How to use OrificeMAC

OrificeMAC gives a preliminary free-discharge flow through a circular orifice from the standard orifice equation, in US customary units. Enter the discharge coefficient and orifice diameter, pick the unknown to solve for — flow, diameter, or head — and read the result, the head–discharge rating curve, and a scaled orifice elevation as you type.

1The equation it solves

OrificeMAC evaluates the standard orifice relationship in its US customary form, and rearranges it to return whichever of the three unknowns you choose. The diameter you enter (in inches) is converted to an area in square feet, A = πD²/4:

Q = C · A · √(2 · g · H)
H = ( Q / (C · A) )² / (2 · g)
D = √( 4A / π ) (then ×12 for inches)
A = π · D² / 4 · V = Q / A · g = 32.2 ft/s²

Q: Discharge (flow), cfs
C: Discharge coefficient — always an input; defaults to 0.6 for a sharp-edged orifice
A: Orifice area, ft² — computed from the diameter, πD²/4
D: Orifice diameter, inches (converted internally to feet)
H: Head measured to the orifice centroid, ft
g: Gravitational acceleration, fixed at 32.2 ft/s²
V: Mean jet velocity (Q ÷ A), ft/s
H/D: Head-to-diameter ratio, dimensionless — a range check

Note on the coefficient. C is not solved for — it defines the orifice type. The default 0.6 suits a sharp-edged circular orifice; values run roughly 0.60–0.65 for sharp edges, ~0.80 for a rounded entrance, and up to ~0.98 for a well-designed bellmouth. OrificeMAC does not correct C for approach velocity or submergence; that judgment stays with you.

2Orifice & flow parameters

The first panel is a ledger of parameters. Enter the discharge coefficient C, then fill in the two values you know among orifice diameter D, head H, and flow Q. D is in inches; H is in feet, measured to the orifice centroid; Q is in cfs.

Every field is color-coded so you always know what you are looking at:

yellow — editable. Type or change the value here.
green — computed. The value OrificeMAC solved for; it is read-only.
A red outline means a value is missing or out of range — the message under the panel says what to fix.

3Solve for flow, diameter, or head

Three rows — Flow Q, Orifice diameter D, and Head H — each carry a solve toggle. Pick the one you want OrificeMAC to compute; that row is highlighted and its field turns green, while the other two become yellow fields you fill in.

Solve for flow. You enter D and H; OrificeMAC returns the discharge Q.
Solve for diameter. You enter Q and H; OrificeMAC returns the required diameter D.
Solve for head. You enter Q and D; OrificeMAC returns the head H.

Everything recomputes as you type — there is no calculate button.

4Read the results

The dark result tile shows the variable you solved for — discharge Q (cfs), orifice diameter D (in), or head H (ft) — with a line beneath it recapping the configuration.
Two cells below report the mean jet velocity V = Q/A (ft/s) and the head-to-diameter ratio H/D. The H/D cell turns amber when the head is not large relative to the opening (H < 2D — the large-orifice regime) or below ~0.2 ft of head (low-head accuracy), and red when the head to the centroid is less than the orifice radius, meaning the opening is not fully submerged and the equation no longer applies.
Open Calculation audit for the full worked set: coefficient, diameter, area, head, the velocity-head term √(2gH), discharge, jet velocity, and H/D, alongside the governing equations.
The orifice elevation panel draws a side section: a gray tank wall holding water, the head H dimensioned from the water surface down to the orifice centroid, the opening D on the wall, and a free jet discharging to atmosphere. Head and diameter are scaled independently so a deep-head, small-orifice case stays readable.
The rating curve plots head H (vertical axis) against discharge Q (horizontal axis) at your current C and D. Drag the marker along the curve to set a new operating point — because H and Q are locked by Q = C·A·√(2gH), moving one drives the other, and the input fields and result update live.

5Scope & limits

Results assume steady free discharge through a fully submerged, sharp-edged circular orifice venting to atmosphere, with the head measured to the orifice centroid, and are reported in English (US customary) units only. The equation treats the orifice as “small” — head effectively uniform across the opening — so accuracy degrades as the opening grows relative to the head. OrificeMAC does not correct the coefficient for approach velocity or submergence (drowned outlets), does not handle non-circular openings or partial submergence, and does not check structural adequacy. The elevation drawing is a schematic aid, not a design check.

Intended for use by or under the supervision of a registered professional engineer. The engineer is responsible for confirming that results are correct and appropriate for the problem being solved.