Round to Rectangular Flue Area Converter
Round and rectangular flues trade by area, not by width — convert a diameter to its square-inch cross-section and the equal square side.
1 Enter your numbers
Round and rectangular flues are interchangeable by AREA, not by width. A 8.0" round flue is about 50.3 in² — the same as a square tile about 7.1" on a side. Convert by area so a reline or a cap fits the real cross-section.
People compare flues by the number on them — “an 8-inch round versus an 8-by-8 tile” — and get it wrong, because flues match by cross-sectional area, not by any single dimension. An 8-inch round flue is about 50 square inches, and to carry the same gas a square flue needs roughly 7 inches of clear side, not 8 nominal.
Enter a round diameter and the tool returns the area in square inches and the side of the square that holds the same area. Use it when you are relining a rectangular clay flue with a round liner, spec’ing a cap for an odd flue, or checking whether a “bigger-looking” tile is actually bigger. It is pure geometry, so it never goes out of date.
Formula
One area, two shapes:
- Round area
A = π × (d ÷ 2)² - Equal square side
s = √A - Reverse (area back to a round bore)
d = 2 × √(A ÷ π)
The equal square side is the clear inside dimension — a nominal clay tile is larger than its clear opening because of wall thickness.
Worked example
Convert a 9-inch round flue. Its area is π × 4.5² = 63.6 in². The equal square side is √63.6 = 7.98″ of clear opening. So a 9″ round liner and a roughly 8″-clear square flue move the same gas — even though “9” and “8” sound a size apart. When you drop a round liner into that square flue, this is why a 9″ liner will not physically fit an 8″-clear tile; you would step down to fit and then check the appliance still has enough area.
Compare clear areas, not nominal numbers
Clear opening, not nominal. A “12×12” clay tile does not have 144 in² inside — the walls eat into it, leaving roughly 87–99 in². Convert with the clear inside numbers. Watch for:
- Comparing a round’s inside diameter to a tile’s nominal outside size — not the same basis.
- Assuming a round liner of the “same” number fits a square flue — check the clear side first.
- Forgetting a liner also needs room for insulation inside the old flue.
One more practical use: this converter tells you whether a proposed round reline actually keeps enough area. If a fireplace calls for 87 in² of flue and someone spec’s a 10″ round liner, run the numbers — a 10″ round is only 78.5 in², short of the target. The label said “10” and sounded generous; the area says otherwise. Convert first, then commit to a liner.
Reference table
Every standard round flue and the square-flue side that carries the same area. Flues match by area, not by the number stamped on them.
| Round dia. | Area | Equal square side |
|---|---|---|
| 4″ round | 12.6 in² | 3.5″ |
| 5″ round | 19.6 in² | 4.4″ |
| 6″ round | 28.3 in² | 5.3″ |
| 7″ round | 38.5 in² | 6.2″ |
| 8″ round | 50.3 in² | 7.1″ |
| 10″ round | 78.5 in² | 8.9″ |
| 12″ round | 113.1 in² | 10.6″ |
| 14″ round | 153.9 in² | 12.4″ |
| 15″ round | 176.7 in² | 13.3″ |
| 18″ round | 254.5 in² | 16.0″ |
Frequently asked questions
Is an 8-inch round flue the same as an 8x8 tile?
Not quite. An 8″ round is about 50 in²; the equal square side is 7.1″ of clear opening. A nominal 8×8 clay tile has thick walls, so its clear area is smaller still — compare on area, not the label.
How do I convert a round flue to a rectangular one?
Find the round area with π×(d/2)², then match a rectangular flue with at least that clear area. The equal square side, √A, tells you the ballpark clear dimension to look for.
Why does the area matter more than the width?
Draft depends on how much gas the flue can move, which is set by cross-sectional area. Two flues with the same area draw similarly regardless of whether they are round or rectangular.
What is the clear opening of a clay tile?
Roughly: 8×8 ≈ 28–33 in², 12×12 ≈ 87–99 in², 12×16 ≈ 120–135 in². Wall thickness varies by maker, so treat these as planning typicals.
A 10-inch round liner sounds big — is it enough for my fireplace?
Check the area, not the name. A 10″ round is only 78.5 in². If your opening needs 87 in² of flue, that liner is short — you would step up to a 12″ round. Always convert the requirement to area before you pick a liner.