Drainage Slope Calculator
Calculate drainage grade, total fall, slope ratio, angle, pipe sections and pipe cost for a straight run.
Default project preview
1 % slope
Total fall
1 ft
Fall per 10 units
1.2 in per 10 ft
Slope angle
0.57°
Ending elevation
99 ft
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Horizontal run
Enter this project assumption in ft. The current imperial default is 100 ft.
Total fall
Enter this project assumption in ft. The current imperial default is 1 ft.
Starting elevation
Enter this project assumption in ft. The current imperial default is 100 ft. Minimum: 0.
Pipe-length allowance
Enter this project assumption in %. The current imperial default is 5 %. Minimum: 0. Maximum: 30.
Stock pipe section
Enter this project assumption in ft. The current imperial default is 20 ft.
Price per pipe section
Enter this project assumption in a unitless value. The current imperial default is 15. Minimum: 0.
How to read the outputs
Total fall
start to end. Default preview: 1 ft.
Fall per 10 units
grade reference. Default preview: 1.2 in per 10 ft.
Slope angle
from horizontal. Default preview: 0.57°.
Ending elevation
starting elevation minus fall. Default preview: 99 ft.
Pipe sections
105 ft order length. Default preview: 6.
Pipe cost
full stock sections. Default preview: $90.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Slope percent = total fall ÷ horizontal run × 100. Ratio = run ÷ fall. Pipe length uses the sloped hypotenuse plus the entered allowance.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
Use this calculator to convert elevation difference and run length into slope, grade percentage and fall per unit length. It reports geometry only and does not prescribe an acceptable drainage slope.
Calculation steps
- 1Measure horizontal run, not sloped pipe length.
- 2Measure start and end elevations from one datum.
- 3Calculate fall ÷ run.
- 4Express the result as percent, ratio or fall per unit length.
Worked numerical example
A 60 ft run dropping 9 in has a grade of 1.25%, equal to 1.5 in of fall per 10 ft.
Purchasing and rounding
This calculator does not generate a material order. Use the verified slope in pipe, swale, gutter or grading takeoffs.
Interpret the result
A positive percentage indicates fall in the intended direction. Small measurement errors can materially affect shallow slopes.
Page-specific assumptions
- Straight uniform run.
- Elevations use the same reference.
- Horizontal distance is measured accurately.
- No intermediate high or low points.
Common mistakes
- Using sloped distance instead of horizontal run.
- Mixing inches and feet.
- Reversing start and end elevations.
- Assuming one slope is suitable for every system.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Drainage Slope Calculator questions
What does a one-percent slope mean?+
It means one unit of fall for every one hundred equal units of horizontal run.
Can a drain be too steep?+
Yes for some systems. Excessive grade can create installation or flow problems, so follow the applicable design standard.
Should pipe length equal horizontal run?+
The sloped pipe is slightly longer than the horizontal run, and connections or routing usually require additional allowance.