How to use WeirMAC
WeirMAC gives a preliminary discharge for a weir from the standard weir
equation, in US customary units. Enter the weir coefficient, pick the unknown to solve for —
flow, crest length, or head — and read the result, the head–discharge rating curve, and
a scaled weir elevation as you type.
1The equation it solves
WeirMAC evaluates the standard weir relationship in its US customary form, and rearranges it to
return whichever of the three unknowns you choose:
Q = C · L · H3/2
L = Q / (C · H3/2)
H = ( Q / (C · L) )2/3
q = Q / L
- Q
- Discharge (flow), cfs
- C
- Weir coefficient — always an input; ~3.33 for a sharp-crested rectangular weir
- L
- Effective crest length, ft
- H
- Head measured on the crest, ft
- q
- Unit discharge per foot of crest (Q ÷ L), cfs/ft
- H/L
- Head-to-length ratio, dimensionless — a range check
2Weir & flow parameters
The first panel is a ledger of parameters. Enter the weir coefficient C, then fill
in the two values you know among crest length L, head H, and
flow Q. L and H are in feet; 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 WeirMAC 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, length, or head
Three rows — Flow Q, Crest length L, and Head H
— each carry a solve toggle. Pick the one you want WeirMAC 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 L and H; WeirMAC returns the discharge Q.
- Solve for length. You enter Q and H; WeirMAC returns the crest length L.
- Solve for head. You enter Q and L; WeirMAC 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),
crest length L (ft), or head H (ft) — with a line beneath it recapping the configuration.
- Two cells below report unit discharge q = Q/L (cfs/ft) and the
head-to-length ratio H/L. The H/L cell turns amber when head exceeds about L/3
(end contractions / approach velocity may matter) or below ~0.2 ft of head (low-head accuracy),
and red when head exceeds the crest length — a prompt to check C, not a pass/fail.
- Open Calculation audit for the full worked set: coefficient, length, head,
H3/2, discharge, unit discharge, and H/L, alongside the governing equations.
- The weir elevation panel draws a front view of the crest, with the overflow sheet
L wide and H tall, a gray buffer framing the weir and a floor beneath it, plus freeboard above the
water surface. Length and head are scaled independently so a long, shallow weir stays readable.
- The rating curve plots head H (vertical axis) against discharge Q (horizontal axis)
at your current C and L. Drag the marker along the curve to set a new operating
point — because H and Q are locked by Q = C·L·H3/2, moving one drives
the other, and the input fields and result update live.
5Scope & limits
Results assume steady free-flow (unsubmerged) discharge over a weir with a well-ventilated nappe, and
are reported in English (US customary) units only. WeirMAC does not correct the coefficient for
approach velocity, end contractions, or submergence, and does not check crest structural adequacy,
upstream freeboard, or downstream conditions. 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.
