about 5/8” per foot.

In practice, most builders grade at 2-3% beyond the initial 10-foot zone. Less than 2% and water pools in low spots. More than 5% and you’ve got erosion problems during heavy rain.

Sewer and Drain Pipe

IPC Table 704.1 (and IRC P3005.3) minimum slopes for horizontal drain pipe:

• 2-1/2” pipe and smaller: 1/4” per foot (2.08%)
• 3” to 6” pipe: 1/8” per foot (1.04%)
• 8” and larger (IPC): 1/16” per foot

Those are minimums. The codes don’t set a maximum slope for horizontal drains, and building drain capacity in IPC Table 710.1(1) goes up with slope.

For more on drain pipe sizing, see the IPC DFU table guide.

Concrete Flatwork

Patios, sidewalks, and garage aprons should slope away from the building at 1/4” per foot minimum. If you’re pouring a slab, the concrete calculator handles the volume math. ADA requires wheelchair ramps at 1:12 (8.33%, or 1” per foot) maximum with 1:20 (5%, or 5/8” per foot) for accessible routes.

A 4-foot wide sidewalk with 1/4” per foot cross-slope drops 1 inch from one edge to the other. Enough to shed water, not enough to feel tilted when walking.

French Drains

Minimum 1% grade (1/8” per foot) for the perforated pipe. Most contractors run at 1-2%. The pipe collects water along its length, so even with minimal slope the water volume increases downstream and keeps things flowing. A 50-foot French drain at 1% drops 6 inches from inlet to outlet.

A Real Grading Problem

You’ve got a backyard patio that’s 20 feet from the house and 40 feet wide. The grade from the house to the property line (another 30 feet past the patio) needs to be consistent.

From the foundation to the patio edge (20 feet) at 2% grade: 20 x 0.02 = 0.4 feet = 4.8 inches of fall

From the patio edge to the property line (30 feet) at 2% grade: 30 x 0.02 = 0.6 feet = 7.2 inches of fall

Total fall over 50 feet: 12 inches. One foot of grade change from the house to the back fence.

If the existing grade only gives you 8 inches of total fall, you’ve got two options: steepen the slope close to the house (where it matters most) and flatten it farther out, or bring in fill to build up the grade near the foundation. Bringing in fill is easier than cutting grade away from a property line where the neighbor’s lawn might be higher.

Setting Grade Stakes

For a 50-foot sewer trench at 1/8” per foot:

Total drop: 50 x 0.125 = 6.25 inches

Set your reference stake at the high end (the building). Drive stakes every 10 feet. Each stake should be 1.25 inches lower than the previous one (10 x 0.125).

Use a laser level or a transit if you have one. A string line works over short distances but sags over runs longer than 20-25 feet, and the sag gives you a false reading that makes the middle of your trench too deep.

The Math That Trips People Up

Converting between formats confuses a lot of people because the units aren’t obvious.

If someone tells you “2% grade” and you need inches per foot: 2 x 0.12 = 0.24 inches per foot. That’s just under 1/4”.

If the plans show “1/4 inch per foot” and the inspector asks for percent: 0.25 / 0.12 = 2.08%.

If you measured a 3-inch drop over 25 feet and need percent: 3 inches = 0.25 feet. 0.25 / 25 = 0.01 = 1%.

Rise and run must be in the same units before you divide. Mixing inches and feet is where the errors happen.

Steep Slopes

Once you get past about 33% (1 vertical in 3 horizontal), you’re out of ordinary grading and into slope stabilization. IRC R403.1.7 sets building and footing setbacks from slopes steeper than that. Retaining walls, terracing, or engineered fill come into play.

For landscape reference:

• 0-5%: Easy to mow, easy to drain
• 5-10%: Still mowable, water moves noticeably
• 10-25%: Difficult to mow, needs ground cover or terracing
• 25-50%: Usually needs retaining walls or erosion control
• Over 50%: Get a geotechnical or civil engineer involved

Running the Numbers

Trussly has a slope calculator that converts rise and run into percent grade, inches per foot, ratio, and degrees, and flags grades under the IRC R401.3 foundation drainage minimum. The Drain Slope calculator uses the IRC P3005.3 minimums for pipe. Faster than looking up the conversion table every time you’re on a different kind of job.

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