How to use PipeMAC

PipeMAC solves the Manning gravity-flow equation for a circular pipe flowing full. Set any three of slope, size, and flow, and solve for the fourth — a quick, auditable capacity check for storm and sanitary gravity mains.

1The equation

PipeMAC uses the Manning formula for a pipe flowing full, in US customary units:

Q = (1.486 / n) · A · R^2/3 · S^1/2

with cross-sectional area A = πD²/4 and hydraulic radius R = D/4 for the full bore, where Q is discharge (cfs), D is inside diameter (ft), S is slope (ft/ft) and n is Manning's roughness. All inputs and outputs are reported in English units.

2Pick what to solve for

Each of the three variables — slope, size, and flow — has a solve toggle on the left of its row. Select exactly one. That row is boxed and turns green to show the computed result; the other two stay as sliders you set. Everything recalculates live as you drag — there is no calculate button.

3Set the known values

Slope (S) — slider from 0.25% to 10% in 0.25% steps.
Size (D) — slider that snaps only to available stock diameters, so it can never land on a size you can't order.
Flow (Q) — slider from 5 to 200 cfs in 5-cfs steps.

Stock sizes (in): 18 · 24 · 30 · 36 · 42 · 48 · 54 · 60 · 66 · 72 · 84

4Manning's roughness (n)

Manning's n lives in a collapsible row that starts closed, with its current value shown in the header as Manning's roughness (n) [0.013]. Click the + to open the yellow field and change it; the header value updates as you type. It defaults to 0.013, typical for concrete pipe.

5Read the results

The dark tile shows the solved variable: capacity in cfs, required slope in %, or the selected pipe size.
Full-flow velocity (ft/s) and full capacity (cfs) are reported alongside. Velocity is flagged if it falls below 2 ft/s (sedimentation) or above 15 ft/s (erosion).
Slope is reported to two decimals, flow and velocity to one decimal.
Open Calculation audit to see every intermediate value — area, hydraulic radius, and the rest — in English units.

When solving for size, the required theoretical diameter is rounded up to the next stock size, so the selected pipe always meets or exceeds the required flow. If the required diameter exceeds 84 in, PipeMAC shows the largest stock size and flags that its capacity falls short. Results are preliminary and must be reviewed by a registered professional engineer.