How to Measure a Roof with Our Roofing Shingle Calculator


  • Reshingling a roof is a complex and expensive project, one that’s usually best left to the professionals.

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Tips to remember:

  • A square foot is equal to 100 square inches.
  • Roofs are typically measured in squares, not feet or yards.
  • “Square Footage” refers to the dimensions of a roof’s surface area, NOT its height or depth.
  • The average cost for materials needed to cover your house can vary depending on size and type of material used.
  • A roofing square or ruler is an important measuring tool for roofs and can help you estimate how much materials will be needed, as well as pricing out the cost of labor. It’s usually easier to compute with a 100-square inch long card than it is without one.

The Drip Cap

  • Before you do any roofing work, you must calculate the roof’s square footage to determine how much material you need.
  • Calculate each plane separately.
  • For example, if a triangle-shaped roof plane measures 60 feet along the base and is 30 feet high, we have 60 x 30 = 1,800 feet, divided by 2 = 900 square feet for the triangle-shaped plane.
  • The square footage would be 40 x 40 = 1,600 square feet (the square) plus 20 x 40 = 800 / 2 = 400 square feet (the triangle).

Why is it important?

A roofing square is an important measurement for roofers. It is the most common way to measure a roof’s size. Roofers use this measurement to calculate the amount of material they will need to cover a roof. This measurement is also used to price a new roof. A roofing square is equal to 100 square feet.

Roofers will measure the length and width of a roof and divide it by 100. This will give them the roof’s square footage. Most roofs are not perfect squares, therefore this measurement may be difficult to calculate without a roofing square ruler or tool such as a credit card that is 100-square inches long.

STEP 4: Determine the number of shingle bundles required

Determining the number of shingle bundles that you will need is the next step in the process. Usually, a bundle of shingles contains enough material to cover ⅓ of a roofing square. This makes it relatively easy to figure out how many shingle bundles your roof requires: simply multiply the number of roofing squares by three. If your roof’s area were equivalent to about 20 roofing squares, then you would need about 60 bundles of shingles.

Similarly, you can use your roofing square measurement to calculate the amount of roofing felt required for the project. This material is laid underneath roof shingles and comes in 15-pound or 30-pound rolls. A 15-pound roll covers about 4 squares, while a 30-pound roll is enough for 2 squares. This means that if you are using a 15-pound roofing felt then you can determine the number of rolls required by dividing the number of roofing squares by four. Alternatively, if you are using a 30-pound roofing felt, you calculate how many rolls you’ll need by dividing the number of roofing squares by two.

Whats your roof slope?

You will also need to know the slope of your deck.

You will also need to know the slope of your deck.

To determine this, measure the vertical rise of your deck in inches over a 12″ horizontal distance.

If this rise is 4″, then your roof slope is 4 in 12.

Roof slopes are always expressed with the vertical rise mentioned first and the horizontal run (12″) mentioned second.

What if you have a steep roof?

To measure a steep roof, use one of the following alternate methods:

Method 1

Calculate the roof length by measuring the exterior walls plus the overhang for the length of the house parallel to the ridge.

Next, throw a rope over the ridge and mark it where it meets each eave. This will give the width dimension to use in figuring your area. This should be done on each roof section containing a horizontal ridge.

Method 2

Determine the roof area by using a mathematical formula that accounts for the roof length, total span, and roof pitch:

  1. Determine your roof pitch by using a pitch gauge (available at most home improvement stores) or a smartphone app (available free through any app store).
  2. Measure the length of the roof surface, including overhangs.
  3. Measure the span of the roof, including overhangs.
  4. Multiply the length x the span. The result will be the plane area. Use the chart below to calculate the total area of the roof.
  5. Calculate the total roof area – Plane Area x Correction Factor = Roof Area

Slope Correction Factor

SlopePlaneCorrection Factor
PitchApproximate AngleMultiply by Projection Length

Now how about nails?

You will also need to determine the amount of nails you’ll need. Generally, you should use four nails per shingle. For regular three-tab shingles, this would require 320 nails per square. For high wind areas or when shingles are being applied to a mansard, six nails per shingle are required (480 nails per square). This is based on 80 shingles per square. Other styles of shingles may require more or fewer nails per shingle and may have more or less than 80 shingles per square. Refer to the application instructions on your shingle wrappers for the correct nailing pattern.

Nails are purchased by the pound, so ask your dealer for the correct amount of nails for your size roof, in the length you specify.

Measure the lengths of your rakes and eaves to determine the amount of drip edge needed.

Measuring Your Roof

All right so there are a few things you should do before you even attempt to measure your roof up.

Make sure you’re secure– Before you start trying to take any sort of measurement it is critical that you find a ladder that is good repair, and make sure it is secure so there is no danger of you falling off on your way up to the roof. Once on the roof, it is critical that you are wearing shoes with proper traction such as work boots.

