Drainage Slope Calculator
Enter your rise (in inches or feet) and horizontal run to find the slope percentage, ratio, angle, and whether it meets typical drainage minimums.
Enter the elevation change over the measured run — e.g. rise of 6 in over a 50 ft run.
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Code guidance
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Details
A 6 in rise over a 50 ft run works out to a 1.00% slope (a ratio of 1 in 100, or 0.57°) — this meets the typical minimum grade for drainage away from a structure.
Slope percentage for common rise-and-run combinations
Run is held at 50 ft in each row so you can see how rise alone changes the slope percentage, ratio, and verdict.
| Rise | Run | Slope % (ratio) | Verdict |
|---|---|---|---|
| 3 in | 50 ft | 0.50% (1 in 200) | Below typical minimum |
| 9 in | 50 ft | 1.50% (1 in 66.7) | Meets typical minimum |
| 18 in | 50 ft | 3.00% (1 in 33.3) | Good |
| 4 ft | 50 ft | 8.00% (1 in 12.5) | Steep |
| 8 ft | 50 ft | 16.00% (1 in 6.3) | Very steep |
How to calculate drainage slope
- Measure your rise. Find the vertical drop or rise between your two points — in inches for smaller grade changes or feet for larger ones.
- Measure your run. Find the horizontal distance between the same two points, in feet.
- Convert rise to feet if needed. If you measured rise in inches, divide by 12 to convert it to feet so it matches your run’s units.
- Divide rise by run and multiply by 100. This gives you the slope as a percentage — the standard way drainage and grading slopes are specified.
- Compare against typical drainage minimums. A slope of at least 1-2% is the typical minimum for gravity drainage; 2-5% is considered a good working grade for most drainage applications.
Worked example
Using the calculator’s defaults: a 6 in rise over a 50 ft run.
- Rise entered6 in
- Run50 ft
- Rise converted to feet (6 ÷ 12)0.5 ft
- Slope percentage (0.5 ÷ 50 × 100)1.00%
- Slope ratio (50 ÷ 0.5)1 in 100
- Slope angle0.57°
- Drop per foot of run (1.00% × 12)0.12 in/ft
- VerdictMeets typical minimum
Choosing the right drainage slope
The right slope depends on what you’re draining — too flat and water pools, too steep and it erodes or runs too fast to be useful.
- Grading away from a foundation.A minimum of 2% slope (1/4 in per foot) is a common standard for the first 6-10 ft of grade away from a house.
- Gravity drain pipe (French drains, sewer lines).A minimum slope of 1-2% is typical, with 2% often preferred so solids and debris don’t settle and clog the pipe over time.
- Swales and drainage ditches.2-5% is a common working range — enough to move water without the flow becoming fast enough to erode the channel.
- Patios, walkways, and driveways.1-2% is typical for hard surfaces — enough to shed water without feeling noticeably tilted underfoot.
- When a slope reads 'very steep'.Above about 12%, consider terracing, retaining walls, or erosion-control measures rather than a single continuous graded slope.
Common mistakes
- Mixing inches and feet without converting.Rise and run must be in the same units before dividing — forgetting to convert inches of rise to feet gives a slope 12x too steep.
- Confusing slope percentage with slope angle.A 1% slope is not 1 degree — percentage and angle are related but different measures; a 100% slope is a 45° angle, not 100°.
- Assuming flat ground drains fine.Standing water and soggy spots are usually a sign the grade is at or below a 0-1% slope, which is too flat for reliable gravity drainage.
- Overcorrecting to a very steep slope.A slope steeper than about 12% can erode, wash out mulch or gravel, and become a fall hazard — aim for the minimum that reliably drains.
- Measuring rise and run over too short a distance.A short measurement is more sensitive to small errors — measure over the longest practical run for a more reliable average slope.
- Ignoring the ratio notation on drawings.Site plans often specify slope as a ratio like ‘1 in 50’ rather than a percentage — confirm which format you’re reading before grading.
What this calculator assumes
Adjust your result if your project differs from these.
- Slope percentage is calculated as rise divided by run, multiplied by 100, both measured in the same units.
- Rise entered in inches is converted to feet (÷ 12) before the percentage is calculated.
- The slope ratio is expressed as ‘run to rise’ (e.g., 1 in 100 means 100 ft of run per 1 ft of rise).
- Verdict bands (below minimum, meets minimum, good, steep, very steep) reflect general drainage guidance, not a specific code requirement.
- The calculation assumes a constant, uniform grade between the two measured points.
- Local codes and manufacturer specs (for pipe, for example) may set stricter or more specific minimum slope requirements than these general bands.
Common questions
What is a good slope for drainage?
A 1-2% slope is the typical minimum for reliable gravity drainage, with 2-5% considered a good working grade for most applications.
How do I convert slope percentage to a ratio?
Divide 100 by the slope percentage — a 2% slope is a ratio of 1 in 50 (100 ÷ 2 = 50).
What slope should a French drain have?
Most French drains are installed with a minimum 1% slope, with 2% preferred so sediment doesn’t settle and clog the pipe.
How much should ground slope away from my house?
A common minimum is 2% (about 1/4 in per foot) for at least the first 6-10 ft away from the foundation.
Is a 1% slope the same as a 1-degree angle?
No — a 1% slope is about a 0.57° angle. Percentage and angle both describe steepness but are calculated differently.
What does '1 in 100' mean for a slope?
It means the surface drops 1 ft in elevation for every 100 ft of horizontal run — equivalent to a 1% slope.
When is a slope considered too steep for drainage?
Above roughly 12%, a slope is considered very steep — water can move too fast, eroding soil or washing out loose surfaces like mulch or gravel.
Can a slope be too flat to drain properly?
Yes — below about 1%, gravity flow becomes unreliable and water is more likely to pool rather than drain away.
Do I measure rise and run in the same units?
Yes — convert both to the same unit (typically feet) before calculating slope percentage; this calculator converts inches of rise to feet automatically.
What's the difference between slope percentage and grade?
They’re the same thing — ‘grade’ and ‘slope percentage’ both refer to rise divided by run, expressed as a percentage.
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Drainage Slope Calculator Result
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materialcalculate.com/tools/drainage-slope-calculator/
Result
| Rise | — |
| Run | — |
| Slope percent | — |
| Ratio | — |
| Angle | — |
Notes
Estimate generated by materialcalculate.com.