Drainage Inlet Spacing Calculator

Example data table

Use this sample set to understand expected inputs and outputs.

Formula used

• Rational runoff (peak flow): Q = C · i · A. For English units, A is in acres; for Metric, include the factor 0.00278.
• Contributing area between inlets (rectangular approximation): A = W · L (converted to acres or hectares).
• Triangular gutter flow at spread (integrated Manning): Q = (k/n) · Sx^5/3 · S^1/2 · T^8/3, where k = 0.56 (English) or 0.377 (Metric).
• Spacing from limiting flow: solve for L so the segment runoff does not exceed the smaller of (spread-limited flow) and (inlet-capacity flow), both reduced by the efficiency factor.

How to use this calculator

1. Select a unit system and choose whether to limit by spread, capacity, or both.
2. Enter contributing width (the surface draining to your gutter line).
3. Enter rainfall intensity directly, or use your IDF coefficients and time of concentration.
4. Provide an inlet capacity if you want the capacity-based spacing check.
5. Provide spread, slopes, and Manning n if you want the spread-limited gutter check.
6. Set an efficiency factor to reflect clogging and safety margin.
7. Click Calculate spacing. Review governing criteria and stationing.
8. Download CSV/PDF to document the design snapshot.

Technical guidance for inlet spacing decisions

1) What the calculator is sizing

Inlet spacing is the distance between capture points along a gutter line or edge drain path. This tool estimates a recommended spacing that keeps the segment runoff at or below a governing limiting flow. The limiting flow can be set by inlet capture capacity, by an allowable water spread, or by using both checks together for a conservative preliminary layout.

2) Runoff inputs and rainfall intensity

Peak runoff is estimated with the Rational Method using a runoff coefficient (C), rainfall intensity (i), and contributing area. For convenience, you may enter i directly, or compute i from an IDF relationship with coefficients a, b, and c and a time of concentration t (minutes). Always select coefficients that match your local design standards and storm return period.

3) Spread-limited flow in a triangular gutter

Where ponding must be controlled, the spread check converts allowable spread (T), cross slope (Sx), longitudinal slope (S), and Manning roughness (n) into an allowable gutter flow. This is a practical approach for straight gutter sections and typical roadway cross sections. When your site includes sag points, transitions, or complex geometry, verify with a detailed capture and bypass analysis.

4) Efficiency factor and design margin

Real inlets rarely perform at perfect conditions. Leaves, sediment, partially blocked grates, and construction debris reduce capture. The efficiency factor reduces both inlet capacity and spread-based flow to provide margin. Values such as 0.80–0.95 are often used for planning, but confirm maintenance expectations and agency guidance.

5) Turning spacing into a layout

After the recommended spacing is computed, the optional project length generates a station list for quick takeoff. Use the result as a starting point, then adjust to match driveways, utilities, curb ramps, and grading constraints. Keep an eye on the governing criterion; if it is spread-limited, improving slopes or reducing allowable spread can tighten spacing more than increasing inlet capacity.

FAQs

1) Which design mode should I use?

Use both checks when you have spread limits and known inlet capacity. If you only know one constraint, select spread-only or capacity-only for a preliminary estimate.

2) What does “contributing width” represent?

It is the effective width draining to the gutter between inlets, such as pavement lanes, shoulder, or a paved yard strip that sheds flow to the same collection line.

3) How do I choose a runoff coefficient C?

Select C based on surface type and drainage characteristics. Impervious pavement is high, lawns are lower, and mixed areas fall in between. Local manuals may provide recommended ranges.

4) Why does the spacing change when I adjust allowable spread T?

Allowable spread affects the gutter flow permitted before water encroaches on traffic lanes or walkways. A smaller T reduces allowable flow, which typically reduces the computed spacing.

5) What slopes should I enter for S and Sx?

Enter the gutter’s longitudinal slope for S and the cross slope draining toward the gutter for Sx. Use the controlling (flattest) slopes where ponding is most critical.

6) Can I rely on the inlet capacity value alone?

Capacity-only spacing can be optimistic if spread control, bypass flow, or clogging is important. Include efficiency, consider spread limits, and confirm inlet type and capture assumptions for final design.

7) Does this handle sag inlets and sump conditions?

No. The spread check is intended for gutter flow in straight grades. For sag points, evaluate ponding depth, inlet capacity curves, and emergency overflow routes using applicable agency procedures.