Calculator
Example data
| Scenario | Run | Slope | Fall | Notes |
|---|---|---|---|---|
| French drain to soakaway | 12 m | 1.5% | 180 mm | Steady grade helps reduce standing water. |
| Raised bed edge drain | 8 m | 2% | 160 mm | Short runs often allow a stronger fall. |
| Downspout extension | 25 ft | 1:80 | 3.75 in | Longer paths can use gentler slopes. |
Formula used
- Fall = Run × Slope
- Slope% = (Fall ÷ Run) × 100
- Ratio = 1 : (Run ÷ Fall)
Inputs convert to meters internally, calculations run in base units, then outputs convert back to your selected elevation unit for display and exports.
How to use this calculator
- Measure the run length along your drain route.
- Select a mode based on what values you already know.
- Choose units, then enter slope, fall, or elevations.
- Press Calculate to see results above the form.
- Use CSV or PDF downloads to share or save reports.
Recommended garden drainage gradients
For most beds and paths, a working fall of 1–2% moves water without scouring soil. That equals 10–20 mm per meter, or roughly 1:100 to 1:50. Clay soils and compacted lawns often benefit from 2% near surface inlets. Sandy profiles may perform well at 1% if outlets stay open.
Interpreting percent, ratio, and drop rate
Percent slope is drop divided by run, multiplied by 100. Ratio expresses the same idea as 1:N, where N is run divided by fall. A 1:60 grade equals 1.67%, while 1:40 equals 2.5%. The drop rate outputs fall per meter and per foot, helping you mark stakes consistently even on mixed‑unit sites.
Using elevations to verify grade
If you know start and end elevations, the calculator derives total fall and slope for your run length. This supports audits of swales, French drain trenches, and downspout extensions before backfilling. Measure elevations with a laser, water level, or string line over fixed benchmarks. If the computed slope is under target, consider adding a shallow catch basin or rerouting to a lower outlet rather than forcing a steep cut near plants.
Managing infiltration and outlet conditions
A slope that is correct on paper can fail if the outlet is restricted. Confirm that your discharge point, soakaway, or rain garden can accept the expected flow during storms. Where infiltration is slow, add gravel, increase perforated pipe diameter, or extend the trench length to reduce surcharge. Incorporate cleanouts at bends and maintain a minimum cover depth to prevent crushing and future settlement.
Recording results for repeatable installation
Saving entries builds a log for zones such as vegetable beds, patios, and lawn edges. Export CSV to share with installers, compare alternatives, and document the chosen gradient. Export PDF to keep a dated field record aligned to stake marks and finished elevations. Recheck grades after the first heavy rain and after seasonal mulch changes, then update the log so maintenance stays consistent year to year.
What slope is typical for a French drain in a garden?
Many installs use 1–2% fall to keep water moving. That is about 10–20 mm per meter, or roughly 1:100 to 1:50. Use site conditions and outlet capacity to finalize.
Can I calculate using feet and inches?
Yes. Choose feet for run length and inches for elevation, or any combination offered. The calculator converts internally, then reports fall, slope percent, and ratio in your selected units for easy marking.
Why does the 1:N ratio change when I edit inputs?
The ratio is derived from slope, not units. If you change slope percent, run, or fall, the slope changes and the ratio updates. Switching only units keeps the underlying slope the same.
How do I handle drains with multiple segments?
Split the route into straight segments, calculate each segment’s fall, then sum the falls for the total. This mirrors field staking and helps you adjust slopes around obstacles while keeping a consistent overall drop.
How precise should my measurements be?
Match precision to your tools. For short runs, small errors matter, so use a level and a tight string line. For longer runs, rounding to one or two decimals is usually adequate for planning and documentation.
Does steeper always mean better drainage?
No. Too steep can cause erosion, undermining, and sediment movement into pipes. Aim for steady flow with minimal scouring. Improve performance by widening outlets, adding gravel, and keeping inlets clear, not only by increasing slope.