Infiltration Rate Calculator
Free Infiltration rate Calculator for hydrology & water resources. Enter variables to compute results with formulas and detailed steps.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Rate = V / (A x t)
The infiltration rate formula Rate = V / (A x t) calculates how fast water enters the soil. V is the volume of water (mL) that infiltrated during the test, A is the inner ring area (m²) of the infiltrometer, and t is the elapsed time (minutes). The result, expressed in mL/m²/min, can be converted to mm/hr by multiplying by 0.06. A higher rate indicates more permeable soil, which supports greater groundwater recharge and reduces surface runoff in stormwater design.
Worked Examples
Example 1: Double-Ring Infiltrometer Test
Problem:Inner ring area = 0.071 m² (30 cm dia), water volume added = 850 mL over 15 minutes
Solution:Rate = V / (A × t) = 850 mL / (0.071 m² × 15 min) = 850 / 1.065 = 798 mL/m²/min; convert: 798 × 0.06 = 47.9 mm/hr
Result:Infiltration rate ≈ 48 mm/hr — sandy loam at early test stage
Example 2: Depth-Change Method
Problem:Ring inner diameter 30 cm (A = 0.0707 m²), initial depth = 150 mm, final depth = 95 mm after 30 min
Solution:Depth change = 150 − 95 = 55 mm; Volume = 55 mm × 0.0707 m² × 1000 = 3888 mL; Rate = 3888 / (0.0707 × 30) = 1832 mL/m²/min = 110 mm/hr
Result:Rate = 110 mm/hr (initial phase) — will decline toward steady state
Frequently Asked Questions
What is infiltration rate and how is it different from infiltration capacity?
Infiltration rate is the actual volume of water entering the soil per unit area per unit time (mm/hr or mL/m²/min), measured directly in the field. Infiltration capacity is the maximum rate the soil can absorb water under ponded conditions. When rainfall intensity exceeds capacity, water ponds and runoff begins. Infiltration Rate Calculator computes actual rate from volume, area, and time measurements.
How does the formula Rate = V / (A × t) work in a ring infiltrometer test?
V is the volume of water (mL) that infiltrated, A is the inner ring area (m²), and t is the measurement time interval (minutes or seconds). The result gives volume flux per unit area. To convert mL/m²/min to mm/hr, multiply by 0.001 (mL → L), then by 1000 (L/m² = mm), then by 60 (per hr): Rate (mm/hr) = [V(mL) / (A(m²) × t(min))] × 0.06.
How is the steady-state infiltration rate used in Green Infrastructure design?
Rain gardens, bioretention cells, and infiltration basins are designed so overflow occurs only when rainfall intensity exceeds soil infiltration rate. Measured fc values from infiltrometer tests set the design drawdown rate. Typical bioretention media targets fc > 25 mm/hr. Regulatory guidelines (e.g., EPA, local stormwater manuals) require site-specific infiltration testing before facility sizing.
What factors reduce infiltration rate over time during a test?
Soil pore spaces fill with water, reducing capillary suction. Fine particles migrate and clog pores (surface sealing). Air trapped in pores resists downward movement. Swelling of clay minerals closes pathways. For these reasons, rates always decline from f0 toward fc during the first 30–120 minutes of a test. Running the test to steady state is essential for design purposes.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy