Concrete Slope Calculator

How to use it

1. Decide which way the water goes. Run is measured along that direction, not along the longest side.
2. Work out the fall: run in feet × slope in inches per foot. A 12 foot patio at a quarter inch per foot needs 3 inches of fall.
3. If you are pitching the top and leaving the base flat, enter your nominal thickness at the low end and that thickness plus the fall at the high end.
4. If you are pitching the base instead, enter the same thickness in both fields, or just leave the high end blank. The volume does not change, because the slab is the same thickness everywhere; it only follows a tilted plane.

That second case is the whole decision this page exists to inform, and it is worth a lot of money on a big pour.

Slope conversion chart

Slope gets published four different ways and the trade mixes them freely. These are exact, not rounded.

Every entry in that last column is low by the same 4.17 percent of its own value, and the trade's literature does it systematically rather than occasionally. Going the other way, a true 2.00 percent is 0.24 inches per foot, which is 1:50 rather than 1:48, and that 0.01 inch per foot is an eighth of an inch of fall across a twelve foot patio. Invisible on site, decisive on an inspection report.

ACI publishes the rounding itself. ACI 302.1R gives its minimum drainage slope as "1/4 in. per ft (20 mm/m)". Twenty millimeters per meter is exactly 2.00 percent; the true conversion of a quarter inch per foot is 20.83 mm/m. So nobody is misquoting ACI when they say "ACI says 2 percent." ACI rounded its own figure down by four percent in a parenthetical and the entire industry inherited it. The one publisher we found getting it right is the Department of Energy's Building America Solution Center, which renders the building code's 2 percent as "approximately 0.24 inch per foot" and explicitly not as a quarter.

How much slope does a slab actually need?

There is no national answer, and the two industry camps differ by a factor of two.

The floors and slabs side says a quarter inch per foot. ACI 302.1R, section 4.4.1, "Establishing grades for adequate drainage": "when positive drainage is desired, the forms and screed guides should be set to provide for a minimum slope of 1/4 in. per ft (20 mm/m) to prevent ponding." The same document says positive drainage "should always be provided for exterior slabs." The Wisconsin Ready Mixed Concrete Association agrees: "a minimum slope of 1/4" per foot." So does essentially every contractor writing about flatwork.

The cement and ready-mix side says an eighth. The Michigan Concrete Association's 2025 driveway and sidewalk guidelines state that "a minimum slope of 1/8 inch per foot (1-2%) shall be maintained for drainage," and the Portland Cement Association's driveway figure, as reported by Concrete Network, is also a minimum of an eighth of an inch per foot.

Worth noting in passing: that Michigan parenthetical is wrong. An eighth of an inch per foot is 1.0417 percent, and there is no reading on which it is 2 percent. An earlier revision of the same document carries the identical slope sentence without the parenthetical, so a state concrete association added the error to its own guideline in 2025.

The building code does not help, because it is not about the flatwork. IRC section R401.3 requires that "the grade shall fall not fewer than 6 inches within the first 10 feet," which is exactly 5.00 percent, and its exception adds that "impervious surfaces within 10 feet of the building foundation shall be sloped a minimum of 2 percent away from the building." Both figures are about keeping water off the foundation. Neither says anything about a patio twelve feet from the house, a shed slab, or the far end of a driveway. The 2 percent sentence also sits inside the exception rather than in the main body, and building officials differ on whether it reaches a covered patio at all.

The code contradicts itself inside that one section, too: it requires 5 percent of dirt grade and accepts 2 percent on an impervious surface, in the same ten feet, for the same purpose. The concrete beside the lawn is allowed to be two and a half times flatter than the lawn.

For a garage floor the code asks for slope and declines to say how much. IRC R309.1 requires only that the parking area "shall be sloped to facilitate the movement of liquids to a drain or toward the main vehicle entry doorway." Garage floor and garage slab cover what practitioners actually use and why a quarter inch per foot across a 24 foot garage, which is six inches of fall, is more than most people want.

The only document we found that separates the design slope from the as-built slope is ACI 362.1R, for parking structures, and it is the honest way to think about all of this. It asks for a design minimum of 1.5 percent in any direction between a floor drain and any point on the slab, states that "the minimum slope as constructed should be 1 percent," and adds that the minimum effective slope "after volume changes and deflections have taken place" should also not be under 1 percent. Design slope, constructed slope and remaining slope are three different numbers, and only the third one drains water.

Why a sidewalk is nearly impossible to build

Put the two rulebooks side by side.

