Free estimate — verify against local code before building
Drainage Slope Calculator
Convert drainage run and fall into percent slope, ratio, angle, and material length, or design a run by application with a stake table.
What this calculator includes
Check a measured drainage run by converting horizontal distance and vertical fall into slope percent, ratio, angle, and fall per 10 feet, or enter a project-specified target slope to calculate required fall. An optional stock-length takeoff carries the same sloped distance into a simple pipe or channel material count. A separate design-a-run mode turns a run length and a chosen application — French drain, downspout extension, or building sewer — into required fall, end depth, and a per-10-ft grade-stake table, plus French drain trench gravel, filter fabric, and pipe quantities.
Next step in your project
Build the trench, gravel, pipe, and fabric takeoff
Carry the verified drainage run, available target slope, and stock length into the French drain calculator, then confirm site-specific outlet and hydraulic design separately.
Open French Drain Calculator →Compatible measurements are carried into the next calculator; product-specific assumptions remain editable.
How to use this drainage slope calculator
- 01
Measure horizontal run
Use the horizontal plan distance between the start and end points, not the sloped tape distance.
- 02
Choose what to solve
Enter measured vertical fall to check existing slope, or enter a target slope supplied by the drainage design to calculate required fall.
- 03
Keep the sign and outlet clear
This tool treats the entered value as positive fall toward the intended outlet; verify elevations, direction, structures, and discharge in the field.
- 04
Plan stock separately
Use waste and stock length for an early pipe or channel count, then add fittings, structures, bedding, fabric, cleanouts, transitions, and outlet components from the approved system.
- 05
Design a run by application
In design-a-run mode, choose French drain, downspout extension, or building sewer to apply that application's default target slope, enter a start depth and run length, and read the required fall, end depth, and per-10-ft grade-stake table.
Worked example
Example: 12 inches of fall over 100 feet
A 100 ft horizontal run with 12 in of vertical fall has a 1% slope, a 1-in-100 ratio, an angle of about 0.57 degrees, and 1.2 in of fall per 10 ft.
Practical buying and overage guidance
Confirm pipe or channel type, diameter, stiffness, joint system, stock length, fittings, structures, cleanouts, bedding, cover, fabric, aggregate, outlet protection, transitions, delivery, and compatible system components before purchasing.
Scope and safety note
Geometry and material-planning tool only. It does not design drainage capacity, choose a slope, pipe, trench, inlet, outlet, bedding, cover, erosion control, or legal discharge. Design-a-run application slopes, including the IPC 704.1 building sewer breakpoints, are planning defaults pending verification against the locally adopted code. Drainage performance and requirements are site-specific; obtain survey, utility locating, permits, and qualified design where needed.
Continue the project
Plan a Gravel or Asphalt Driveway
Measure a driveway, plan excavation and base, compare gravel or asphalt quantities, estimate truck loads, and build a complete contractor price.
Open the project workflow →Continue the project
Upgrade Roof Drainage
Plan roof area, gutter runs, downspouts, fittings, extensions, drainage stone, grading, excavation, supplies, and contractor pricing.
Open the project workflow →Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula and test check recorded July 15, 2026. What this review covers
Formula version 2026-07-17.1
No named qualified reviewer is recorded for this calculator. Treat the result as planning support and obtain the professional or code review identified in its safety note.
- USDA NRCS National Engineering Handbook (opens in a new tab)
Site drainage and engineering context.
- EPA — Soak Up the Rain (opens in a new tab)
Runoff management and site-specific drainage context.
- International Plumbing Code Section 704.1 (or the locally adopted plumbing code edition) — minimum horizontal drainage pipe slope (opens in a new tab)
Minimum building sewer/horizontal drainage pipe slope breakpoints used by design-a-run mode; the locally adopted edition and amendments control.
Frequently asked questions
How is drainage slope percentage calculated?
Slope percent equals vertical fall divided by horizontal run, using the same units, multiplied by 100. For example, 1 ft of fall over 100 ft of run is 1%.
What does a 1-in-100 slope mean?
It means one unit of vertical fall for every 100 units of horizontal run, equivalent to 1%. The calculator reports the ratio from your entries.
What slope should I use for a drain or driveway?
There is no universal value. Pipe type and size, solids, flow, surface, accessibility, outlet, erosion, frost, soil, rainfall, local requirements, and the approved design all matter.
Does the calculator account for pipe sag or settlement?
No. It calculates a straight line between elevations. Bedding, compaction, trench conditions, structures, deflection, settlement, construction tolerance, and field survey control must be addressed separately.
Can I discharge water anywhere downhill?
No. Property rights, easements, erosion, foundations, septic systems, streets, storm systems, waterways, permits, and local drainage rules can restrict where and how water may discharge.
Does design-a-run mode assume one universal slope?
No. Each application — French drain, downspout extension, or building sewer — has its own selectable default slope, not a single universal value; building sewer uses the International Plumbing Code Section 704.1 breakpoints (1/4 in per foot for pipe 3 in and smaller, 1/8 in per foot for pipe 4 in and larger). The locally adopted plumbing code edition and any local amendments control, so treat these as planning values pending verification.