What is 'radius of influence' used for?

It estimates how far from a dewatering well the groundwater drawdown effect extends — used to check for interference with adjacent wells, potential settlement risk to nearby structures, and whether dewatering could affect neighboring properties.

Why is this formula called a 'first-pass' estimate?

It only uses two simple parameters (drawdown and permeability) and ignores factors like aquifer boundaries, pumping duration, and recharge — real dewatering systems are typically monitored and adjusted using observation wells rather than relying on the formula alone.

Does higher permeability mean a larger or smaller radius of influence?

Larger — more permeable soil transmits the drawdown effect further from the well, which is why coarse sand/gravel sites need wider well spacing (or more wells) than silty or clayey sites for the same target drawdown.

Can I treat the answer as a boundary line — affected inside it, unaffected outside?

No, and that is the single most common misuse of this number. Drawdown decays continuously with distance and is already down to millimetres long before you reach R, so the figure is a fade-out distance rather than a fence line, and this page returns it as one bare number with no uncertainty band and no drawdown-versus-distance curve behind it. Nothing in two inputs can describe direction either — a gravel lens, a fissured layer, a river or a leaking main on one side — so the real effect can reach much further one way than another, and the answer is a circle only because the arithmetic has nothing to make it any other shape. Use it to decide where your instruments go, not to assure a neighbour they sit outside it: take baseline groundwater levels before the pumps start, set piezometers both inside the predicted radius and well beyond it, and get a geotechnical engineer's assessment where compressible ground, a shallow-founded structure, or a services corridor falls within it. That handover point is the honest limit of what this page can tell you.

Does the hydraulic conductivity field follow the metric/imperial switch?

No — it is the one input on this page the switch does not reach. Drawdown is a dimensioned length, so on an imperial page that box is in feet and the conversion to metres is done for you before the arithmetic runs. Hydraulic conductivity is a bare number read as metres per second whichever system is selected, so convert before you type: 1 cm/s = 0.01 m/s, 1 m/day = 1.16e-5 m/s, 1 ft/day = 3.53e-6 m/s. Its accepted range is the only guard, and it is a partial one — type below 1e-7 or above 0.01 and the box resets to the nearer limit when you leave it and tells you it did, but that window spans five orders of magnitude and anything inside it is taken as typed. A figure entered in cm/s is the one that slips through quietly: a hundred times too large, still in range. The square root softens that less than people assume, since a k out by a factor of 100 moves the radius by a factor of 10, while drawdown enters linearly, so doubling it doubles the answer.

The drawdown field goes to 15 m — can a wellpoint ring actually lower water that far?

Not in one stage. Wellpoints are suction-lift kit — header main and vacuum pump sitting at ground level — so a single ring commonly gets somewhere around 5 to 6 m of drawdown, and less in fine soils, at altitude, or with air leaking into a badly sealed header. Ask for 10 or 12 m and you are really specifying a staged installation, with a second and sometimes third ring set on lower benches as the dig goes down, or you have moved on to deep wells with submersible pumps, or to ejector wells in low-permeability ground. This page does not know which of those you are building; it returns 3000 x s x sqrt(k) for any drawdown the field accepts. For a staged scheme, enter the total lowering you need rather than one stage's share, since the far-field cone is driven by the full drawdown — and budget the space early, because each lower ring is installed from a bench cut inside the one above it, so the excavation has to open out considerably wider at the top than the base you are digging for.

Materials & Quantities

Wellpoint Dewatering Radius of Influence Calculator (Sichardt)

Estimate the radius of influence for a dewatering wellpoint system using the Sichardt empirical formula.

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Every formula citedNamed published standards
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Imperial · sales tax

Required Drawdown (s)

How far you need to lower the water table at the well.

The difference between the natural groundwater level and the target lowered level needed for dry excavation.

Hydraulic Conductivity (k, m/s)

The soil's hydraulic conductivity, from a pump test or published typical values for the soil type.

Fine sand is roughly 1e-5 to 1e-3 m/s; silt is roughly 1e-7 to 1e-5 m/s — permeability varies enormously by soil type, so a site-specific pump test is strongly preferred over a textbook estimate.

Estimated radius of influence

300 ft

Low confidence

The Sichardt formula is a widely-used but approximate empirical estimate — it's most useful for early planning and tends to underestimate the true radius of influence compared to more rigorous pump-test-based methods. Verify with an observation well or pump test once dewatering begins, especially near property lines or sensitive structures.

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Sichardt's empirical formula (unconfined conditions): R ≈ 3000 x s x sqrt(k), where R is the radius of influence in meters, s is the drawdown in meters, and k is the soil's hydraulic conductivity in m/s. Widely used as a first-pass planning estimate, though it's known to be approximate and typically most reliable for early-stage (first few days) dewatering planning.

Inputs used

Required Drawdown (s): 10 ft
Hydraulic Conductivity (k, m/s): 0

Final result300 ft

Confidence note: The Sichardt formula is a widely-used but approximate empirical estimate — it's most useful for early planning and tends to underestimate the true radius of influence compared to more rigorous pump-test-based methods. Verify with an observation well or pump test once dewatering begins, especially near property lines or sensitive structures.