• The 2010 ADA Standards, section 403.3: "the cross slope of walking surfaces shall not be steeper than 1:48." The 2023 PROWAG final rule, R302.5.1, repeats it as "1:48 (2.1%) maximum."
• ACI 302.1R, section 4.4.1: minimum slope for positive drainage, 1/4 inch per foot.
• A quarter inch per foot is 1:48. One inch in forty-eight inches.

Build the drainage minimum and you are sitting on the accessibility maximum with no margin in either direction. The only published construction tolerance on slope we could find, a 2017 Colorado DOT memo adopting "a maximum tolerance on ramp slopes of +0.5 percent beyond the stated maximum," is five times wider than the window it would have to fit inside.

The two rulebooks do not even round the same slope the same way. PROWAG renders 1:48 as 2.1 percent, rounding 2.0833 up. ACI renders a quarter inch per foot as 20 mm/m, rounding it down to 2.00. The identical physical slope is published as 2.0 by one body and 2.1 by the other, and that tenth of a point is five percent of the entire allowance.

Practitioners have written about this and they do not agree with each other either. Jean Tessmer, a barrier consultant writing in Construction Specifier in 2014, put the real window at "between 0.5 and 2.1 percent" and recommended designing at 1 percent to leave room to succeed. Frank Salzano and Bruce Suprenant, two professional engineers writing in the same journal a year later, recommended "an absolute minimum slope of 1.5 percent (3/16 in./ft)" and observed that finishers "will have a difficult time consistently achieving a slope of less than one percent, if they can achieve it at all." One article's recommended design slope is the slope the next article says cannot reliably be built. Both are worth reading; neither is wrong.

And nobody official knows where 2 percent came from. TxDOT's own researchers wrote in 2001 that the requirement "possibly derives from construction standards for a minimum required slope for drainage purposes, but review of the literature has not determined the original basis for this requirement."

The basis exists, and the same agency paid for it thirty years earlier. In May 1971 Bob Gallaway, Robert Schiller and Jerry Rose published Texas Transportation Institute report 138-5 for the Texas Highway Department, measuring water depth on pavement at six cross slopes: 1/16, 1/8, 1/4, 3/8, 1/2 and 1 inch per foot. At 1.5 inches per hour of rainfall over a 24 foot drainage length, going from a sixteenth to a quarter inch per foot cut water depth by 62 percent. Going on to half an inch bought another 20 points, and a full inch another 15. The returns flatten sharply right around a quarter inch per foot, and the report writes that slope as "1/48" itself.

So in 1971 a highway drainage study identified 1:48 as the point where more slope stops paying, and in 2010 the Department of Justice set 1:48 as the steepest cross slope a wheelchair user should be asked to traverse. Two agencies, thirty-nine years apart, solving opposite problems, landed on the same ratio, and there is no evidence either knew about the other. One caution on the common reading of this: the Gallaway data shows water depth continuing to fall past a quarter inch per foot, to 82 and then 97 percent reduction. The marginal returns diminish sharply. They do not stop.

Our sidewalk calculator already carries the PROWAG width and cross slope rules, the state DOT targets, and the practical conclusion of building at 1.5 percent: concrete sidewalk calculator. For ramps, where the governing number is the 1:12 running slope rather than the 1:48 cross slope, use the ramp calculator.

What sloping a slab costs in concrete

A slab whose thickness runs linearly from one value to another is a prism with a trapezoidal cross section, and its volume is the length times the width times the average of the two thicknesses. That is the average end area method used in highway earthwork, reduced to its simplest case. It is also the same geometry as a ramp: set the thin end to zero and the trapezoid becomes a triangle and the formula collapses to half the rise, which is the solid wedge formula on the ramp page. One rule, two pages.

Sloping the top surface adds half the total fall to the average thickness, no matter how thick the slab is. So the extra concrete as a percentage depends only on the nominal thickness, the slope, and the run. On a nominal 4 inch slab held at 4 inches on the low side, at a quarter inch per foot:

A 2 percent fall over sixteen feet of run adds half again as much concrete. That is why the decision between pitching the top and pitching the base is not a detail.

To check the arithmetic against a published rate: Kim Basham gives the conversion as about 0.39 cubic yards per 1,000 square feet for every 1/8 inch the grade runs low, which is one inch of extra average thickness per 1,000 square feet costing 3.09 cubic yards. Our own 20 by 10 foot driveway example on the driveway calculator runs the same way: 3.09 cubic yards at a flat 5 inches, 3.40 once you slope it from 5 to 6 and average 5.5.

