This guide shows drainage contractors how to use our free calculators to create professional, data-backed proposals that win more jobs and justify your pricing.
Why Data-Backed Proposals Win
Homeowners get multiple quotes for drainage work. The contractor who can explain why a specific solution is needed — with numbers, not just opinion — wins the job more often and can justify higher pricing.
Instead of saying “You need a French drain,” you can say: “Your 5,000 square foot drainage area produces 0.23 cubic feet per second during a 10-year storm. A 4-inch perforated pipe in a 12-inch by 18-inch gravel trench at 1% slope will handle that flow with a safety factor of 1.8.”
That level of specificity builds trust and separates you from competitors who estimate by gut feel.
Step-by-Step Proposal Workflow
Step 1: Assess the Site
Before you open any calculator, document the basics:
- Total contributing drainage area (pace it off or measure with a phone GPS app)
- Approximate impervious percentage (roof, driveway, patio vs. lawn)
- Soil type (ribbon test or local soil survey data)
- Available fall from problem area to discharge point
- Location and condition of any existing drainage structures
Step 2: Calculate the Design Flow
Use the Rational Method Calculator to determine the peak flow:
Example: Client has a 10,000 sq ft backyard area (0.23 acres) with mixed lawn and patio (C = 0.45), and the local 10-year rainfall intensity is 4 in/hr.
- Q = C x i x A = 0.45 x 4 x 0.23 = 0.41 cfs
For the proposal: “Your backyard generates approximately 0.41 cubic feet per second (about 185 gallons per minute) during a 10-year design storm. That is equivalent to two fire hoses running at the same time.”
Step 3: Size the Solution
Based on the flow, use the appropriate calculator:
| Problem | Calculator | What to Size |
|---|---|---|
| Standing water in yard | French Drain | Pipe diameter, trench dimensions, gravel quantity |
| Surface runoff from driveways | Catch Basin | Grate size, basin dimensions |
| Need to convey water to outlet | Manning’s Pipe | Pipe size and slope |
| Swale along property line | Swale | Bottom width, side slopes, depth |
| Rain garden for downspouts | Rain Garden | Garden area and depth |
| Roof drainage | Roof Drain | Drain size and count |
Step 4: Export and Document
Each calculator provides:
- A step-by-step calculation breakdown showing the math
- A shareable URL with all inputs pre-filled
- Results you can reference in your proposal
Save the calculator URL for each calculation you run. Include it in your proposal so the client (or their engineer) can verify your work.
Step 5: Build the Proposal
Use the calculator outputs to build a professional proposal with these sections:
Proposal Template
Section 1: Site Assessment
Describe what you observed. Use specific measurements.
“The rear yard has a low area approximately 40 feet by 25 feet that collects surface runoff from the roof (2,400 sq ft), the patio (600 sq ft), and the surrounding lawn (7,000 sq ft). The total contributing area is approximately 10,000 sq ft. Soil is clay loam (estimated infiltration rate 0.1 in/hr based on field observation).”
Section 2: Problem Analysis
Explain why the problem exists, with numbers.
“The drainage area produces 0.41 cfs during a 10-year storm (Rational Method, C=0.45, i=4 in/hr, A=0.23 ac). The clay loam soil infiltrates at only 0.1 in/hr, meaning it would take over 10 hours for 1 inch of rainfall to soak in. Without a positive drainage outlet, this water has nowhere to go except sit on the surface.”
Section 3: Proposed Solution
Describe your solution and the engineering behind it.
“We propose a 75-foot French drain from the low point to the front yard swale, consisting of a 6-inch perforated PVC pipe in an 18-inch by 24-inch gravel trench at 1.5% slope. This system has a capacity of 0.62 cfs (Manning’s equation, n=0.013), providing a safety factor of 1.5 over the design flow.”
Section 4: Materials and Cost
| Item | Quantity | Unit Cost | Total |
|---|---|---|---|
| 6” perforated PVC pipe | 75 ft | $1.50/ft | $113 |
| 3/4” washed gravel | 4.5 cy | $45/cy | $203 |
| Non-woven geotextile | 100 ft | $0.40/ft | $40 |
| Fittings, cleanouts | Lump sum | $65 | |
| Pop-up emitter | 1 | $12 | $12 |
| Materials Subtotal | $433 | ||
| Labor (2 crew x 2 days) | 32 hrs | $45/hr | $1,440 |
| Equipment (trencher rental) | 1 day | $250 | $250 |
| Total | $2,123 |
Section 5: Warranty and Maintenance
“We provide a 5-year warranty on materials and workmanship. The system requires minimal maintenance: flush cleanouts with a garden hose once per year and inspect the outlet emitter for blockage.”
