Free area, not band area
A 12 by 12 inch flue tile with three quarter inch walls is 13.5 inches outside, so the distance round the cap is 54 inches. An 8 inch band gives 432 square inches of mesh. At 55% open that is 237.6 square inches of actual hole, against 144 square inches of flue. Change the mesh to something with 30% open area and the same cap delivers 129.6 — less than the flue it sits on, with no visible difference from the driveway.
That is the whole reason to type an open area percentage rather than a mesh description. Wire diameter and spacing together decide it, and two meshes described the same way can differ by a factor of two.
Two flues, two caps, two answers
On the default crown the round 6 inch flue is 28.3 square inches and its cap band delivers 103.7. The 12 by 12 fireplace flue is 144 square inches and its band delivers 237.6. Both clear at a ratio of 1, but they clear by very different margins, and a whole-crown cap sized on the total would hide that. The cap-by-cap section exists because a total can pass while a cap does not.
The plate is the cheap part
A top plate at a 2 inch overhang comes to about 3.05 square feet of blank for the pair. It is the band height, and the mesh, that decide whether the cap works, and both are set before any metal is cut.
Related pages
For the flue itself against what it vents, the flue size calculator compares cross-section to an appliance outlet, and the chimney draft calculator covers the stack. For the flashing where the chimney meets the roof, the roof penetration page counts step pieces and curbs. For the other openings on the same building, the gap survey page ranks them, and the vent screening take-off handles vents and soffits.
Questions people ask
Do I enter inside or outside flue dimensions?
Inside, in every case. The inside dimensions give the flue area, which is the number the screen is being compared against. The tile wall thickness field then adds twice itself to each dimension to get the outside the cap has to fit over, so both dimensions come from one measurement plus one thickness rather than from two measurements that may not agree.
What open area percentage should I use?
The one on the mesh data sheet, and only that. Open area depends on wire diameter as well as spacing, and the same nominal mesh in a heavier wire has noticeably less hole in it. If a supplier cannot give the figure for the exact mesh, that is worth knowing before ordering rather than after.
What free area ratio should I put in?
This page does not have an opinion and will not fabricate one. The multiple comes from the appliance manufacturer instructions, the cap maker, or the person who inspects and sweeps the chimney, and it can be affected by what the flue serves and by the code the local authority has adopted. The field is there so their number drives the arithmetic instead of an invented one.
One cap over the crown, or one per flue?
Both are built. A single cap is one plate, one band and one set of fixings, and it is often cheaper. The catch is that the band then has to carry every flue under it, and the cap-by-cap check disappears — a shared cap sized on the total can be short for the biggest flue while the total looks fine. If the flues serve different appliances, that is worth thinking about before choosing.
Is a screened cap a wildlife measure?
This page will not present it as one. It sizes a band against a free area figure you supply and counts the material. Chimneys are a common roost, bats and most birds are legally protected, and there are times of year when closing one traps animals inside. That is a matter for your state wildlife agency and a licensed wildlife control operator, and nothing here substitutes for either.