Calculate soil infiltration rate from the water depth absorbed over a timed test in mm/h or in/h, or find the depth or time at a known rate.
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Soil Infiltration Rate Formula
The soil infiltration rate describes how quickly water enters the soil over a measured period of time. This value is useful in irrigation planning, drainage evaluation, stormwater design, runoff estimation, and field testing of soil performance.
- SIR = soil infiltration rate
- WD = water depth infiltrated during the test interval
- T = elapsed time
If you know any two values, you can rearrange the equation to solve for the third:
How to Use the Calculator
- Enter the water depth that infiltrated into the soil during the test.
- Enter the elapsed time for that measurement.
- Select the proper units for each field.
- The calculator returns the soil infiltration rate in depth per unit time.
- If solving for water depth or time instead, enter the other two values and let the calculator compute the missing variable.
The selected result is a rate, depth or elapsed time. The calculator accepts depth in mm, cm or inches; rate in mm/h, cm/h or in/h; and time in minutes or hours. Primary metric results use mm/h or mm; imperial results use in/h or inches. Elapsed-time results use minutes. A measured rate is an average over its observation interval; depth and time estimates assume that rate remains constant.
What the Result Means
A higher observed rate means faster water entry during that test interval. Conditions, initial moisture, soil structure and test method matter. Historical NRCS rate bands provide a descriptive comparison, not a measurement of saturated hydraulic conductivity (Ksat), permanent impermeability or proof of drainage suitability. The USDA Soil Quality Test Kit Guide uses older permeability bands for this comparison; the 2024 NRCS Field Book recommends Ksat in place of permeability classes.
| Observed Rate | General Interpretation |
|---|---|
| Low | Water enters slowly; runoff, crusting, or compaction may be more important concerns. |
| Moderate | An intermediate observed entry rate; suitability depends on the site, purpose and test conditions. |
| High | Fast observed entry during the test; this alone does not predict storm runoff or water movement through the entire soil profile. |
Examples
If 5 inches of water infiltrate over 30 minutes, the average rate is 10 in/h (254 mm/h), equivalent to approximately 0.167 in/min:
At an assumed constant rate of 0.25 in/min (15 in/h) for 12 minutes, the infiltrated water depth is:
For 4 inches at an assumed constant rate of 0.2 in/min (12 in/h), the time required is:
How Soil Infiltration Is Measured in Practice
In a suitable field test, use the water depth that actually infiltrated during a measured interval, not simply the initial water applied. Account for other water gains or losses. Single-ring lateral flow can increase the apparent rate, and initial moisture and changing conditions affect comparisons. Repeat measurements using a documented method rather than treating one short test as a site-wide design capacity.
- Measure the water depth lost to infiltration during a known time interval.
- Keep units consistent when entering values.
- Repeat tests at multiple locations if the site is variable.
- Note that early-time infiltration is often faster than later-time infiltration.
Factors That Affect Soil Infiltration Rate
| Factor | Typical Effect on Infiltration |
|---|---|
| Soil texture | Coarser soils usually infiltrate faster than finer soils. |
| Compaction | Compacted soil reduces pore space and typically lowers infiltration. |
| Soil structure | Stable aggregates and connected pores usually improve water entry. |
| Organic matter | Higher organic matter often supports better aggregation and pore continuity. |
| Initial moisture | Dry soil may absorb water faster at first; wet soil may infiltrate more slowly. |
| Vegetation and roots | Roots and biological activity can create channels that increase infiltration. |
| Surface sealing or crusting | A sealed surface can sharply reduce the rate of water entry. |
| Macropores and cracks | Large channels can cause very rapid localized infiltration. |
Common Mistakes
- Using the total water applied instead of the depth that infiltrated during the measured interval.
- Mixing units without converting them properly.
- Comparing results from tests run under very different moisture conditions.
- Assuming one short test represents long-term soil behavior everywhere on the site.
Why This Calculation Matters
Knowing the soil infiltration rate helps determine how quickly water can move from the surface into the ground. That makes it valuable for selecting irrigation application rates, estimating runoff potential, assessing drainage performance, and comparing different soils or site conditions with a simple and consistent metric.
Additional Notes
Interpret the result in its soil, land-use and test context. Optional ponding removal time is target depth divided by an unchanged rate, with no continuing inflow: zero target depth takes zero time, while a positive target at zero rate has no finite completion time. Potential rainfall excess is max(applied intensity minus assumed infiltration rate, 0); it is not predicted actual runoff because surface storage, vegetation, changing moisture and rate, and spatial variability are omitted. One-hour and 24-hour depths are arithmetic constant-rate scenarios, not storm or long-term forecasts.
