Do you mean the amount of inlets, or the grate capacity?

I guess the grate capacity.  My problem is determining what spread to use in my calcs.  Should it be the entire width of the parking area between curbs? And do I abritrarily pick a grate width to use for the flow capacity of the inlet and then iterate the dimensions of the grate until the flow capacity is equal to or greater than what is coming in?

Well, here are some formulas to help in the grate capacity.
For ponding up to 0.4 ft:
Q=CPd^1.5
C=3.0
P=Permieter of grate minus side against curb
d=depth

For ponding over 0.4 ft:
Q=CA(2gd)
C=0.67
A=clear grate opening
g=32.2 ft/sec^2

Hope this helps a little

Thanks but my inlets are in the middle of the parking lots.  There isn't a curb within 24' of the inlet on both sides.

Google "Visual Urban" for a free program which will do these calc's for you.

good lick

Neenah Foundry has an Inlet Grate Capacity Handbook and even some old DOS software.  Wildwings27 has given you the formulas for weir and orifice flow.  Around 0.4' is the change over between which one limits flow through the inlet.  Use the lesser.  

I assume you've graded the parking to a series of low points which you are placing inlets.  How much depth of water can you tolerate at the inlet?  0.1'?  Back check the spread of water at that depth.  It would also be nice to leave some "dry" isles for people to get to their cars without stepping ankle deep during a rain shower.  Tweak the grading plan.

I’m guessing your next task is storm detention.

newbie -- wildwing was givong you equations for sump/sag conditions.  Are your inlets on grade or in sag?  If they are on grade chk out FHWA's HEC-22.  If in sag use what ww provided.

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I suggest the following:

1) Determine if there is a regulatory requirement for maximum allowable ponding depth at a specific storm interval/type (10yr 24hr Type II; 25 yr 6hr Type III,; etc).
2) Determine if you client has a requirement for maximum allowable ponding depth at a specific storm interval/type.
3) Determine the maximum allowable ponding depth at a specific storm interval/type for the most active governmental body in your area.
4) Use the more restrictive of the ponding depths for design unless there is a specific reason not to do so (such as direction from your client to use the maximum allowable ponding depth).
5) Make sure that any depth selected protects the health, safety, and welfare of the public regardless of other requirements.
6) Determine the traffic types: heavy vehicular, light vehicular, bicycle, pedestrian, wheelchair, motorcycle, etc.
7) Develop the runoff hydrograph for the watershed contributing to each inlet.
8) Use the traffic types to determine the appropriate grate type (maximum grate spacing to allow safe passage of tires without dropping into grate, minimum grate load capacity to carry heaviest traffic).
9) Route the inlet watershed runoff hydrograph through the inlet/grate (and subsurface system) to determine if it is adequate. (You can develop a routing spreadsheet to automate this analysis using the various appropriate orifice, pipe, weir, and channel equations/models. There are several software package that will aid in this analysis as well. See “http://hhwq.blogspot.com”.)
10) Verify adequacy of grate
11) Repeat steps 8, 9, and 10 as necessary.

Let me know if you need more information. There are lots of documents (manuals, guides, etc) available that will walk you step-by-step through this process.

Also, Why do you have a grate in the middle of the traffic lane? Why not have depressed islands between the rows of spaces. The depressed islands can eliminate or reduce the number and size of inlets and pipes necessary while providing detention, retention, and associated water quantity and water quality management benefits. For images of this, see “http://www.lid-stormwater.net/bioretention/biocomind_home.htm”, “http://ohioline.osu.edu/cl-fact/1000.html”, “http://www.urbanext.uiuc.edu/lcr/LGIEN2002-0017.html”, “http://clean-water.uwex.edu/plan/subdivision.htm”, and “http://nemo.uconn.edu/images/reducing_runoff/possib6.jpg”.

.

tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation
http://hhwq.blogspot.com