Checklist and calculator
Soil drainage test checklist
Plants in soggy soil often look thirsty, so drainage trouble is easy to misread. This page walks you through the standard percolation test, which takes about an hour of watching and costs nothing, and then turns your two measurements into a rate. Tick the steps as you go, or print the page and work from paper.
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Drainage rate calculator
Worked example (shown when scripts are off): water that dropped 1.5 inches in 60 minutes is draining at 1.5 inches per hour, which is in the range that suits most garden plants.
When to stop and call someone
If water pools near your foundation, if you see damp basement walls, or if the fix would mean changing the grade near the house or digging near a septic field, get a licensed professional. Do not dig or build over a septic drain field yourself.
How to read the result
The rate is the drop in water level divided by the time, scaled to one hour. The calculator uses these bands: under 0.5 inches per hour is poorly drained, 0.5 to 1 is slow and borderline, 1 to 3 suits most garden plants, and over 3 is fast enough that plants may dry out. Sources differ a little. One garden publication calls 1 to 2 inches an hour good drainage and says over 3 is too fast, while others describe 1 to 3 as fine and about 2 as ideal. A rate under half an inch is called poor in these sources. Because the 0.5 to 1 zone falls between the published points, the tool calls it borderline and asks you to repeat the test.
A slow result is a reason to act, not a verdict on the whole garden. Compost worked into the top 8 to 12 inches helps mild cases, raised beds help moderate ones, and severe cases may need grading or a drain. Fast results can be eased with compost too, which holds water.
Limits
- One test measures one hole on one day. Two or three spots on different days are far more reliable.
- Soil that is already wet gives a falsely slow rate, and very dry soil can give a falsely fast one on the first fill, which is why the hole is pre-wet.
- This is general information. Local rules on drainage and on moving water off your property apply, and a professional should assess anything that touches the house or utilities.
Where this comes from
The 12-inch hole, fill-and-drain-twice method and the inches-per-hour bands come from garden-society and gardening-publisher percolation guides and a municipal stormwater programme worksheet, which agree on the method and give slightly different cut-offs, as described above. We compared several guides and did not rely on a single source for the thresholds. The checklist adds the utility-locating step and the foundation and septic warnings that the guide on this site also includes. The rate arithmetic was tested in code, including unit conversion and invalid inputs.