Chimney Cap Size Calculator

A chimney cap is an essential component of any fireplace or wood stove system. It protects the flue from rain, snow, animals, birds, and wind-driven debris while also helping to prevent spark escape from the chimney top.

Why Chimney Caps Matter

Without a cap, rain and snow enter the flue and accelerate deterioration of the mortar joints, liner, and damper. Animals (raccoons, squirrels, birds) frequently nest in uncapped chimneys, creating fire hazards and blockages. A good chimney cap extends the life of the chimney system significantly.

Flue Types and Their Standard Sizes

Chimney flues come in two shapes:

Rectangular / Square Flues (most common in older homes)

• 8×8 inches: Small fireplaces, wood stoves
• 8×12 inches: Standard fireplace
• 12×12 inches: Larger fireplace
• 12×16 inches: Large fireplace
• 16×16 inches: Very large or double fireplace
• 16×20 inches: Oversized or commercial

Round Flues (used with factory-built fireplaces and HVAC)

• 6-inch diameter: Small inserts, gas logs
• 8-inch diameter: Standard insert
• 10-inch diameter: Medium fireplace
• 12-inch diameter: Large fireplace

Cap Sizing Rules

The chimney cap should overhang the flue by at least 2 inches (50 mm) on all sides to redirect rain away from the flue opening. Therefore:

Cap Width = Flue Width + 4 inches (100 mm minimum) Cap Length = Flue Length + 4 inches (100 mm minimum)

For round flues: Cap Diameter = Flue Diameter + 4 inches (100 mm)

Mesh and Screen Selection

• ⅝-inch mesh: the standard. Blocks animals and large debris, and it is what most codes call for as a spark arrestor
• ¼-inch mesh: finer, so it also stops small birds and insects. It clogs with creosote faster and needs cleaning more often
• No mesh: only for some high-draft applications, and it gives you no animal or spark protection at all

Multi-Flue Caps

If you have multiple flues, a single large cap can cover all of them. Measure the total chimney crown area and select a cap that covers all flues with at least 2-inch overhang on all sides.