Stormwater Calculator

Size a storage tank, pipe rows, chambers or seepage pits for a New Jersey site. Every design is checked against all six storms the rules require — the current and the projected (future) 2-, 10- and 100-year storms — using the same NRCS hydrograph method an engineer's software uses. How it works · ← All tools

1

Site

Where the project is decides which rainfall and which rules apply. Looking up the address fills in the county, town, soil and the exact-site rainfall.

Soil group hydrologic soil group

Does the stormwater rule apply?

Lot width and depth are used only for the plan drawing.
Usually the whole lot on a new building.
Is existing paving or roof being torn out and rebuilt? reconstruction
Leave blank if none. Existing hard surface that did not drain to a pipe before counts toward the thresholds once it does.
Does the project need site plan or subdivision approval?
In Newark this alone makes a project minor development (§41:17-2).
Rules
2

Storms

New Jersey requires six design storms: the 2-, 10- and 100-year storms at today's rainfall (current, Table 5-5) and at the rainfall expected for 2050–2099 (projected, Table 5-6). All six are always run; the projected storms usually decide the size.

Rainfall source
Rain pattern and time step
How often the calculator steps through each storm. Shorter steps are more exact for roofs and paving, which drain in a minute or two, and a little slower.
3

Surfaces

List what covers the lot before and after construction. Each surface is modeled separately, as the state rule requires (N.J.A.C. 7:8-5.7(a)4). Uncheck To system for areas that drain straight to the street, and tick Cars for surfaces that cars drive or park on.

Has the existing ground cover been in place for five years or more?
How much runs off: CN (NRCS curve number) and C (Rational runoff coefficient) both measure how much of the rain runs off instead of soaking in; the minutes are how long water takes to reach the drain.

How water travels across the site

4

Storage system

Choose what holds the water back. Enter a size, or let the calculator find the smallest one that meets every storm.

A Green Stories margin, not a code requirement. Enter 0 to size to the code minimum.
5

Outlet

Openings in the outlet structure let water out slowly. The lowest one handles the small storms; higher ones start working only in bigger storms. Heights are measured from the floor of the storage system.

An overflow slot (weir) is a wide opening near the top of the outlet structure that lets big storms out.

Outlet pipe to the sewer or street

Smoother pipe carries more water. Manning n: HDPE 0.012 · PVC 0.010 · concrete 0.013
6

Elevations

Optional. With a few elevations the calculator checks soil cover, groundwater separation and whether the outlet pipe can drain to the sewer by gravity. Leave any field blank to skip its check.

From the soil borings.

Where the outlet pipe goes

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Results

7

Report

A printable study in the order an engineer's stormwater report uses: site, rules, method, rainfall, surfaces, storage and outlet, results, drawings and maintenance notes. Print it or save it as a PDF from the preview.

These calculations are preliminary estimates for design and coordination. They follow N.J.A.C. 7:8 (amended January 20, 2026), the NJ Stormwater BMP Manual and Newark §41:17, but they do not replace analysis and certification by a licensed professional engineer. Green Stories LLC is a building design and consulting firm; projects that require a licensed professional are completed with registered design professionals.