What This Solves
Calculates foundation drain pipe sizing and sump pump requirements for basement and crawlspace waterproofing based on groundwater inflow and soil permeability.
Best Used When
- You are designing a perimeter foundation drain system for a new building
- You need to calculate the groundwater inflow rate to size the drain pipe and sump pump
- You want to verify that an existing foundation drain system has adequate capacity
Do NOT Use When
- You need a yard or field drainage system rather than a building foundation drain — Use French Drain Calculator
- You need to size only the sump pump without foundation drain calculations — Use Sump Pump Calculator
Key Assumptions
- Groundwater inflow is estimated using Darcy's Law with the surrounding soil hydraulic conductivity
- The drain pipe is installed at or below the footing elevation
- Filter fabric or graded aggregate prevents soil migration into the drain
- The sump pump runs intermittently based on pit volume and inflow rate
- No significant artesian pressure or confined aquifer conditions
Input Quality Notes
Soil hydraulic conductivity is the most uncertain input. Use field percolation tests at footing depth if possible. Seasonal high groundwater elevation should be the design condition, not average.
Size a foundation (footing) perimeter drain for a basement, crawl space or slab. Enter your foundation geometry, soil type and pipe details to estimate groundwater and surface-water inflow, check pipe capacity with Manning's equation, and — when there is no gravity outlet — size the required sump pump.
Calculate Foundation Drain Design
For educational purposes only. Not a substitute for professional engineering judgment.
Foundation Drain Design Overview
Foundation drains (footer drains) collect and remove groundwater and surface water from around building foundations. They protect against hydrostatic pressure, basement flooding, and foundation damage.
- Groundwater Flow - Calculated using Darcy's Law with soil hydraulic conductivity
- Surface Water - Calculated using Rational Method (Q = CiA)
- Pipe Capacity - Determined using Manning's equation for pipe flow
- Sump Pump - Required when gravity outlet is not available
Soil Hydraulic Conductivity Values
| Soil Type | Min (ft/day) | Typical (ft/day) | Max (ft/day) |
|---|---|---|---|
| Gravel | 2800 | 14000 | 28000 |
| Sand | 28 | 140 | 560 |
| Sandy Loam | 2.8 | 14 | 56 |
| Silt Loam | 0.28 | 1.4 | 5.6 |
| Clay Loam | 0.028 | 0.28 | 1.4 |
| Clay | 0.00028 | 0.0028 | 0.028 |
Source: Mays, L.W. (2011), Water Resources Engineering, Table 3.4
Standard Sump Pump Sizes
Common residential and commercial sump pump capacities (GPM):
A 25% safety factor is applied when selecting pump size.
How the foundation drain calculator works
The design flow a footing drain must carry is the sum of steady-state groundwater infiltration and peak surface-water runoff. The pipe is then checked against its full-flow capacity, and a sump pump is sized only when gravity discharge is unavailable.
1. Groundwater inflow — Darcy's Law
Qgw = K × i × A
- K = soil hydraulic conductivity (ft/day, converted to ft/s)
- i = hydraulic gradient, taken as 1.0 (conservative)
- A = perimeter length × depth of footing below the water table
If the groundwater table is below the footing, Qgw = 0.
2. Surface-water inflow — Rational Method
Qsw = C × i × A
- C = runoff coefficient, set to 0.95 (near-impervious backfill/paving against the wall)
- i = design rainfall intensity (in/hr)
- A = contributing surface area directed to the drain (converted to acres)
3. Pipe capacity — Manning's equation
Qpipe = (k / n) × A × R2/3 × S1/2
- k = unit conversion factor (1.486 for US customary, 1.0 for SI)
- n = Manning's roughness (material-dependent, see table below)
- A = full pipe cross-sectional area = πD²/4
- R = hydraulic radius = D/4 for a full circular pipe
- S = pipe slope (ft/ft)
The drain is adequate when Qpipe ≥ Qgw + Qsw.
For a sump outlet, the design flow is converted to gpm (1 cfs = 448.831 gpm) and a standard
pump is selected at a 25% safety factor.
Manning's roughness (n) for foundation drain pipe
Pipe material sets the roughness coefficient used in the capacity calculation. Smooth-wall PVC carries noticeably more flow than corrugated HDPE of the same diameter and slope.
| Pipe material | Manning's n | Relative capacity |
|---|---|---|
| Perforated PVC, SDR-35 (smooth wall) | 0.010 | Highest |
| Perforated PVC, Schedule 40 (smooth wall) | 0.010 | Highest |
| Rigid PVC | 0.010 | Highest |
| Cast iron | 0.013 | Moderate |
| Corrugated HDPE | 0.020 | Lowest (~half of smooth PVC) |
Source: FHWA HEC-22 (2009), Urban Drainage Design Manual, Table 7-1. Because capacity scales with 1/n, a corrugated-HDPE drain (n = 0.020) carries roughly half the flow of smooth PVC (n = 0.010) at the same size and slope.
Frequently asked questions
What size pipe do I need for a foundation drain?
Most residential footing drains use 4 in (100 mm) perforated pipe, which is also the minimum interior diameter required by the International Plumbing Code (IPC 2021, Section 1111) for subsoil drains. At a 1% (0.01 ft/ft) slope, a 4 in smooth-wall PVC drain (Manning's n = 0.010) carries on the order of 0.25 cfs (about 110 gpm) when flowing full — comfortably more than the inflow from a typical house. This calculator sizes the pipe by comparing Manning's full-flow capacity against the combined groundwater and surface-water design flow, so step up to 6 in pipe or a steeper slope if the capacity check fails.
How is groundwater inflow to a footing drain calculated?
The calculator uses Darcy's Law, Q = K · i · A, where K is the soil hydraulic conductivity (ft/day), i is the hydraulic gradient (taken as 1.0 as a conservative assumption), and A is the wetted infiltration area along the foundation. The area is the perimeter length multiplied by the depth the footing extends below the water table. If the groundwater table sits below the footing, the groundwater inflow is zero and only surface water governs. Conductivity values come from Mays, Water Resources Engineering (2011), Table 3.4 — ranging from roughly 140 ft/day for clean sand down to 0.003 ft/day for clay.
When do I need a sump pump instead of a gravity drain?
A gravity outlet is preferred whenever the footing drain can discharge to daylight or to a storm sewer below the drain invert. A sump pump is required when there is no gravity outlet lower than the footing — common with deep basements on flat sites or below a storm main. When you select a sump-pump or combined outlet, the calculator converts the design flow to gpm, sizes the pump with a 25% safety factor against standard capacities (20, 33, 50, 75, 100, 150, 200 gpm), and computes total dynamic head as static lift plus friction losses.
What slope should a foundation drain be laid at?
Footing drains are typically laid at a minimum of about 1% (0.01 ft/ft, or roughly 1/8 in per foot) to keep water moving toward the outlet while staying buildable around the foundation perimeter. The IPC permits subsoil drains to be laid level when surrounded by an approved gravel envelope, but a positive slope improves self-cleansing and outlet performance. This calculator accepts slopes from 0.001 to 0.1 ft/ft and uses Manning's equation to confirm the chosen slope and diameter deliver enough capacity. (Subsoil drain requirements, including the level-in-gravel-envelope allowance, are in IPC 2021, Section 1111.)
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Last verified: February 2026