What this calculation does not cover

No input describes the excavation or the wellpoint ring, so the answer is a distance out from the dewatering source, not a radius measured from a defined centre. Layering, horizontal-versus-vertical permeability, and wellpoints that penetrate only part of the aquifer all sit outside the two numbers this uses.
Time is not in the formula. There is no pumping duration and no aquifer storage behind the figure, so it does not tell you how far the cone has spread after a week, a month, or the length of the job — only a single planning distance.
Boundaries and recharge are excluded. A river, canal, leaking main or other recharge source near the excavation holds the water table up and keeps the real effect well short of this figure, while a cut-off wall, sheet-pile box or clay layer redirects it. Nothing about the surroundings reaches the arithmetic.
The formula is written for unconfined, water-table conditions. Confined or artesian ground, perched water sitting on a clay layer, and pressure relief of a deeper aquifer behave differently, and this number does not describe them.
This is not a dewatering design and not a damage assessment. It returns no well count, spacing, flow rate or pump duty, and it says nothing about settlement of compressible ground, timber piles kept sound by the water table, or services and shallow foundations inside the radius — that needs a geotechnical engineer, baseline levels and monitoring. Abstraction and discharge consents are a separate matter it does not touch.

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 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Sichardt's empirical formula (unconfined conditions): R ≈ 3000 x s x sqrt(k), where R is the radius of influence in meters, s is the drawdown in meters, and k is the soil's hydraulic conductivity in m/s. Widely used as a first-pass planning estimate, though it's known to be approximate and typically most reliable for early-stage (first few days) dewatering planning.

Cite this page

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Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Dewatering an Excavationuses this calculator
Below the water table you either pump the ground down and pay for it outside your boundary, or hold the hole with head and pay for it in wall.
Tanking a Basement Wall Against Standing Water

Still deciding? Wellpoints vs Sump Pumping — the factors that actually differ, with no invented prices.

How to calculate wellpoint dewatering radius of influence (Sichardt) in 3 steps

Required Drawdown (s)How far you need to lower the water table at the well.
Hydraulic Conductivity (k, m/s)The soil's hydraulic conductivity, from a pump test or published typical values for the soil type.
Estimated radius of influenceThe tool computes the estimated radius of influence from those figures and shows the formula, its sources, and a confidence rating alongside it.

Frequently asked questions

What is 'radius of influence' used for?: It estimates how far from a dewatering well the groundwater drawdown effect extends — used to check for interference with adjacent wells, potential settlement risk to nearby structures, and whether dewatering could affect neighboring properties.
Why is this formula called a 'first-pass' estimate?: It only uses two simple parameters (drawdown and permeability) and ignores factors like aquifer boundaries, pumping duration, and recharge — real dewatering systems are typically monitored and adjusted using observation wells rather than relying on the formula alone.
Does higher permeability mean a larger or smaller radius of influence?: Larger — more permeable soil transmits the drawdown effect further from the well, which is why coarse sand/gravel sites need wider well spacing (or more wells) than silty or clayey sites for the same target drawdown.
Can I treat the answer as a boundary line — affected inside it, unaffected outside?: No, and that is the single most common misuse of this number. Drawdown decays continuously with distance and is already down to millimetres long before you reach R, so the figure is a fade-out distance rather than a fence line, and this page returns it as one bare number with no uncertainty band and no drawdown-versus-distance curve behind it. Nothing in two inputs can describe direction either — a gravel lens, a fissured layer, a river or a leaking main on one side — so the real effect can reach much further one way than another, and the answer is a circle only because the arithmetic has nothing to make it any other shape. Use it to decide where your instruments go, not to assure a neighbour they sit outside it: take baseline groundwater levels before the pumps start, set piezometers both inside the predicted radius and well beyond it, and get a geotechnical engineer's assessment where compressible ground, a shallow-founded structure, or a services corridor falls within it. That handover point is the honest limit of what this page can tell you.
Does the hydraulic conductivity field follow the metric/imperial switch?: No — it is the one input on this page the switch does not reach. Drawdown is a dimensioned length, so on an imperial page that box is in feet and the conversion to metres is done for you before the arithmetic runs. Hydraulic conductivity is a bare number read as metres per second whichever system is selected, so convert before you type: 1 cm/s = 0.01 m/s, 1 m/day = 1.16e-5 m/s, 1 ft/day = 3.53e-6 m/s. Its accepted range is the only guard, and it is a partial one — type below 1e-7 or above 0.01 and the box resets to the nearer limit when you leave it and tells you it did, but that window spans five orders of magnitude and anything inside it is taken as typed. A figure entered in cm/s is the one that slips through quietly: a hundred times too large, still in range. The square root softens that less than people assume, since a k out by a factor of 100 moves the radius by a factor of 10, while drawdown enters linearly, so doubling it doubles the answer.
The drawdown field goes to 15 m — can a wellpoint ring actually lower water that far?: Not in one stage. Wellpoints are suction-lift kit — header main and vacuum pump sitting at ground level — so a single ring commonly gets somewhere around 5 to 6 m of drawdown, and less in fine soils, at altitude, or with air leaking into a badly sealed header. Ask for 10 or 12 m and you are really specifying a staged installation, with a second and sometimes third ring set on lower benches as the dig goes down, or you have moved on to deep wells with submersible pumps, or to ejector wells in low-permeability ground. This page does not know which of those you are building; it returns 3000 x s x sqrt(k) for any drawdown the field accepts. For a staged scheme, enter the total lowering you need rather than one stage's share, since the far-field cone is driven by the full drawdown — and budget the space early, because each lower ring is installed from a bench cut inside the one above it, so the excavation has to open out considerably wider at the top than the base you are digging for.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.