Calculator
Example Data Table
| Surface | Run | Basis | Fall | Slope |
|---|---|---|---|---|
| Garden bed | 6.00 m | 1:80 | 0.075 m | 1.25% |
| Garden path | 10.00 m | 1.5% | 0.150 m | 1.50% |
| Drainage trench | 18.00 m | Levels 100.00 to 99.73 | 0.270 m | 1.50% |
Formula Used
Fall from ratio: Fall = Run ÷ Ratio Denominator
Fall from percent: Fall = Run × (Slope Percent ÷ 100)
Slope percent: Slope % = (Fall ÷ Run) × 100
Gradient ratio: Ratio = Run ÷ Fall
Angle: Angle = arctan(Fall ÷ Run)
Use the same unit for run and levels before calculating.
How to Use This Calculator
- Choose a calculation mode.
- Select the garden surface and your working unit.
- Enter the run length of the area or drain line.
- Enter a ratio, slope percent, or surveyed levels.
- Add a safety allowance when using ratio or percent mode.
- Click calculate to view fall, slope, ratio, levels, and angle.
- Review the graph and export the result as CSV or PDF.
FAQs
1. What is drainage fall?
Drainage fall is the vertical drop across a measured run. It helps water move away from beds, paths, edging, and foundations instead of pooling on the surface.
2. What slope works for most garden areas?
Many garden surfaces work near 1% to 2%, but the right value depends on soil, finish level, paving type, drain location, and how fast you want water removed.
3. Is a 1:80 gradient enough?
A 1:80 gradient equals 1.25%, which is often suitable for general garden drainage. Check the finish material and local practice before fixing the final slope.
4. Why show both percent and ratio?
Percent slope is quick for design checks. Ratio is common on site drawings. Showing both helps you compare survey notes, layout plans, and installation targets.
5. Should levels be measured from finished surfaces?
Yes. Finished surface levels are usually best because they reflect the true drainage path. Subgrade levels matter too, but finished levels confirm how water will behave.
6. Can I work in feet instead of meters?
Yes. This file accepts meters and feet. The calculator still reports extra fall conversions in millimeters, centimeters, and inches for easier checking.
7. What if the end level is higher than the start?
That creates a reverse fall. Water may run back toward the starting point or pond in the middle, so the levels should be reviewed before construction.
8. Does a steeper slope always drain better?
No. Too much slope can look awkward, affect walking comfort, expose edges, and move water too fast. Aim for a practical range that suits the surface.