Soil Infiltration Rate Calculator converts a water level drop over time into in/hr or mm/hr from ring, perc and pit tests, then sets a design rate and ponding drawdown for a basin.
Pre-soak the ring or pit, then read the water level twice.
Design Factor, Basin & Rainfall
Infiltration Test Cross-Section
Test Readings Table
Ponding Drawdown Table
| Ponding Depth | At Measured Rate | At Design Rate | 48 hr Limit |
|---|---|---|---|
| 3 in | 2.78 hr | 5.56 hr | Drains in Time |
| 6 in | 5.56 hr | 11.1 hr | Drains in Time |
| 9 in | 8.33 hr | 16.7 hr | Drains in Time |
| 12 in | 11.1 hr | 22.2 hr | Drains in Time |
| 18 in | 16.7 hr | 33.3 hr | Drains in Time |
| 24 in | 22.2 hr | 44.4 hr | Drains in Time |
| 36 in | 33.3 hr | 66.7 hr | Too Slow |
Soil Type Rate Reference
Step-by-Step Math
- Water infiltrated = H₁ + refill - H₂ = 6 in + 0 in - 5.55 in = 0.45 in.
- Elapsed time = 0.417 hr (25 min).
- Rate = depth ÷ time = 0.45 in ÷ 0.417 hr = 1.08 in/hr.
- Design rate = 1.08 in/hr ÷ 2 = 0.54 in/hr; drawdown of 12 in = 12 in ÷ 0.54 in/hr = 22.2 hr.
- Result: 1.08 in/hr Infiltration Rate.
What the Soil Infiltration Rate Calculator Measures
Infiltration rate is how fast water enters the soil surface. It is written as a depth of water per hour, in inches per hour in the US or millimeters per hour elsewhere. A rate of 1 in/hr means a 1 in layer of standing water soaks in over one hour.
The basic field formula takes the water that disappeared and divides it by the time it took. Any water you poured in to top up the ring counts as water that soaked in.
H1 is the starting water depth, H2 the final depth, R any refill added mid-test and t the elapsed time in hours. In the default example, water falls from 6 in to 5.55 in over 25 minutes. That is 0.45 in over 0.417 hr, or 1.08 in/hr (27.4 mm/hr).
Most online tools stop at that one number. The Soil Infiltration Rate Calculator carries it on to the figures a basin, rain garden or trench design needs. It divides the rate by a safety factor, finds how long your ponding depth takes to drain and checks whether design rain will run off.
To log readings in Excel, the same math works in one cell. With H1 in A2, H2 in B2 and minutes in C2, the formula =(A2-B2)/(C2/60) returns in/hr. The calculator adds the refill, unit handling and design checks you would otherwise build by hand.
Ring, Borehole and Pit Test Methods
Field tests differ in how water leaves the hole. A ring forces it downward, a borehole lets it out through the walls and a pit drains through both. Each needs its own math, so the Soil Infiltration Rate Calculator offers a separate mode for each.
Ring infiltrometer tests
The FAO field method drives a 30 cm inner ring at least 15 cm into the soil, with a 60 cm outer ring or earth bund around it. Water between the rings keeps the inner water moving straight down, so the reading reflects vertical flow.
In the soil infiltration rate calculator, use Water Level Drop when you read the level falling. Use Water Volume Added when you hold the level steady and log how much water you pour. The default 444 mL in a 6 in ring works out to 0.96 in of water, or 2.30 in/hr over 25 minutes.
The 444 mL figure comes from the NRCS soil infiltration test, where it equals one inch of water in its ring. That ring is close to 5.875 in inside, so enter the true inside diameter of your own ring for an exact depth.
Readings until the rate is steady
Dry soil takes water fast at first, then slows as the pores fill. FAO calls the final, level rate the basic infiltration rate, and it is the one to design with. The Multi-Reading Test Log mode shows each interval's rate so you can see it settle.
With drops of 1, 0.6, 0.4, 0.3 and 0.28 in every 15 minutes, the rates fall from 4.0 to 1.12 in/hr. The last reading changed only 6.7%, inside the 10% band the tool uses to call the rate steady. Using the first reading would overstate the soil almost fourfold.
Borehole perc test and the Porchet method
A septic perc test times water falling in an unlined auger hole. Most of that water leaves sideways through the hole walls, so the raw drop rate is much higher than the true downward infiltration rate. The Porchet method corrects for the wetted wall area.
Here r is the hole radius and H_avg the average water depth during the reading. A 0.45 in drop in 25 minutes in a 6 in hole with about 5.8 in of water gives a raw rate of 1.08 in/hr. Porchet cuts it to 0.22 in/hr, about 21% of the raw figure.
Soakaway pit test, BRE Digest 365
For soakaways, BRE Digest 365 times the water level falling from 75% to 25% of the effective depth in a trial pit. The rate f is the stored volume divided by the wetted area at half depth and the fall time.
A 2 × 0.6 m pit with 1 m of effective depth that drains in 60 minutes stores 0.6 cubic meters across 3.8 square meters of wetted area. That gives f = 4.39 × 10-5 m/s, or 158 mm/hr. Enter a fill porosity below 1 if the pit is already packed with gravel.
