Dormer Volume Calculator

Estimate the total interior volume of a dormer for construction, HVAC sizing, or space planning.

Dormer Width (feet)

Dormer Depth (feet)

Dormer Wall Height (feet)

Dormer Roof Height (feet)

function calculateDormerVolume() { // Get input values from the form var dormerWidth = parseFloat(document.getElementById('dormerWidth').value); var dormerDepth = parseFloat(document.getElementById('dormerDepth').value); var wallHeight = parseFloat(document.getElementById('wallHeight').value); var roofHeight = parseFloat(document.getElementById('roofHeight').value); var resultDiv = document.getElementById('result'); // Validate inputs if (isNaN(dormerWidth) || isNaN(dormerDepth) || isNaN(wallHeight) || isNaN(roofHeight) || dormerWidth <= 0 || dormerDepth <= 0 || wallHeight <= 0 || roofHeight < 0) { resultDiv.innerHTML = 'Please enter valid, positive numbers for all dimensions.'; return; } // Calculate the volume of the rectangular base (the "box") var rectangularVolume = dormerWidth * dormerDepth * wallHeight; // Calculate the volume of the triangular prism roof // Formula: (0.5 * base * height) * length // Here, base = dormerWidth, height = roofHeight, length = dormerDepth var triangularVolume = (0.5 * dormerWidth * roofHeight) * dormerDepth; // Calculate the total volume var totalVolume = rectangularVolume + triangularVolume; // Display the result resultDiv.innerHTML = 'Total Dormer Volume: ' + totalVolume.toFixed(2) + ' cubic feet'; }

Understanding Your Dormer's Volume

This calculator helps you find the interior volume of a standard gable-fronted dormer, which is a common addition for creating more usable space in an attic or loft. The calculation is essential for various aspects of a construction or renovation project.

How to Calculate Dormer Volume

To find the total volume, we treat the dormer as two distinct geometric shapes and add their volumes together:

The Rectangular Body: This is the main "box" of the dormer with vertical walls. Its volume is calculated with the standard formula for a rectangular prism.
Formula: Volume = Dormer Width × Dormer Depth × Wall Height
The Triangular Roof: This is the pitched roof section on top of the walls, shaped like a triangular prism. Its volume is the area of the triangular face multiplied by the depth of the dormer.
Formula: Volume = (0.5 × Dormer Width × Roof Height) × Dormer Depth

The Total Volume is the sum of the rectangular body's volume and the triangular roof's volume.

Practical Example

Let's say you are planning a dormer with the following dimensions:

Dormer Width: 8 feet
Dormer Depth: 12 feet
Dormer Wall Height: 7 feet
Dormer Roof Height (from top of wall to peak): 4 feet

Using the formulas:

1. Rectangular Body Volume: 8 ft × 12 ft × 7 ft = 672 cubic feet

2. Triangular Roof Volume: (0.5 × 8 ft × 4 ft) × 12 ft = 16 sq ft × 12 ft = 192 cubic feet

3. Total Volume: 672 + 192 = 864 cubic feet

Why is Dormer Volume Important?

Calculating the volume of your dormer is a critical step for several reasons:

HVAC Sizing: The volume of a space is a primary factor in determining the required capacity for heating, ventilation, and air conditioning (HVAC) systems. An accurately sized system ensures energy efficiency and comfort.
Material Estimation: Knowing the volume helps in estimating the amount of insulation needed. It also informs calculations for the surface area, which is used to estimate drywall, paint, and flooring materials.
Space Planning: Understanding the total volume gives you a tangible sense of the living space you are adding, which is more descriptive than square footage alone, especially in rooms with sloped ceilings.
Building Regulations: Some local building codes have requirements related to room volume or ceiling heights, making this calculation a necessary part of the planning and permitting process.