Chimney Height Calculator

1. Enter the roof pitch as the rise per 12 inches of run.
2. Enter the distance to the ridge, measured horizontally.
3. Read the minimum height above the roof.
4. Check which rule governs and the rise within 10 feet.

Here is what each result line means:

Building the chimney in block or brick? Size the units with the concrete block calculator or the brick and mortar calculator.

The 3-2-10 rule is the building-code requirement for how tall a chimney must be. It comes from IRC section R1003.9, which says a chimney shall extend not less than 2 feet higher than any portion of a building within 10 feet, but not less than 3 feet above the highest point where it passes through the roof.

The two parts work together, and the taller result wins. The rule keeps the chimney top clear of the roof so smoke draws properly and sparks and gases are carried away from the building, not blown back onto it.

The roof rises toward the ridge. The highest roof point within 10 feet of the chimney is either the ridge, if it is closer than 10 feet, or the roof at 10 feet away. The chimney must clear that point by 2 feet, and never be less than 3 feet above where it exits the roof.

1. Find the roof pitch as a ratio, the rise over 12.
2. Take the distance within 10 feet toward the ridge.
3. Multiply for the roof rise, then add 2 feet.
4. Use the greater of that and 3 feet.

A steep roof or a chimney far down the slope raises the required height, because the roof climbs more within those 10 feet toward the ridge.

Suppose the chimney passes through a 6/12 roof, 5 feet from the ridge measured horizontally.

Calculation: a 6/12 pitch is a ratio of 0.5. Over the 5 feet to the ridge the roof rises 0.5 times 5, or 2.5 feet. Add 2 feet and you get 4.5 feet, which beats the 3-foot minimum, so the chimney must rise at least 4.5 feet above the roof.

The lesson: when the chimney is near a higher ridge, the 2-foot rule usually governs and the 3-foot minimum is just the floor.

Roof Pitch

A steeper roof climbs faster, so the 2-foot rule demands a taller chimney for the same distance to the ridge.

Distance to the Ridge

The farther down the slope the chimney sits, up to 10 feet, the more the roof rises within reach, raising the required height. Beyond 10 feet the rule caps at the 10-foot point.

Nearby Structures

Any part of the building within 10 feet counts, not just the chimney’s own roof. A higher wing, dormer or parapet nearby can govern.

Local Code and Listing

Your jurisdiction, or a factory-built unit’s instructions, may require more than the minimum. The code is a floor, not a target.

Meeting the rule is the start, not the whole story.

A chimney that just meets the rule can still draw poorly if the flue is short or wind swirls around nearby obstructions. Build the structure with the cement calculator for the mortar, and raise the height if draft is a problem.

Planning a Chimney

Set the minimum height before you build or extend a masonry chimney.

Inspecting a Roof

Check whether an existing chimney meets the 3-2-10 rule.

Adding to a Roof

See how a new dormer or addition within 10 feet changes the required height.

Troubleshooting Draft

Rule out a too-short chimney as a cause of a smoky fireplace.

1. Meeting Only One Rule

Both the 3-foot and the 2-foot-within-10-foot tests must pass. The taller result governs; satisfying one is not enough.

2. Measuring to the Ridge Wrong

The 10 feet is horizontal, not along the slope. Measure the level distance to the ridge or nearby high point.

3. Ignoring Nearby Structures

A taller wing or parapet within 10 feet can set the height, even if the chimney’s own roof is lower.

4. Forgetting Draft

The rule is about clearance, not performance. A short flue can still draw poorly even if it meets the rule.

5. Skipping the Local Code

The IRC is a model code. Your jurisdiction or a unit’s listing may require more, so confirm before building.

The geometry is exact for the inputs, but the code has conditions this tool does not model, so treat the result as a planning minimum and confirm with your building department.

A chimney must extend at least 3 feet above where it passes through the roof and at least 2 feet above any part of the building within 10 feet, measured horizontally. The taller result governs.

At least 3 feet above the roof penetration, and 2 feet above any roof or structure within 10 feet. For a 6/12 roof 5 feet from the ridge, about 4.5 feet above the roof.

Horizontally. Measure the level distance from the chimney to the ridge or other high point, not the distance along the sloped roof surface.

Yes. A steeper roof rises faster, so the 2-foot-within-10-foot test requires a taller chimney for the same distance to the ridge.

The 2-foot test uses the roof at 10 feet away, since that is the highest point within reach. Beyond 10 feet the required height does not keep climbing with distance.

In the United States the International Residential Code, section R1003.9, sets the 3-2-10 rule for masonry chimneys. NFPA 211 and local codes also apply.

Usually a taller flue drafts better, up to a point. But meeting the 3-2-10 rule is about clearance and safety; draft also depends on flue size and nearby obstructions.

They must follow the manufacturer listing and instructions, which often include a height table. The 3-2-10 rule is the baseline for masonry chimneys.

From the point where the chimney passes through the roof up to the top of the chimney. Codes are not always specific about the exact top point, so confirm with your inspector.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• IRC / California Residential Code R1003.9: Chimney Termination (the 3-2-10 chimney height rule).
• Chimney Safety Institute of America (chimney safety and performance).
• BIA Technical Note 10, Dimensioning and Estimating Brick Masonry (Brick Industry Association; masonry chimney construction).