BRE 365 asks for the pit to be filled and drained three times. The first run usually drains fastest because the soil starts dry. Enter up to three timings and the tool designs with the slowest one, which gives the lowest f.
Soil Infiltration Rate by Soil Type
Texture sets the broad range. Sand has large pores and passes water fastest, while clay has tiny pores and can take days to absorb a few inches. Three published tables cover most uses, and the soil infiltration rate calculator matches your result against each.
| Soil Type | FAO Basic Rate (mm/hr) | FAO Basic Rate (in/hr) | NRCS Steady Rate (in/hr) |
|---|---|---|---|
| Sand | Above 30 | Above 1.18 | Above 0.8 |
| Sandy loam | 20 to 30 | 0.79 to 1.18 | 0.4 to 0.8 |
| Loam | 10 to 20 | 0.39 to 0.79 | 0.2 to 0.4 |
| Clay loam | 5 to 10 | 0.20 to 0.39 | 0.04 to 0.2 |
| Clay | 1 to 5 | 0.04 to 0.20 | 0.04 to 0.2 |
| Sodic clay | Not listed | Not listed | Below 0.04 |
The FAO figures are basic rates from ring tests for irrigation planning. The NRCS figures are steady-state rates for soil health work, and NRCS groups sandy and silty soils into one 0.4 to 0.8 in/hr band. FAO's own printed table lists sand as "less than 30" mm/hr, which reads as a typo, since sand passes water faster than sandy loam.
For stormwater design, the Minnesota Stormwater Manual design rates are the most widely quoted US values. They are meant for sizing when only soil borings are available, and they are already conservative.
| Soil Group | Textures | Design Rate (in/hr) | Ponding Drained in 48 hr | Time to Drain 12 in |
|---|---|---|---|---|
| A | Gravel, sandy gravel, silty gravels, gravelly sands, sand | 1.63 | 78.2 in | 7.4 hr |
| A | Sand, loamy sand, sandy loam | 0.8 | 38.4 in | 15 hr |
| B | Silty sands | 0.45 | 21.6 in | 26.7 hr |
| B | Loam, silt loam | 0.3 | 14.4 in | 40 hr |
| C | Sandy clay loam, silts | 0.2 | 9.6 in | 60 hr |
| D | Clay loam, silty clay loam, sandy clay, silty clay, clay | 0.06 | 2.9 in | 200 hr |
Read across the last two columns. A loam basin can hold about 14 in of water and still drain in two days, while a C soil manages under 10 in. On D soils, 12 in of ponding would stand for more than eight days, so those sites need an underdrain or a different practice.
Minnesota also lowers a compacted soil by one row in the table. A compacted silty sand, for example, is sized at the 0.3 in/hr loam rate instead of 0.45 in/hr.
Turning a Test Result Into a Design Rate
A single test is a small sample of a variable site, and soil surfaces clog over time. The Minnesota infiltration design criteria require dividing field-measured rates by 2 as a safety factor. The soil infiltration rate calculator uses 2 by default and accepts any factor from 1 to 10.
The same criteria require infiltration practices to drain within 48 hours. From the design rate, the tool works out how long your ponding depth takes to drain and the deepest ponding that still clears in time.
With the default 1.08 in/hr test, the design rate is 0.54 in/hr. Twelve inches of ponding drains in 22.2 hours, and the most the basin can hold within 48 hours is 25.9 in. A 100 sq ft basin floor at that rate absorbs about 33.7 gallons per hour.
Very fast soil is a problem too. Minnesota prohibits infiltration where the field rate tops 8.3 in/hr unless the soil is amended, because water that fast gets little filtering before it reaches groundwater. The tool flags any result above that line.
The rainfall check compares a design storm intensity with the design rate, the same rate used for drawdown. When rain arrives faster than that, the difference ponds or runs off. For the runoff side of that storm, use the storm water runoff calculator, and for a gravel trench to hold the excess, the French drain calculator. If the basin drains through an outlet pipe, the drainage fall calculator sets its pitch, and the pond calculator gives storage volume for a wet basin.
Infiltration Rate Unit Conversions
Lab reports, septic logs and groundwater models each use a different unit for the same soil. The Soil Infiltration Rate Calculator shows your result in several of them at once. This table converts common rates, including the Minnesota limits.
| in/hr | mm/hr | Perc Rate (min/in) | cm/s | ft/day |
|---|---|---|---|---|
| 0.04 | 1.0 | 1,500 | 2.82 × 10-5 | 0.08 |
| 0.2 | 5.1 | 300 | 1.41 × 10-4 | 0.4 |
| 0.4 | 10.2 | 150 | 2.82 × 10-4 | 0.8 |
| 0.8 | 20.3 | 75 | 5.64 × 10-4 | 1.6 |
| 1 | 25.4 | 60 | 7.06 × 10-4 | 2 |
| 2 | 50.8 | 30 | 1.41 × 10-3 | 4 |
| 8.3 | 210.8 | 7.2 | 5.86 × 10-3 | 16.6 |
Perc rate in minutes per inch is simply 60 divided by in/hr. Keep in mind that a raw borehole perc rate is not an infiltration rate, as the Porchet section shows. Convert it with the Borehole Perc Test mode before using it to size a basin.
