Formula used
- Grade (%) = (Drop ÷ Run) × 100
- Drop = (Grade ÷ 100) × Run
- Angle (°) = arctan(Drop ÷ Run)
- Slope ratio = 1 : (Run ÷ Drop)
How to use this calculator
- Pick a method based on what you can measure.
- Enter run length, then grade, drop, or elevations.
- Select a project use to view a suggested range.
- Press Calculate; the result appears above the form.
- Download CSV or PDF for notes and sharing.
Example data table
| Run | Grade | Drop | Typical garden use |
|---|---|---|---|
| 10 m | 1% | 0.10 m | Gentle surface drainage |
| 10 m | 2% | 0.20 m | Move water away from beds |
| 20 m | 0.5% | 0.10 m | Swale shaping with mild flow |
| 30 ft | 2% | 0.60 ft | Downhill edge drainage line |
| 50 ft | 3% | 1.50 ft | Path shedding water in storms |
Why garden grade matters
Grade controls how water moves across soil, mulch, and hardscape. A small change in slope can prevent ponding, reduce algae on paths, and keep plant crowns healthier. For beds, gentle grade supports even infiltration, while still guiding excess water toward a safe outlet.
Common target ranges in landscapes
Many landscape crews use 0.5% to 2% for mild drainage lines, swales, and bed edges. Areas near structures often use 2% to 5% to keep runoff moving away. Steeper grades can work, but erosion control and surface stability become more important as the percentage increases.
Soil behavior and adjustment
Sandy soil drains quickly, so a slightly lower grade can still perform well. Clay holds water longer and may benefit from a modest increase in grade or added drainage layers. The soil option here nudges the suggested range, helping you compare practical field expectations before you stake the line.
How measurements translate on site
Grade is simply drop divided by run. Over 10 meters, a 1% grade equals a 0.10 meter drop. Over 50 feet, a 2% grade equals a 1.00 foot drop. Use string lines, a level, and clear reference points so the finished surface matches the design intent.
Using outputs for planning and records
The result tiles include grade percent, slope ratio, and angle for quick interpretation. Export the summary to document decisions, share targets with installers, and keep a baseline for maintenance. Consistent records help when you regrade after settling, add edging, or extend drainage runs later.
FAQs
1) What grade is best for moving water away from a patio?
A common target is 2% to 5% away from the surface. Verify local conditions, surface material, and outlet location so runoff does not create erosion or pooling.
2) How do I calculate drop if I only know run and grade?
Use Drop = (Grade ÷ 100) × Run. For example, 2% over 10 m gives 0.20 m of drop. Enter run and grade to compute it automatically.
3) Why does clay often need a slightly higher grade?
Clay drains slowly and can stay saturated, so mild increases in grade help prevent long-term ponding. Pair grade with soil improvement, mulch management, and protected flow paths.
4) What does “1 : N” slope ratio mean?
It means 1 unit of drop for every N units of run. A higher N is flatter. Ratios are useful for communicating slope to crews without using percentages.
5) Can a grade be too steep in a garden?
Yes. Steeper slopes can increase erosion, wash mulch, and expose roots. If you must go steep, use check dams, groundcovers, stabilized surfaces, and controlled outlets.
6) Which method should I choose: grade, drop, or elevations?
Choose what you can measure reliably. Use grade when you have a target, drop when you’ve measured fall, and elevations when you have start and end reference heights.