Want to measure your house for siding? It’s a lot more difficult than simply plugging numbers into an online siding calculator. Better to take the “measured approach,” to avoid getting ripped off by vinyl siding salespeople.
How One Hour of Work Saves You $500
The biggest mistake that homeowners make is to rely on the siding salesman to measure the house. Think about this in another area of your life. Would you let the auto salesman tell you how much your tradein car is worth? No, you would research this ahead of time.
Vinyl siding salesmen make their commissions based on the cost of the job. So, if your siding job costs $10,000, they get a cut of that price. It’s just plain dumb to let someone measure your house who has a vested interest in seeing your cost go up. Got that now?
Truth be told, this will take about an hour to measure and calculate the figures. But you might save $500 just by taking the time to do this. Unless you make $500 per hour in your job, it’s well worth your time to do this.
Tools You Need:
 Buy or borrow a 50foot or greater tape measure.
 Buy or borrow a “wide” 25foot tape measure such as the Stanley FatMax. The FatMax is a cool tape, so even though it’s expensive you might want to consider buying one.
 Painter’s telescoping aluminum pole (optional).
 Calculator
 Pencil and paper.
Step 1: Measure “The Square Parts”
With the 50foot measuring tape, measure the height and width of the square or rectangular parts of your exterior walls. Multiply length by width to get area.
In the example shown here, the height is 10 feet and the width is 30 feet. So, the area is 300 square feet.
This is the easy part. But remember that most houses are not made of perfect squares and rectangles. The siding people add in every single conceivable weird shape. But they also subtract “cutouts” for windows.
Step 2: Measure “The Triangular Parts”
With your “wide” tape measure (i.e. the FatMax), measure the triangular portions of the walls. Since the triangular portions are ten feet high or greater, the “wide” tape measure provides the stability you need to push the tape measure up to the very top of the triangle.
Still can’t reach it? Alternatives:

Affix the end of the tape measure to a telescoping aluminum painter’s pole with duct tape. Push it up that way.

Stand on a ladder to gain a few extra feet. Be careful: looking up can cause you to lose your balance.
Multiply width by half of the height to get the area. In the example, the width is 30 feet. The height of the triangle is 10 feet. 30 x (1/2 x 10) = 30 x 3 = 90. So the area of the triangular portion of this wall is 90 square feet.
Step 3: Measure “The Weird Stuff”
This is where it gets tricky. Little things can add up. Most houses have all sorts of things jutting out that are hard to measure, This is a laborious process, but this is the part that many siding salesmen neglect to measure–and where you can get the upper hand on them.
Measure the oddsized parts of the exterior such as dormers.
Here, the area of the dormer is 6 square feet.
Step 4: Measure the “Cut Outs”
Now that you’ve added, it’s time to measure the subtractions. Yes, there are certain things, like windows and doors, which will not be covered in siding. To be precise, and to save money, you need to measure them.
The easy part? You only need to measure one door and one window. Each door and window will stand in for the others.
But of course, if you have larger or smaller doors (such as French doors or sliding doors) or windows of different shapes, you will need to measure each one individually.
In our example, we have measured one window and found that it is 14 square feet. The other two windows are the same size. So, 3 x 14 = 42 square feet.
Step 5: Arrive at Total Area of Wall
Add Steps 13, minus the cutouts in Step 4.
Step 1 – Add Square Parts  300 
Step 2 – Add Triangle Parts  90 
Step 3 – Add Weird Parts  6 
Subtotal  396 
Step 4 – Subtract CutOuts  42 
Total  354 
Step 6: Measure Rest of House
Repeat these steps for the rest of the house. Add all amounts to get a total square footage for your house.
Step 7: Important: Add in 10% Wastage
Wastage. Sounds pretty bad, doesn’t it. Actually, wastage is a normal part of estimating siding. Yes, sometimes the installers will make a bad cut, and that will become wastage. But what wastage really means are the ends of the siding that are are cut off to make a perfect fit. These waste ends can sometimes be reused in other places, sometimes not.
You will need to buy more siding than is actually used to allow for wastage. Take the total square footage of the house, multiply by .10 on the calculator. The amount shown on the calculator is your wastage amount. Add the wastage amount to the total you have at this point.
Step 8: Divide Everything by 100 to Find Number of “Squares”
Siding companies and manufacturers speak in terms of “squares.” A square is 100 square feet of siding material. They generally do not talk in terms of individual square feet because the measurements they are dealing with are so large.
So, for our example: the total wall area for the house is 3200 square feet. Divide 3200 by 100. The siding installer will use 32 squares of siding material on the house.
Step 9: Should You Put the Salesman on the Spot?
Don’t tell the salesman the square footage upfront. Let them tell you their measurements first.
If your measurements match theirs–great. Fine and dandy.
If your measurements are different, you can either:

Call them on it (“Why did you get 23 squares when I only got 7 squares?” Or,

Cross them off your list, because obviously they are trying to cheat you.