Tips for Better Proposals
Include Before/After Photos
If the client’s yard is currently flooding, photograph it during or after rain. Show what the problem looks like today, and reference similar projects you have completed.
Reference Local Rainfall Data
Using the local 10-year rainfall intensity (from NOAA Atlas 14) instead of a generic number shows you understand the local conditions. Mention the rainfall intensity and its source.
Provide Options
Give the client two or three options at different price points. For example:
- Option A: Surface grading only ($800)
- Option B: French drain with grading ($2,100)
- Option C: French drain with dry well and grading ($3,200)
Use the calculators to justify each option’s sizing.
Show Your Work
Including calculator screenshots or URLs in your proposal lets technically minded clients verify your calculations. This builds trust and differentiates you from competitors.
Proposal Checklist
Before you send a proposal, confirm every item below. Missing pieces are the most common reason clients stall or shop your bid around:
- Measured areas — roof, driveway, patio, and lawn square footage (not eyeballed)
- Soil condition — ribbon test result or local soil-survey HSG
- Available fall — measured drop from problem area to discharge point
- Design storm stated — return period and source (e.g., 10-year, NOAA Atlas 14)
- Runoff coefficient / curve number cited — with the HEC-22 or TR-55 source
- Design flow (Q) — calculated, with the calculator URL saved
- Solution sized to Q — pipe/trench/basin dimensions with the safety factor stated
- Itemized cost table — materials, labor, and equipment as separate line items
- Two or three priced options — so the client chooses, rather than saying no
- Warranty and maintenance terms — years covered and what the client must do
- Permit note — who pulls permits, or a flag that a licensed engineer is required
- Verification links — calculator URLs or screenshots the client’s engineer can check
Sample Scope-of-Work Line Items
Reviewers and clients expect these standard scope items in a residential drainage proposal:
| Scope Item | Typical Detail |
|---|---|
| Utility location | Call 811; hand-dig within 2 ft of marked utilities |
| Excavation | Trench to specified depth/width; haul or spread spoil |
| Geotextile | Non-woven filter fabric lining trench walls and top |
| Pipe | Perforated PVC/HDPE, size and slope per calculation |
| Aggregate | Washed 3/4-inch stone envelope, quantity per calculation |
| Outlet | Daylight, pop-up emitter, dry well, or tie-in |
| Cleanouts | At the head and at each direction change |
| Restoration | Topsoil, seed/sod, and site cleanup |
Quick Calculator Links for Common Jobs
| Common Job | Start Here |
|---|---|
| Yard drainage / French drain | French Drain Calculator |
| Driveway / parking lot drainage | Catch Basin Calculator |
| Pipe sizing | Manning’s Pipe Calculator |
| Detention requirement | Pond Sizing Calculator |
| Roof drainage | Roof Drain Calculator |
| Gutter / downspout sizing | Downspout Calculator |
Frequently Asked Questions
What should a drainage proposal include?
A strong drainage proposal has six parts: a site assessment with measured areas, a problem analysis backed by a design-flow calculation, the proposed solution with the engineering behind it, an itemized materials-and-labor cost table, warranty and maintenance terms, and the calculator links or screenshots that let the client verify your math. Data-backed proposals win more jobs and justify higher pricing.
How do I calculate the design flow for a residential drainage proposal?
Use the Rational Method: Q = C x i x A, where C is the composite runoff coefficient (HEC-22 Table 3-1), i is the local 10-year rainfall intensity from NOAA Atlas 14, and A is the contributing area in acres. For a 0.23-acre mixed backyard with C = 0.45 and i = 4 in/hr, Q = 0.45 x 4 x 0.23 = 0.41 cfs (about 185 gpm).
How much should I charge for a French drain?
Price from your itemized cost, not a per-foot rule of thumb. The 75-foot 6-inch French drain in the example above totals about $2,100: roughly $433 in materials plus labor and equipment. Build the estimate from measured quantities so you can defend every line if the client asks.
Do I need to be an engineer to use these calculators for proposals?
No. The calculators are free reference tools for sizing and estimating. For projects that require stamped drawings, permits, or work in a floodplain or wetland, involve a licensed professional engineer — the calculator output gives them a documented starting point and speeds up review.
Related Calculators
- Rational Method Calculator — Calculate design flows for proposals
- French Drain Calculator — Size French drains with material quantities
- Manning’s Pipe Flow Calculator — Verify pipe sizing
- Catch Basin Calculator — Size catch basins and inlets