What is soil infiltration rate?
Soil infiltration rate is how fast water moves from the surface into the soil, expressed as a depth per hour, the same way rainfall or a sprinkler's output is measured. It's not the same as how much water a soil can eventually hold; it's how quickly that water gets in before it either soaks past the root zone, pools on the surface, or runs off.
The standard field method, described in the USDA NRCS Soil Quality Test Kit Guide, is to drive an open-ended ring a few inches into the soil, pond a measured depth of water inside it, and time how long that depth takes to disappear. Dividing depth by time, then converting to a common hourly rate, gives the infiltration rate.
| Group | Final infiltration rate | Typically |
|---|---|---|
| A — high | more than 0.30 in/hr (7.6 mm/hr) | Deep, well-drained sands and gravels. |
| B — moderate | 0.15–0.30 in/hr (3.8–7.6 mm/hr) | Moderately deep to deep soils, moderately fine to moderately coarse texture. |
| C — slow | 0.05–0.15 in/hr (1.3–3.8 mm/hr) | Soils with a layer that slows downward movement, or moderately fine to fine texture. |
| D — very slow | less than 0.05 in/hr (1.3 mm/hr) | Swelling clays, a high water table, a claypan, or shallow soil over an impervious layer. |
These bands come from the USDA NRCS National Engineering Handbook, Part 630, Chapter 7: Hydrologic Soil Groups. They were developed for runoff and flood modeling, not as a lawn or garden grading scale, but the underlying rate is exactly what this calculator works out, so the groups are a useful way to read your result. A single-ring test measures a point in your yard; soil survey maps built from these groups describe a much larger area and may not match your exact spot.
This is different from a septic percolation test. "Perc" tests for septic system design follow separate procedures set by local health departments, with their own ring sizes, timing and pass/fail standards. Check with your local permitting office for that requirement instead of using this tool's result.
How this calculator works
The calculator divides the depth of water you measured by the time it took to infiltrate, then scales that to a rate per hour. If you enter two readings, it uses only the second one as your result, because the first reading is inflated by dry soil.
1. One reading
Infiltration rate
Rate = Depth ÷ Time × 60
Depth is in inches or millimeters; time is in minutes. Multiplying by 60 converts minutes to an hourly rate.
2. Two readings (basic infiltration rate)
Basic rate
Rate (basic) = Depth₂ ÷ Time₂ × 60
Only the second interval is used for the headline result. The first interval's rate is shown separately, along with how much it dropped, so you can see whether the soil had reached a steady rate.
Decline between readings
Decline = (Rate₁ − Rate₂) ÷ Rate₁ × 100%
Unit conversions
1 inch = 25.4 millimeters exactly, so an in/hr rate converts to mm/hr by multiplying by 25.4. Switching units on this page converts your entered depths and any sprinkler rate the same way; time in minutes doesn't need converting.
Rounding policy
Infiltration rates and sprinkler rates are shown to 2 decimal places in US units and 1 decimal place in metric units, matching the precision used in the calculation. Nothing here is a modeled estimate, so results aren't rounded to a "nice" number or shown with "about" — they're the direct result of the depth and time you entered.
When this calculator isn't the right tool
It measures a point-in-time surface infiltration rate from a simple ring or can test. Use a different tool for:
- How much water your soil can store, rather than how fast it takes water in — see the available water capacity calculator.
- How deep a given watering will wet the soil — see the wetting front depth calculator.
- Checking current soil moisture rather than intake speed — see the soil moisture calculator.
- Splitting a sprinkler run into cycles once you know your rate — see the cycle and soak calculator.
- Septic system design, which uses a local health department's percolation test procedure, not this one.
Infiltration rate calculation example
A homeowner drives a 6-inch ring 3 inches into a compacted flower bed, following the NRCS test kit method. They pour 1 inch of water and it takes 90 minutes to soak in. They refill it with a second inch, which takes 240 minutes.
- First reading rate: 1 in ÷ 90 min × 60 = 0.67 in/hr.
- Second reading rate: 1 in ÷ 240 min × 60 = 0.25 in/hr. This is the reported infiltration rate.
- Decline: (0.67 − 0.25) ÷ 0.67 ≈ 62.5%, showing the soil was still drying out on the first pass.
