Soil Infiltration Rates by Soil Type: How Fast Sand, Loam and Clay Actually Drain

The one number that sorts every drainage fix

Every yard drainage solution belongs to one of two families. The first family stores water in the ground and lets it soak away — dry wells, rain gardens, infiltration trenches, the gravel-filled hole people dig on instinct. The second family moves water across or out of the yard — grading, swales, catch basins piped to daylight, French drains with a real outlet. Which family applies to your yard is decided almost entirely by one number: how many inches of water your soil can absorb in an hour.

This page collects the typical published rates by soil type, the federal soil groups they map to, and the presets the site's calculators use, so a number from a test hole, a soil survey or a contractor can be read against the same baseline.

Typical infiltration rates by soil texture

Ranges are for saturated soil — the state it is in during a storm — and come from the sources listed at the end. Real yards sit anywhere inside a range, and a compacted lawn can read a class or two slower than its texture suggests.

The two thresholds worth memorising: 1 inch per hour, above which storage fixes behave as designed, and half an inch per hour, below which they are usually ruled out. The band between is where a dry well or rain garden can still work if it is oversized, kept shallow, and given an overflow — the rain garden calculator and dry well calculator both check this for you.

The rates the site's calculators assume

When you pick a soil preset instead of entering a measured rate, the tools use one deliberately cautious figure per class:

A measured rate from the soil drainage rate calculator always beats a preset, and every sizing tool accepts one.

Hydrologic soil groups, in plain English

The USDA sorts every mapped soil in the country into four hydrologic soil groups by how fast water enters it when thoroughly wet. Engineers use them to estimate runoff; homeowners can use them to know what to expect before digging a test hole.

• Group A — sand and gravel; very low runoff, very high infiltration. Rare in the North Carolina Piedmont, common on the coastal plain and the Sandhills.
• Group B — loamy soils; moderate infiltration. The soil most drainage advice is written for.
• Group C — soils with a layer that slows water down, or moderately fine textures. Common across the Piedmont where topsoil sits on clay subsoil.
• Group D — clay, shallow soil over rock, or a high water table; very slow infiltration and high runoff. The typical reading for a graded, compacted Piedmont lot.

Your county's soil survey, free through the USDA Web Soil Survey, gives the group for every mapped area. It describes the native soil, though — a builder's lot that was stripped, graded and compacted often behaves a full group worse than the map says, which is exactly why the site insists on the test hole.

Why a test hole beats a table

Texture sets the ceiling; what has happened to the yard sets the actual number. Compaction from construction traffic, a buried layer of fill, a seasonal high water table, and a hardpan a foot down all pull the real rate below the table value, sometimes by an order of magnitude. The one-hour method on how to test soil drainage takes a shovel and a bucket and measures the yard you actually have. Dig it where the water sits, pre-soak the hole, and read the drop after an hour.

Two cautions when reading the result. A test hole drains through its sides as well as its bottom, so it reads a little faster than a broad basin will; the calculators already assume a hole test and build in a margin. And ground that is already wet from a week of rain reads slower than it will in a normal season — retest in a dry spell before ruling anything out.

FAQs

What is a good infiltration rate for soil?+

For yard drainage, anything above about 1 inch per hour is good: dry wells and rain gardens empty between storms as designed. Half an inch to an inch is workable with oversizing and an overflow. Below half an inch, storing water in the ground is fighting the soil, and fixes that move water on the surface — grading, swales, a piped basin — serve you better. Septic design uses a different scale expressed in minutes per inch.

What is the infiltration rate of clay soil?+

Roughly 0.02 to 0.05 inches per hour for true clay, and 0.05 to 0.1 for clay loam — a twentieth of what loam manages and a hundredth of sand. At that rate 6 inches of ponded water takes days to soak away, which is why a rain garden in unamended clay becomes a pond and a dry well fills and stays full. Amending the top foot helps a rain garden; it does nothing for a dry well sitting three feet down in the clay.

What are hydrologic soil groups A, B, C and D?+

The USDA's four-way sort of soils by how fast water enters them when wet. A is sand and gravel with very high infiltration; B is loam with moderate infiltration; C has a layer that slows water or a finer texture; D is clay, shallow soil over rock or a high water table, with very slow infiltration. Most of the North Carolina Piedmont maps as C or D, the coastal plain and Sandhills as A or B.

Why is my measured rate different from the table?+

Because the table describes soil texture and your yard has a history. Grading and construction traffic compact the top foot; a buried layer of fill or a hardpan blocks water below it; a seasonal water table stops infiltration altogether; and a hole tested in a wet week reads slow while one tested without a pre-soak reads fast. Trust the measurement over the table — but repeat it in a different season if the decision is expensive.

Can I improve my soil's infiltration rate?+

At the surface, yes: core aeration, compost and deep-rooted planting can double the rate of a compacted lawn over a season or two, and replacing the top 12 inches with a sandy mix is how a rain garden is made to work in loam. You cannot change the dense subsoil a dry well sits in or the water table beneath it, so improvements help surface-fed fixes far more than deep ones. The clay-soil page covers what does and does not help.