Tile Flow & Pipe Capacity Calculator — free.
Two questions, answered in seconds. How much water must the outlet move for a given acreage and drainage coefficient — and whether a pipe at a given grade can carry it. Results in cubic feet per second and gallons per minute, so you can match a main to the flow before you order pipe.
A full-flow planning check on one pipe — not a hydraulic design. Real capacity depends on grade along the run, sediment, surcharge and the outlet. To size every main from a network you draw, that’s the full app (a flat membership — unlimited fields, no per-acre fees).
A free drain tile flow rate calculator and pipe capacity calculator for agricultural drainage. Convert a field’s acreage and drainage coefficient into required flow in cfs and gpm, and check a pipe’s full-flow Manning capacity by diameter, grade and wall type (single-wall vs dual-wall smooth). Built by Geo-Surface Pro — the flat-fee membership for surface drainage, tile design and 3D landforming, all in your browser, on unlimited fields with no per-acre charges across the US and Canada.
Estimate in seconds
Match the pipe to the water — before you order it
Two questions, one tool. Work out the flow an outlet has to move for a given acreage, or check whether a pipe at a given grade can carry that flow. Switch between them with the tabs.
Field & drainage coefficient
The coefficient is the depth of water the system removes in a day. Pattern tile usually runs 1/4"–1/2"; depressional or targeted ground that takes surface runoff warrants more. Many regions cap the allowable coefficient for licensing — check your local drainage rules.
Pipe, grade & wall
This only sets how many acres this pipe can serve at full flow — it doesn’t change the pipe’s capacity. Pattern tile usually runs 1/4"–1/2"; regional licensing caps may apply.
Your result
Required flow at the outlet
≈ 377 gpm
Full-flow, theoretical maximum. Real systems need margin for surcharge, sediment and biofilm — design below this number, not right at it.
How these numbers are calculated
The same two equations the paid product's flow and pipe-sizing engine uses:
- Required flow (tab 1):
Q (cfs) = acres × drainage coefficient × (43,560 ÷ 12 ÷ 3,600 ÷ 24). The coefficient is the inches of water the system removes in 24 hours; the constant converts acre-inches per day into cubic feet per second. - Gallons per minute:
gpm = cfs × 448.831(1 cubic foot per second = 448.831 US gallons per minute). - Pipe capacity (tab 2): a full-pipe Manning calculation —
Q = (1.49 ÷ n) × A × R^(2/3) × √S, with areaA = πr², hydraulic radiusR = r ÷ 2for a full circular pipe (r= diameter ÷ 2), and slopeS = grade ÷ 100. - Roughness (n): dual-wall smooth =
0.012; single-wall corrugated steps up with size —0.015at 8" and under,0.017above 8" through 12",0.020above 12". Smoother pipe carries more water at the same grade. - Acres served: capacity divided by the flow one acre needs —
acres = Q ÷ (drainage coefficient × 43,560 ÷ 12 ÷ 3,600 ÷ 24). It's the exact inverse of tab 1, so the two tabs always agree.
Read this before you order pipe
It's a full-flow check, not a hydraulic design
The capacity number is the theoretical maximum for a clean pipe running exactly full at one steady grade — the most water it could ever move. A real tile line rarely runs there: sediment and biofilm roughen it, joints and inlets add loss, and the outlet can back it up. Treat the result as a ceiling to design under, not a target to hit — leave margin, and confirm the numbers over the real field before you commit.
Good enough to
- Ballpark the flow an outlet has to move
- Check whether a pipe size is in the right range
- Compare single-wall vs dual-wall at the same grade
- See roughly how many acres one main can serve
Not built to
- Set a final pipe size or order materials
- Account for sediment, surcharge or partial flow
- Size a main stepwise as acres accumulate
- Replace a real layout or an RTK survey
Plan and quote from elevation data — survey and grade with RTK when the job goes ahead. Geo-Surface Pro turns a flat contributing area into a sized network over the real field, stepping each main up as acres accumulate, with elevation built in so there's no survey just to plan. The committed job is then surveyed run-by-run with your machine-control RTK and built however you already work.
Quick answers
What size tile pipe do I need for X acres?
It depends on more than acres alone — the drainage coefficient you're designing to and the grade the pipe runs at both move the answer. That's exactly what the calculator above solves for one pipe: enter the acres and DC for required flow, or the pipe, grade and wall type for capacity.
For a whole system, the right size shifts main-by-main as laterals add acres along the run — which is why Geo-Surface Pro sizes every main from the real contributing area at each junction, not one flat number for the field.
What is a drainage coefficient?
A drainage coefficient is the depth of water a tile system is designed to remove from the field in 24 hours, expressed in inches per day. It's the "how fast," not just the "how much" — the number a drainage coefficient calculator like the one above turns directly into a required flow in cfs and gpm.
Pattern tile commonly runs 1/4"–1/2" per day; depressional or targeted ground taking surface inlet water usually warrants more. Many regions cap the allowable coefficient for licensing, so check your local drainage rules before you settle on one.
Should I use cfs or gpm for drain tile?
Both — they're the same flow in different units, and the calculator above shows them side by side. Cubic feet per second (cfs) is the standard for pipe sizing and Manning's equation, the number you'll see in most drainage references. Gallons per minute (gpm) is the easier number for pumps, and for picturing how much water is actually moving. Use cfs when you're sizing pipe; use gpm when you're explaining the flow to someone else.
This checks one pipe. The app sizes every main you draw.
Here you size a single pipe against a flat contributing area. Geo-Surface Pro sizes the real thing: draw the network and it steps each main up automatically at every junction as acres accumulate, surface inlets included — on as many fields as you want, for $499/yr (new signups pay $799/yr from September 1, 2026). One flat membership, unlimited fields and acres, no per-acre fees, ever.
One membership, three jobs: surface drainage · tile design · 3D landforming — all in the browser.
- Automatic Manning's pipe sizing. The Auto Size Tile Mains wizard sizes every run from real contributing area, drainage coefficient and grade — no chart lookups.
- Stepped main sizing at every junction. Diameter steps up automatically as each lateral adds acres, with segment-by-segment review and manual override.
- Live profile charts. See cut and fill along the run as you grade — not just a single-pipe number.
Price lock: sign up at $499/yr now and your renewal stays $499/yr for as long as you keep your membership — every feature we ship after that is included, forever, at that price.
Plan and quote from elevation data. Survey and grade with RTK when the job goes ahead.