The failure mode nobody plans for is the opposite one. Screed a fall into forms set for a flat 4 inches and you do not get extra concrete, you get a thin low end. Holding the average at 4 inches over a 20 foot run at a quarter inch per foot puts the low end at 1.5 inches, and over 16 feet at 2 inches. Both are far below any published flatwork minimum, and the low end is exactly where the water collects and the freeze-thaw damage starts.

Slope the concrete, or slope the gravel?

ACI 302.1R specifies the surface. Its instruction is that "the forms and screed guides should be set to provide" the slope, which says nothing about the base and therefore thickens the slab toward the high end. Elsewhere in the same document, though, the fine grading material exists "to better control the thickness of the concrete," and its base tolerances are specified from the slab bottom elevation, which only makes sense if the base is shaped to follow the slab.

Published field practice resolves it in favor of the base. Writing for Fine Homebuilding, Matthew Burkholder lays out a reference elevation and the slopes, calculates top-of-concrete elevations across the pour, then subtracts the slab thickness to get the subgrade elevation, so the gravel runs at the same pitch as the finished surface and the slab stays a constant four inches. His 20 by 28 foot patio "needs to have at least 5 in. of fall to the outer edge," which is a quarter inch per foot over twenty feet.

So: ACI tells you where the top goes, the trade builds the bottom to match, and the concrete bill does not move. We could not find any document stating a threshold at which you should switch from one method to the other, and we are not going to invent one. What can be said with numbers is that the base tolerance is bigger than the thing you are trying to build. ACI 117-10 allows the fine grade under a slab-on-ground to sit within plus or minus three quarters of an inch, a band an inch and a half wide, while the total cross fall across a four foot walk at a quarter inch per foot is one inch. You cannot hold a constant thickness across a four foot walk at the drainage slope inside ACI's own grading tolerance.

The volume consequence of a base that is simply low rather than pitched is on the yardage calculator, where three quarters of an inch of low grade works out to 18.8 percent more concrete on a nominal 4 inch slab.

Pouring on a slope

Match the slump to the slope. That is John Carroll's rule, writing for Fine Homebuilding: a 3 inch slump works at a 1-in-20 slope, and on anything steeper you want a stiffer mix at 1 or 2 inches. The consequence is labor rather than material, because low-slump concrete does not flow, so you need hands to move it into position ahead of the screed.

That sits inside the published range rather than against it. ACI 211.1 and PCA table 9-6 give pavements and slabs a slump of 1 to 3 inches, so the answer to "what slump for a sloped pour" is already in the standard; the slope just tells you where in the range to sit. The Michigan Concrete Association sets a higher jobsite ceiling, 4 inches at the point of placement, rising to 7 with a mid-range or high-range water reducer, which is a different kind of limit: it is an acceptance figure, and the 7 inch version is conditional on an admixture so the water-cement ratio does not move. How to mix concrete has the full slump table and what each extra inch costs in strength.

Finish across the fall, not down it, and use a broom. A smooth trowelled surface is the wrong choice on anything sloped, for the obvious reason. How to pour a concrete slab covers screeding, floating and the order of operations.

On the limits, we are going to be honest about a gap. Every contractor page asserts a slope at which conventional placement stops working and you have to move to a stiffer mix, a top form, or shotcrete. We could not find that threshold in a code, a standard, an ACI document or a DOT specification. The thresholds that circulate, generally expressed in degrees, describe slopes around 30 degrees, which is a 58 percent grade, twenty-eight times a drainage slope. Those are retaining wall faces and spillways, not flatwork, and we are not going to repeat them as if they came from a standard. The same goes for the slope at which fresh concrete creeps downhill before it sets: no published figure, in either direction.

What is published is maximum slope for use rather than for placement. Concrete Network puts a driveway's practical ceiling at 15 percent, "since anything steeper than that can be hazardous to drive on during snowy or icy weather." For parking, ACI 330R's reported figures are 6 percent maximum where cars park and 8 percent at the entrance to stop cars dragging; we could not reach that document directly, so treat those two as secondhand.

If you want to hit a slope repeatably rather than approximately, Tessmer's techniques are the most specific published set we found: digital inclinometers accurate to a tenth of a degree mounted on the screed, intermediate screed guides on any pour wider than 10 to 12 feet, a rest of 12 to 30 minutes after initial screeding before the first consolidation pass, and checking on a 2 by 2 foot grid or continuously at 18 to 24 inches. A useful field calibration from the same article: a penny is a sixteenth of an inch thick, so a penny under one end of a two foot level is about half a percent.

How flat is too flat

The low end of the range has a harder floor than people expect, and one manufacturer's specification sits below all of it.

