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The soil's hydraulic conductivity. Scientific notation is accepted — type 1e-9 rather than counting zeros.
From a pump test, a permeameter test, or published values for the soil type. The range is genuinely enormous: clean gravel is around 1e-1 m/s, clean sand 1e-3 to 1e-5, silt 1e-6 to 1e-9, and intact clay below 1e-9. Ten orders of magnitude separate the ends, so the order of magnitude matters far more than the digits in front of it.
The head loss per unit length of flow path (dimensionless).
Computed as the difference in hydraulic head divided by the flow path length between two points.
The cross-sectional area perpendicular to the flow direction.
The GROSS area of the face, solids included — not the pore area. Darcy's velocity is a bulk figure spread over the whole cross-section, which is why it is smaller than the speed water actually moves through the pores; the two differ by the porosity. Entering a pore area here applies that correction twice and understates the seepage, which is the wrong direction for a dewatering or a cut-off design.
Seepage flow rate
0.108 CFM
Darcy's Law assumes laminar flow through a fully saturated, homogeneous, isotropic soil — it becomes less accurate in highly fractured rock, karst, or coarse gravel where flow may be turbulent.
- Equivalent in m³/day
- 4.41 m³/day
- Equivalent in m³/year
- 1,611.38 m³/year
They open the calculator with your figures already in it
Darcy's Law Seepage Flow Rate Calculator: 0.1083 CFM — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Darcy's Law: Q = k x i x A, where Q is flow rate, k is hydraulic conductivity, i is the hydraulic gradient, and A is the cross-sectional flow area
Inputs used
- Hydraulic Conductivity (k, m/s)
- 0
- Hydraulic Gradient (i)
- 0.05
- Cross-Sectional Flow Area (A)
- 110 sq ft
Intermediate steps
- Equivalent in m³/day
- 4.41 m³/day
- Equivalent in m³/year
- 1,611.38 m³/year
Confidence note: Darcy's Law assumes laminar flow through a fully saturated, homogeneous, isotropic soil — it becomes less accurate in highly fractured rock, karst, or coarse gravel where flow may be turbulent.
What this calculation does not cover
- Darcy's Law here treats the ground as one uniform block with a single hydraulic conductivity, a single gradient and a single area. It does not account for layering, anisotropy (horizontal permeability is commonly several times the vertical), or preferential paths — sand seams, fissures, root channels, backfill around old services — which often carry most of the real flow.
- The answer is a flow rate at one instant under one fixed gradient. It does not model transient drawdown, release from storage, recharge from rainfall, or tidal and seasonal movement of the water table, so it will not tell you how inflow to an excavation changes over the first days of pumping.
- This is a flow rate, not a dewatering design and not a stability check. It says nothing about exit gradient, piping, heave, base stability or the settlement of neighbouring ground that drawdown can cause, and it makes no allowance for wellpoint entrance and filter losses, surface water and rainfall entering the dig, or standby capacity — so it does not size a pump on its own. Discharge of the pumped water is normally consented separately.
- Accuracy is dominated by the conductivity you type, which this calculator does not derive and which spans ten orders of magnitude. A published table value for "sand" can be a long way off the sand on your site, and the law itself loses validity where flow turns turbulent — open-graded gravel, rockfill, fractured rock and karst. The input ceilings (a conductivity of 0.1 m/s — 0.33 ft/s — and a gradient of 1) sit at the edge of that territory and anything beyond them is pulled back to the ceiling.
- The area entered is the gross cross-section, so the result is a bulk Darcy flux across that face. It is not the speed water actually travels between the grains — that requires dividing by effective porosity, which this does not do — so it gives you no contaminant or tracer travel time.
Add the equipment this sizes
This result is a specification — 0.108 CFM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-05 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Darcy's Law: Q = k x i x A, where Q is flow rate, k is hydraulic conductivity, i is the hydraulic gradient, and A is the cross-sectional flow area
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