- Soil group: 0.25 in/hr falls in the 0.15–0.30 in/hr range, Group B, moderate infiltration.
- Sprinkler check: their spray heads apply 1.20 in/hr from a catch-can test. Since 1.20 > 0.25, the sprinklers apply water about 4.8 times faster than the soil absorbs it, so runoff is likely on a single long run.
The same calculation in metric
A 25 mm first reading takes 100 minutes; a 25 mm second reading takes 500 minutes; a rotor zone applies 12 mm/hr.
- First reading rate: 25 mm ÷ 100 min × 60 = 15.0 mm/hr.
- Second reading rate: 25 mm ÷ 500 min × 60 = 3.0 mm/hr.
- Decline: (15.0 − 3.0) ÷ 15.0 = 80%.
- Soil group: 3.0 mm/hr falls in the 1.3–3.8 mm/hr range, Group C, slow infiltration.
- Sprinkler check: 12 mm/hr is about 4 times the 3.0 mm/hr infiltration rate, so this zone needs shorter, split cycles.
How to test your soil's infiltration rate
The USDA NRCS Soil Quality Test Kit Guide describes a straightforward single-ring test that this calculator is built around.
- Drive a ring into the soil. NRCS uses a 6-inch diameter metal ring, beveled edge down, driven about 3 inches into undisturbed soil.
- Mark a fill line. Measure or mark the depth of water you plan to add, commonly 1 inch (or 25 mm).
- Pour and time it. Fill to the line and start a timer. Stop when the surface just glistens dry, and record the minutes elapsed as your first reading.
- Repeat for a second reading. Refill to the same line with the same depth and time it again. NRCS notes this second test is skipped only if the soil was already near field capacity (already moist) before you started.
- Enter both readings above. The calculator reports the second, slower reading as your soil's basic infiltration rate.
Single ring vs. a wider or double ring
Water inside a ring doesn't fall straight down; some of it spreads sideways as it soaks in, and a narrower ring is more affected by that sideways spread than a wide one. Comparisons of infiltration test methods summarized in the Minnesota Pollution Control Agency's Determining Soil Infiltration Rates guidance found meaningful differences between single-ring, double-ring and flood-test results at the same site. A double ring, with an outer ring kept full of water to buffer that lateral spread, or simply a wider single ring, gives a more representative number than a narrow can.
Common mistakes
- Testing right after rain or irrigation. Wet soil infiltrates more slowly; for a schedule-planning rate, test after the soil has had a day or two to dry, or rely on the second reading rather than the first.
- Disturbing the soil surface. Driving the ring in at an angle, or churning up the surface while pouring, opens artificial gaps that let water in faster than it really would.
- Only testing one spot. Infiltration rate can vary widely across a yard, especially where there's been foot or equipment traffic. Test a few representative spots and compare.
- Stopping the timer too early. "Glistening dry" is a thin, matte-wet sheen, not fully dry soil; stopping while a visible puddle remains overstates the rate.
Using your infiltration rate to prevent runoff
Once you know how fast your soil takes in water, the practical question for irrigation is simple: does your sprinkler put water down faster than the soil can absorb it? If it does, the extra water runs off the surface, most visibly on slopes, compacted areas, or anywhere the lawn meets a hard surface, instead of reaching the root zone where it's needed.
Find your sprinkler's application rate with a catch-can test, enter it in the optional field above, and the calculator compares the two rates directly. When the sprinkler rate is higher, split the total run time into two or three shorter cycles with a break in between so each round can soak in before the next one starts; the cycle and soak calculator works out that split from both rates. Uneven coverage compounds the problem, since a zone with poor distribution uniformity often gets run longer to reach dry spots, adding to runoff everywhere else.
For drip irrigation, infiltration rate matters differently: water is applied slowly at a point, and the drip wetted area calculator or wetting front depth calculator are more directly useful there than a runoff comparison.
What changes infiltration rate
A rate measured today at one spot won't necessarily hold everywhere on the property, or next month. These are the main reasons it moves.
| Factor | Effect | What to adjust |
|---|---|---|
| Soil texture and structure | Sand infiltrates fastest, clay slowest; well-aggregated soil infiltrates faster than compacted or crusted soil of the same texture. | Add organic matter over time; check the soil texture calculator. |
| Current soil moisture | Dry soil infiltrates fastest at first, then slows sharply as it wets, per the USDA NRCS test procedure. | Use the second reading, not the first, for scheduling decisions. |
| Compaction | Traffic, construction and heavy equipment collapse pore space near the surface, cutting infiltration. | Aerate or core-cultivate compacted turf and beds. |
| Surface crusting and cover | Bare soil hit directly by rain or sprinklers can crust over, sealing surface pores; residue or turf cover protects them. | Maintain turf density or mulch bare soil. |
| Slope | On sloped ground water can run off before it has time to infiltrate, even where the flat-ground rate is adequate. | Use shorter cycles with soak breaks on slopes. |
| Test method | A narrow single ring can read faster than the true rate because water spreads sideways under the rim. | Use a wider ring (6 in or more) or a double-ring setup. |
Frequently asked questions
What is soil infiltration rate?
Soil infiltration rate is how fast water enters the soil surface, usually measured in inches or millimeters per hour. It's found by ponding a known depth of water inside a ring pushed into the soil and timing how long it takes to soak in. A fast rate means water moves in quickly with little runoff risk; a slow rate means water can pool or run off before it's absorbed.
Why does my infiltration rate change between the first and second reading?
Dry soil has open pores and cracks that let water in quickly at first. As the soil wets up, those pores fill and swell, and the rate settles toward a slower, steady value. The USDA NRCS Soil Quality Test Kit Guide has you pour a second measured depth of water for this reason: the second reading is a better estimate of the soil's real working infiltration rate.
What counts as a good infiltration rate for a lawn or garden?
There isn't one universal "good" number, because it depends on how you're using the result. For irrigation, a soil that infiltrates at or above your sprinkler's application rate can absorb water without runoff; a slower soil needs shorter, split watering cycles. The hydrologic soil group bands in this calculator, from USDA NRCS, describe the range from very slow to high infiltration and can help you see where your soil sits.
What is a hydrologic soil group?
Hydrologic soil groups (A, B, C and D) are a USDA NRCS classification based mainly on a soil's final, steady infiltration rate. Group A soils, typically deep sands, infiltrate quickly; Group D soils, typically swelling clays or soils with a water table or hardpan near the surface, infiltrate very slowly. The groups are used mainly for runoff and flood modeling, but they're also a handy shorthand for how fast a soil takes up water.
How does infiltration rate affect my sprinkler schedule?
If your sprinklers apply water faster than the soil can absorb it, the excess runs off the surface instead of reaching the roots, especially on slopes or compacted ground. Comparing your measured infiltration rate with your sprinkler's application rate, found with a catch-can test, tells you whether you need to split a run time into shorter cycles with soak breaks in between.
See the catch-can test calculator and the cycle and soak calculator.
Can I use a coffee can instead of a metal ring?
Yes, for a rough, single-reading estimate. A straight-sided can pushed a couple of inches into the soil works on the same principle as the NRCS ring test, though a can's smaller diameter is more sensitive to water spreading sideways under the rim, which can inflate the reading. For a more reliable result, use a wider ring, at least 6 inches across, and take a second reading as described above.
Does infiltration rate differ from soil permeability or hydraulic conductivity?
They're related but not identical. Infiltration rate is what you measure at the surface at a point in time, and it's affected by current soil moisture, compaction and surface conditions. Hydraulic conductivity is a more fundamental property of the saturated soil. In practice, a soil's steady-state infiltration rate, the number this calculator focuses on, approximates its saturated hydraulic conductivity.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formula, unit conversions and both worked examples verified by hand.
- Hydrologic soil group thresholds checked against the USDA NRCS source below.
- Test procedure follows the USDA NRCS Soil Quality Test Kit Guide.
Sources
- USDA Natural Resources Conservation Service. Soil Quality Test Kit Guide.
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 630, Hydrology, Chapter 7: Hydrologic Soil Groups (2009).
- USDA Natural Resources Conservation Service. Soil Infiltration, Soil Health Guides for Educators.
- Minnesota Pollution Control Agency. Determining Soil Infiltration Rates, Minnesota Stormwater Manual.