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Yard Drainage Fixes

Soil Drainage Rate Calculator

Percolation calculators are built for septic drain fields: they give you minutes per inch and stop. If you dug a hole in a wet lawn, you want a different answer — can this ground soak water up, and which fixes does that rule in or out? This converts your test into both units, reads it against the thresholds on the soil drainage test page, and hands the rate to the dry well and rain garden calculators.

How far did the water drop, and in how long?After the pre-soak: refill the hole, then measure the drop. One hour is standard; any time works.

Drainage rate

1 in/hr = 60 minutes per inch

1 in/hr — normal, healthy soil

Normal · likely loam or sandy loam

This is the range most drainage advice assumes. The soil itself isn't the problem; look at where the water is coming from — downspouts, a neighbour's runoff, a grade that falls toward the house — before choosing a fix. Every solution on the site is on the table.

What it rules in or out

At this rate

  • A 6-inch-deep rain garden empties in about 6 hours
  • A full 12-inch test hole clears in about 12 hours

Use this rate

Size a dry well with it →Size a rain garden with it →Run the solution finder →

What this doesn't tell you

One hole tells you about one spot, on one day. Skip the pre-soak and the number reads fast; test in a wet week and it reads slow; a buried layer of construction fill or a seasonal water table can make the same yard read differently two holes apart. The rate is a strong hint about which category of fix suits your ground — not a design value. The sizing tools that take it already build in a margin for that.

How to do the test

A shovel, a bucket and a ruler. Dig a hole about 12 inches deep where the problem is. Fill it and let it drain fully — the pre-soak. Refill it, lay the ruler across the top, and read the drop after an hour. In very slow soil measure over a longer time and enter that instead; the tool works with any duration. Two or three holes across the wet area tell you whether the whole yard drains the same way, and the tool averages them.

Three stages. First, a hole about 12 inches deep dug in the ground. Second, the hole filled with water to pre-soak and left to drain. Third, the hole refilled and the drop in water level measured over one hour.
Pre-soak the hole first, then measure how far the water drops in one hour. The pre-soak is what makes the number meaningful.

The formula — check it yourself

inches per hour = drop (in) ÷ time (hr)
minutes per inch = 60 ÷ inches per hour

Two inches in 30 minutes is 4 inches per hour, or 15 minutes per inch. The bands the tool uses — free-draining above 4 in/hr, normal from 1 to 4, slow from 0.5 to 1, poor below 0.5 — are the ones the site's solution pages are written against, so a result here maps directly onto which pages apply to your yard.

Conversion and interpretation chart

Minutes per inchInches per hourBandLikely soil
5 min/in12 in/hrFree-drainingSandy or gravelly
10 min/in6 in/hrFree-drainingSandy or gravelly
15 min/in4 in/hrNormalLoam or sandy loam
30 min/in2 in/hrNormalLoam or sandy loam
60 min/in1 in/hrNormalLoam or sandy loam
120 min/in0.5 in/hrSlowSilty clay loam, or compacted
240 min/in0.25 in/hrPoorHeavy clay, or a water table

What the number decides

Whether your soil absorbs water decides which whole category of fix applies. Above about 1 inch per hour, storing water in the ground works: a dry well for a downspout, a rain garden for a wider area. Below about half an inch, the ground is the problem and water has to be moved instead — regrading, a swale, or a French drain to a real outlet. The comparison of every option is sorted the same way. Heavy clay soil is the usual reason for a slow result, and it is common across the North Carolina Piedmont.

Frequently asked questions

What is a good percolation rate for yard drainage?
For a yard, 1 to 4 inches per hour — 15 to 60 minutes per inch — is normal, healthy soil, and anything faster is free-draining. Below 1 inch per hour soak-away fixes like dry wells and rain gardens get marginal, and below half an inch per hour they are usually ruled out. Septic guides quote a different ideal range because they are sizing a drain field, not deciding how to fix a wet lawn.
How do I do a perc test at home?
Dig a hole about 12 inches deep and 6 to 12 inches wide where the problem is. Fill it with water and let it drain completely — that pre-soak is what makes the number meaningful. Refill it, then measure how far the water level drops in one hour. Enter the drop and the time here; the tool converts it to inches per hour and minutes per inch.
What does 1 inch per hour mean?
The water level in a pre-soaked test hole fell one inch in sixty minutes, which is also 60 minutes per inch. It is the boundary between normal and slow soil. At that rate a 6-inch-deep rain garden empties in about 6 hours and a dry well drains between storms, so soak-away fixes are on the table; below it, they start to need oversizing.
Why convert minutes per inch to inches per hour?
Septic testing reports minutes per inch, and every drainage design figure — infiltration rates, drain-down times, the numbers the sizing tools on this site use — is in inches per hour. They are the same measurement inverted: inches per hour = 60 ÷ minutes per inch. Thirty minutes per inch is 2 inches per hour; 120 minutes per inch is half an inch per hour.
My perc test was slow. Can I still fix my drainage?
Yes — it just changes which fixes make sense. Slow soil rules out storing water in the ground and points you toward moving it: regrading, a swale, a dry creek bed, or a French drain to a real outlet. It also means roof water matters more, because none of it will soak away where it lands. A slow result narrows the field; it does not close it.
Why do I have to pre-soak the hole?
Dry soil pulls water in fast by capillary action, so an un-soaked hole reads far faster than the ground really drains during a storm, when it is already wet. Filling the hole once and letting it empty puts the soil in the saturated state that matters. Skip it and a clay yard can look like sand for the first twenty minutes.

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