Measure twice, order once– One of the biggest mistakes people make when attempting to measure, well, anything really, is that they don’t measure carefully. This is especially important in the case of ordering roofing materials, where mis-measurement can greatly increase your costs.

Calculate the square footage– Once you’ve managed to measure the length and width of your roof without falling off, all that’s left to do before you can calculate the roofing square value you need is to determine the square footage of your roof. To do this simply multiply the length by the width, and voila, you’ve got the square footage of your roof!

Calculate the roofing square to square feet value– This is perhaps the simplest part of the process simple divide the square footage by 100, for example, if you had 1000 square feet of roofing needed, you would need one square of roofing’s worth of material.

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STEP 2: Measure and calculate the square footage of each plane

Walk up to the peak of the roof to get an idea of the layout and make sure to orient the overhead sketch in the notepad to your current position. This will help ensure that you’re labeling each plane accurately. Choose the first section of the roof to be measured and lay your measuring tape out along the length of the plane. Write down the first measurement in the corresponding location in your notepad, then lay the tape out along the width of the plane to find the corresponding measurement.


If you are measuring from the peak of the roof (perpendicular to the peak), you will need to plant your feet on one side of the roof and feed the measuring tape down to the bottom edge. Measure from the edge to the exact peak and record the measurements.

If you are measuring along the peak (parallel with the peak), place the tape measure at the edge of the roof on one side of the peak and run it across to the other edge of the plane along the peak.

Repeat the process of measuring both the length and width of each roof plane, then climb down from the roof using the ladder (with a partner holding it steady). Use these measurements to calculate the square footage of each plane. Keep in mind that some roof planes may not be square or rectangular. Common alternatives are trapezoids and triangles. The formulas for calculating the surface area of rectangles, squares, trapezoids, and triangles are listed below.

Area of a rectangle or square: A = L x W Area = Length x Width Example: 20 feet x 10 feet = 200 square feet

Area of a trapezoid: A = [(B1 + B2) x H] / 2 Area = [(Top + Bottom) x Height] / 2 Example: [(5 feet + 10 feet) x 6 feet] / 2 = 45 square feet

Area of a triangle: A = (B x H) / 2 Area = (Base x Height) / 2 Example: (5 feet x 4 feet) / 2 = 10 square feet

When you have finished calculating the area of each plane, use this information to find your roof’s total square footage. Add the area of each plane together, and the sum will be the total square footage of the roof.

RELATED: How to Hire a Roofing Contractor

How to Measure Your Roof From the Top of the Structure

The most accurate way to measure your roof is by being on top of the roof itself. You can make sure each roof section’s slope area is accounted for. You’ll need some graph paper, measuring tape, and a calculator. For our example, we’ll use a hip roof, though you can use this technique to measure any shape roof with varying complexity. 

Step One: Get Up on Our Roof Safely and Take Initial Measurements

Sketch out the shape of your roof on graph paper.

Sketch out the shape of your roof on graph paper. In this example, you’d only need to measure the overall width and length, the central ridge, and one of the hips (marked above with blue stars). If you have a more complex roof shape with dormers, take all the measurements you can and prepare to use your eraser to refine the image of your overall roof structure. Make sure you have solid footing and if possible, use fall protection to keep yourself safe. Be mindful of hazards like ridge vents. Note the lengths in feet:

Step Two: Split Tricky Shapes Into Squares and Rec

Step Two: Split Tricky Shapes Into Squares and Rectangles

Once you have these figures, return to the safety of your home. Notice in the image above that the hip roof is made of two triangles and two trapezoids. Trapezoids are tricky to get the area measurement of, so we’re going to split each of them into two triangles and a rectangle:

We know enough about the overall measurements of t

We know enough about the overall measurements of the roof to fill in the missing values. Let’s fill the figures in now:

Step 3: Draw Each Shape Individually, Calculate Ea

Step 3: Draw Each Shape Individually, Calculate Each Area, and the Total Square Footage of Your Roof

Now we have eight shapes to find the area of – six triangles and two rectangles. Instead of breaking out the old textbooks to calculate triangle areas, we like to use a triangle calculator. It makes figuring out the area of triangles a lot simpler.

In the example above, we have two triangles with sides of 30, 20, and 20 feet. Pop those figures in the three-sides calculator and you’ll get an area of 198. Since we have two of those triangles, multiply by 2 for a running total of 396 square feet.

We have four remaining triangles, all with dimensions of 15, 20, and 25 feet. That same calculator reveals a result of 150 square feet. Since there are four of those triangles, multiply by 4 to get 600 square feet.

Lastly, we have two rectangles of 15 by 30 feet. This one is easy, just width times height for a total of 450 square feet for each one. Multiply by 2 and their total square footage is 900.

Adding all the areas up yields a total of 1896 square feet (396 + 600 + 900). Since roofing material comes in 100 square foot “squares”, you’ll need at least 19 of them for your roofing project. It’d be wise to get at least 20 to account for mistakes and waste.

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