Gallaway's flattest tested cross slope was a sixteenth of an inch per foot, 0.521 percent, and it was the baseline that left the most water of the six. Salzano and Suprenant say slopes of "1/16 in./ft or less" are where "water will not drain because construction tolerances and deflection offset the shallow slope." ACI 362.1R puts the as-constructed floor at 1 percent.

Against that, hot tub manufacturers specify flatter. Watkins, which makes Hot Spring and Caldera, states that concrete "sloped at 1/2 inch per 10 feet (1 cm per 3 m) is preferred so that rain water and water will drain," which is 0.417 percent. Bullfrog allows a maximum of half an inch across an 8 foot run, 0.521 percent. Those are flatter than the flattest slope in the foundational drainage research, below what two ACI-affiliated engineers say can be built, and below ACI 362.1R's as-constructed minimum.

Watkins is not wrong for its own purpose. It wants the shell in continuous plane contact and it is trading drainage away to get it, which is the right call when a few thousand pounds of water is sitting on the slab. But it calls the figure a drainage slope, and as a drainage slope it is indefensible. Hot tub pad has the manufacturer specifications in full.

Related calculators

The yardage calculator has the same two-depth field and the chart of cubic yards by slab size, plus why an in-specification subgrade can cost you nearly 19 percent extra concrete. The slab calculator adds reinforcement and base. The driveway calculator carries the mix and thickness defaults for vehicle loads and the apron that thickens. The sidewalk calculator is the one to use on a pedestrian route, because it checks width, jointing and cross slope against PROWAG. The ramp calculator handles a deliberate slope you walk or drive up, where 1:12 rather than 1:48 is the governing number. For a curb and gutter, where the gutter cross slope is around 8.33 percent rather than 2, use curb and gutter.

Sources

• ACI 302.1R, Guide to Concrete Floor and Slab Construction, section 4.4.1 for the 1/4 in. per ft minimum drainage slope and section 3.2.2 for the fine grading material. The current edition is paywalled; the wording quoted here was read in the 1996 edition and is reported identically for the 2004 edition by Salzano and Suprenant below.
• ACI 362.1R-12, parking structures, for the 1.5 percent design and 1 percent as-constructed minimums, and ACI 211.1 with PCA table 9-6 for the 1 to 3 inch slump range for pavements and slabs. The 362.1R wording is quoted secondhand from Salzano and Suprenant.
• ACI 117-10 tolerances, via Kim Basham for ForConstructionPros: plus or minus 3/4 in. fine grade below slabs-on-ground, and the 0.39 cubic yards per 1,000 square feet per 1/8 inch rate.
• IRC sections R401.3 (drainage, 6 inches in the first 10 feet, and the 2 percent impervious surface exception) and R309.1 (garage floor slope, no figure). Section numbering has moved between editions; R309.1 is the 2021 numbering. The ICC text is paywalled and was read through permitted plan sets and the Department of Energy's Building America Solution Center, which also carries the ENERGY STAR requirement of at least 0.25 inches per foot on impervious surfaces.
• 2010 ADA Standards section 403.3 (cross slope not steeper than 1:48) and the US Access Board's 2023 PROWAG final rule chapter R3, R302.4 and R302.5.1. PROWAG states no minimum slope for drainage anywhere.
• Gallaway, Schiller and Rose, "The Effects of Rainfall Intensity, Pavement Cross Slope, Surface Texture, and Drainage Length on Pavement Water Depths," Texas Transportation Institute report 138-5, May 1971, for the six tested cross slopes and the measured reductions in water depth.
• Kockelman, Heard, Kweon and Rioux, TxDOT report FHWA/TX-03/4171-1, for the admission that the basis of the 2 percent requirement could not be determined, and for the City of Roseville's practice of setting forms at 1.5 percent to land inside 2.
• Frank Salzano and Bruce Suprenant, "Specifying broomed exterior concrete surfaces," Construction Specifier, 2015, and Jean Tessmer, "Making 'Impossible' ADA Slopes Possible," Construction Specifier, 2014. These two disagree and both are cited for that reason.
• Colorado DOT, construction tolerance for ADA curb ramps memo, 2017, for the plus 0.5 percent acceptance tolerance.
• Association guidelines: Michigan Concrete Association 2025, Wisconsin Ready Mixed Concrete Association, and Concrete Network. The ACI 330R parking figures are reported through a continuing education course rather than from the document itself.
• Field practice: Matthew Burkholder and John Carroll, both Fine Homebuilding.

The conversion chart, the fall-by-run table and the extra-concrete percentages are arithmetic on the sources above rather than figures any of them publish. No source we could find states a slope at which conventional placement must change method, or a slope at which fresh concrete creeps before it sets, and we have said so rather than filled the gap.