1:100 Slope Calculator
Enter either horizontal run or vertical rise/fall. If you enter both, the tool checks how close your values are to an exact 1:100 slope.
Reference: a 1:100 slope equals 1.00% grade and approximately 0.5729°.
What does a 1 in 100 slope mean?
A 1:100 slope means that for every 100 units of horizontal distance, elevation changes by 1 unit. You can think of this as a very gentle slope commonly used in drainage, landscaping, pipe runs, flat roof design, and grading plans.
In plain terms:
- 100 meters run = 1 meter rise (or fall)
- 10 meters run = 0.1 meter rise (or fall)
- 1,000 millimeters run = 10 millimeters rise (or fall)
Core formulas
Use these formulas for any unit system (meters, feet, inches, millimeters):
Rise = Run / 100Run = Rise × 100Percent grade = (Rise / Run) × 100Angle (degrees) = arctan(Rise / Run)
For a perfect 1:100 slope, percent grade is always 1%.
How to use this calculator
If you know the run
Enter the horizontal distance and leave rise blank. The calculator returns the required vertical change to produce exactly 1:100. This is useful for setting levels in site work or checking fall across slabs, pavers, and channels.
If you know the rise or fall
Enter the vertical change and leave run blank. The calculator computes the necessary horizontal distance to maintain a 1:100 gradient.
If you know both values
Enter both run and rise. The calculator will evaluate your actual slope, provide percent and angle, and tell you whether your layout is flatter or steeper than 1:100.
Quick reference table for 1:100 slope
| Horizontal Run | Vertical Change at 1:100 |
|---|---|
| 1 m | 0.01 m (10 mm) |
| 5 m | 0.05 m (50 mm) |
| 10 m | 0.10 m (100 mm) |
| 25 m | 0.25 m (250 mm) |
| 50 m | 0.50 m (500 mm) |
| 100 m | 1.00 m |
Common applications
- Surface drainage: Encourages water to flow without creating a steep walking surface.
- Pipe installation: Helps maintain reliable gravity flow for certain drainage systems.
- Outdoor paving: Prevents ponding around patios, drives, and pathways.
- Roofing and waterproofing design: Supports controlled runoff and minimizes standing water.
Common mistakes to avoid
- Mixing units (for example, run in meters and rise in millimeters without conversion).
- Confusing ratio slope (1:100) with percent slope (1%).
- Reversing rise and run in the formula.
- Ignoring tolerance limits required by local building or civil standards.
Tip: Always verify final grades on site using calibrated equipment and your local code requirements.