Step 1: Measure the length and width of each roof section.

Step 2: Enter the values in the calculator and select the appropriate units.

Step 3: Select the roof pitch and waste factor.

Step 4: Optionally, enter the price per square.

Step 5: The calculator will compute the total square feet and total cost.

Formula for total square feet: $$\text{Total Square Feet} = \sum (\text{Length} \times \text{Width} \times (1 + \text{Waste Factor}) \times \text{Roof Pitch Factor})$$

Formula for total cost: $$\text{Total Cost} = \text{Total Square Feet} \times \text{Price per Square}$$

How to Estimate Roof Size

When replacing your roof, the first thing to consider is the size of the roof so you can accurately estimate the materials needed for the project.

Measuring in Square Feet

In the United States, roofs are measured in square feet, and roofing contractors typically quote prices based on the square footage of the roof. If your roof area is 2,300 square feet, you will need materials for 23 squares (each square is equivalent to 100 square feet).

Collecting Square Footage and Square Numbers

There is no direct correlation between the size or square footage of a house and the size or square footage of its roof. A 2,000-square-foot house may have a simple gable roof, a complex cross-gable roof, or a hip roof, and each will have a different size.

Finding the Roof Footprint

To calculate the roof size, you need to measure the flat roof area, consider the roof pitch, calculate the square footage, and finally determine how many squares of material are needed for the roof.

Remember, you need to do this for each section of the roof. While the illustration above shows a simple gable roof, a cross-gable roof requires you to measure each section's length and width, and complex roofs (like mansard and gambrel roofs) have two sections each.

Using a Calculator

The calculator above can handle all of this; just input the length, width, and pitch, and it will determine the square footage of the roof.

Measuring the Roof's Length and Width

Stand on the ground and measure the length and width of the roof (in feet). Remember, the length and width are for the roof, not the house, so consider any overhang.

Calculating the Roof Area (Assuming Flat)

Multiply the length by the width (in feet) to get the area measurement.

Finding the Total Roof Area

Once you know the roof's footprint area, calculate the total roof area by accounting for the roof pitch. The roof pitch is the amount of rise over 12 inches horizontally.

Most roofs have a pitch between 4/12 and 8/12, but complex roofs may have one section at 20/12 and another at 7/12. This means you need to calculate each area separately.

Also, remember that each section of the roof may have its own pitch. Even a gable roof can be a so-called dual-pitch gable where one side of the roof has a different pitch from the other. In this case, calculate each side's pitch separately.

Using the Roof Pitch Multiplier

Multiply the roof footprint area by the roof pitch multiplier below to find the total roof area.

For example, a 4/12 pitch roof area of 100 square feet:

100 × 1.054 = 105.4 square feet

Roof Pitch Area Multipliers

Common roof pitch multipliers can be used to calculate the total roof area.

Roof Pitch (in/12)	Multiplier
0/12	1
1/12	1.0035
2/12	1.0138
3/12	1.0308
4/12	1.0541
5/12	1.0833
6/12	1.118
7/12	1.1577
8/12	1.2019
9/12	1.25
10/12	1.3017
11/12	1.3566
12/12	1.4142
13/12	1.4743
14/12	1.5366
15/12	1.6008
16/12	1.6667
17/12	1.7341
18/12	1.8028
19/12	1.8727
20/12	1.9437
21/12	2.0156
22/12	2.0883
23/12	2.1619
24/12	2.2361

Finding the Roof's Area in Squares

Roofing contractors estimate projects and materials by the square, so finding this measurement is crucial for accurately estimating the amount of material needed. A square simply refers to a 10-foot by 10-foot roof area, which is 100 square feet of roof.

To calculate the number of squares, divide the total roof area by 100 and round up. For example, if the roof area is 1550 square feet, it is 16 squares.