How it works
Subsurface drains lower the water table by carrying water away. Between two parallel drains the water table forms a hump, highest at the midpoint. The Hooghoudt equation balances the flow into the drains against how easily water moves through the soil, so tighter soils (lower conductivity) need drains closer together, and faster drainage rates do too.
The tile itself has to carry the water from everything upstream. The design flow is the area times the drainage coefficient, and Manning's equation gives the flow a full pipe carries at a given grade and roughness. The calculator finds the smallest standard tile size that can carry the flow.
Worked example
A 20 acre field with 4 inch tile at 3.5 ft, a 1.5 ft required water table depth, 1 in/hr soil, a barrier at 8 ft, a 0.5 in/day drainage coefficient, and 0.2% grade:
- Flow = 20 ac × 0.5 in/day × 18.86 gpm per ac·in/day = 189 gpm
- Hooghoudt: q L² = 8 K_b d_e h + 4 K_a h², with h = 2 ft and d_e = 3.13 ft, gives L = 56 ft
- Tile: smallest size whose full-pipe Manning capacity covers 189 gpm at 0.2% grade = 8 in (263 gpm)
| Input | Value |
|---|---|
| Area drained by this tile line | 20 ac |
| Drainage coefficient (water to remove per day) | 0.5 in/day |
| Soil hydraulic conductivity above the drain | 1 in/hr |
| Soil hydraulic conductivity below the drain | 1 in/hr |
| Drain depth below the surface | 3.5 ft |
| Required water table depth midway between drains | 1.5 ft |
| Depth to the barrier layer below the surface | 8 ft |
| Drain radius | 2 in |
| Tile grade | 0.2 % |
| Pipe roughness (Manning n) | Dual-wall corrugated plastic, smooth interior (0.012) |
| My Manning n | 0.015 |
| Result | Value |
|---|---|
| Drain spacing | 56 ft |
| Hooghoudt equivalent depth | 3.13 ft |
| Design flow for the area | 189 gpm |
| Smallest tile size for that flow at your grade | 8 in |
| Full-pipe capacity of that size | 263 gpm |
| Full-pipe velocity in that size | 1.68 ft/s |
Assumptions and limits
- Steady-state Hooghoudt equation with a uniform soil and drains that all discharge freely. It is a first approximation.
- The equivalent depth uses the closed-form expression of van der Molen and Wesseling (1991) and never exceeds the actual depth to the barrier.
- Tile size uses full-pipe flow at the grade you enter, with nominal size taken as the inside diameter. Actual inside diameters vary by manufacturer.
- One inch of water over one acre is 27,154 gallons, so one inch per day per acre is 18.86 gpm.
- Drainage coefficients, allowable water table depth, and conductivity are inputs. This tool does not choose them for your soil or crop.
Common questions
How far apart should drain tile be?
It depends on the soil. Tighter soils need drains closer together. Enter your conductivity, depth, and drainage coefficient above; the spacing updates with them.
How do I size drain tile for a field?
Multiply the acres it drains by the drainage coefficient to get the flow, then pick the smallest tile whose full-pipe capacity at your grade covers it. One inch per day per acre is 18.86 gpm.
What grade does drain tile need?
Steeper grade carries more water in the same size. Check your state drainage guide and the manufacturer for the minimum grade that keeps the tile from silting.
What is the barrier layer?
A tight layer, such as dense clay or rock, that water cannot easily pass through. A shallow barrier limits how deep the drain flow can reach and shortens spacing.
Sources
- Hooghoudt (1940) steady-state drain spacing equation: q L² = 8 K_b d h + 4 K_a h². Equivalent depth closed form: van der Molen and Wesseling (1991), as given in the standard drainage equation references.
- Manning's equation for pipe flow (US units, 1.486 ÷ n). Smooth-interior dual-wall corrugated plastic pipe: Manning n of about 0.012, per plastic pipe industry references. Use the manufacturer's value if different.
- Unit conversion: 1 acre-inch = 27,154 gallons.
- Read them: Drainage equation (Hooghoudt)
Updated 2026-09-30