Raising a Slow Soil Infiltration Rate
Texture cannot be changed, but structure can. NRCS lists organic matter, surface cover, compaction, restrictive layers, soil moisture and earthworm channels as the main factors that raise or lower infiltration on a given texture.
Its recommended practices include avoiding tillage and traffic on wet soil, confining equipment to set lanes and subsoiling to break compacted layers. Continuous no-till, manure or compost, high-residue rotations, cover crops and contour farming all help pores stay open and connected.
On a building site, the biggest losses come from heavy equipment driving over the future basin floor. Fence the infiltration area off during construction, and test again after grading if it was disturbed.
Soil Infiltration Rate in Practice
Most bad results come from trusting the first reading, testing ground that machines have packed down, or treating a raw borehole perc rate as an infiltration rate. Each of those problems comes up again in the answers that follow.
How do you calculate infiltration rate in soil?
Subtract the final water depth from the starting depth, add any water you poured in during the test, and divide by the elapsed time in hours. A 0.45 in drop over 25 minutes is 0.45 / 0.417 = 1.08 in/hr. Repeat the reading until it stops changing and use that steady value.
What is the infiltration rate in soil?
It is the speed at which water enters the soil surface, given as depth per hour. The first minutes show a high initial rate while dry pores fill. The rate then levels off at the basic or steady-state rate, which reflects how the soil behaves through a long storm or irrigation.
What is a good soil infiltration rate?
For stormwater infiltration, a measured rate between about 0.5 and 8.3 in/hr is workable. After a safety factor of 2, that still drains 12 in of ponding within two days, while staying under the Minnesota limit for groundwater protection. A measured 0.2 in/hr drains only 4.8 in in 48 hours after the safety factor, so slow sites usually need an underdrain.
How do you measure soil infiltration?
Drive a ring 3 to 6 in into the soil, pre-soak it, then fill it and record the water level and clock time at equal intervals. Top up to the starting level after each reading and log how much you added. FAO advises at least two tests per site, keeping on until the drop is the same over each interval.
Which soil has the highest infiltration rate?
Sand and gravel. FAO puts sand above 30 mm/hr (1.18 in/hr), and clean gravels can run far faster. Minnesota uses 1.63 in/hr for gravelly A soils without testing, but requires amending any soil that tests above 8.3 in/hr before it receives stormwater.
What is the infiltration rate of clay soil?
FAO gives clay a basic rate of 1 to 5 mm/hr, about 0.04 to 0.2 in/hr. Minnesota designs D clay soils at 0.06 in/hr, so 3 in of ponding takes about 50 hours to soak in. Sodic clays, rich in sodium, can fall below 0.04 in/hr.
What is the infiltration rate in India?
There is no single national figure, because soils range from desert sands to heavy black cotton clays. Each falls in the FAO band for its texture, from above 30 mm/hr for sand to 1 to 5 mm/hr for clay. Black cotton soils crack when dry and swell shut when wet, so expect the clay band once they are saturated.
Is a high infiltration rate good?
For drainage and erosion, yes, since less rain runs off. Above a point it causes trouble. Water moving faster than 8.3 in/hr carries pollutants and septic effluent to groundwater with little treatment, so fast soils are often amended or lined before use.
What are the different types of infiltration?
Rates are usually split into the initial rate, measured when the soil is dry, and the basic or steady-state rate reached once the pores fill. Infiltration capacity is the most the soil can take at a given moment. The actual rate equals rainfall when rain is lighter than capacity, and equals capacity when rain is heavier.
How do you find infiltration capacity?
Keep water ponded in a double ring so the surface never runs short, and record the drop over equal intervals until it levels out. That steady value is the field infiltration capacity. The Multi-Reading Test Log mode in the Soil Infiltration Rate Calculator plots each interval and marks when the change falls under 10%.
What factors increase infiltration?
Coarse texture, stable crumb structure, high organic matter, surface residue or plant cover, earthworm channels and dry soil at the start all raise infiltration. Compaction, surface crusts, dense subsoil layers, high sodium and already wet soil all lower it.
How can you increase soil infiltration?
Keep traffic off wet soil and break up compacted layers with deep ripping. Then add organic matter through compost, manure or cover crops, and keep the surface covered with residue or plants. Retest after a season, since structure builds slowly.
What is the fastest way to improve soil quality?
Ending compaction and adding compost give the quickest change. Loosening a compacted layer can raise infiltration right away, while a few inches of compost worked into the top layer improve structure within a season. Cover crops and no-till build on that over several years.
How long should an infiltration test run?
Until the rate is steady. On sandy soil that can be under an hour, while clays may need several hours of readings. In the default log, the rate fell from 4.0 to 1.12 in/hr across five 15-minute readings before the last change dropped under 10%.