You're staring at a hole in your wall, or maybe just a drafty old frame, and you realize that "standard size" is a complete myth. Honestly, it’s one of the biggest frustrations in home renovation. You see a beautiful 400 Series or an A-Series in a showroom, but when you look at the andersen casement size chart, it feels like you're trying to decode a secret government transmission. The numbers don't always look like inches. The abbreviations like "CW" or "CR" don't mean anything to the average person just trying to get some fresh air into their kitchen.
Andersen is the heavy hitter in the window world for a reason. They’ve been at this since 1903. But their sizing system is proprietary and, frankly, a little quirky if you aren't a contractor who looks at these grids every day. If you get the measurement wrong by even a quarter-inch, you aren't just out a few hundred bucks; you're stuck with a custom-ordered hunk of glass and wood that won't fit without a chainsaw and a lot of regret.
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The Weird Logic of Andersen Model Numbers
Don't look for inches first. That's the mistake everyone makes. When you look at an andersen casement size chart, you’ll see four-digit codes like 2040, 2440, or 3050. It’s a shorthand. The first two digits usually represent the nominal width, and the last two represent the nominal height. A 2840 doesn't mean it’s 28 inches by 40 inches. It actually means the window is roughly 2 feet 8 inches wide by 4 feet 0 inches tall.
Except it isn't exactly that either.
Andersen builds in a "rough opening" buffer. For a 400 Series Casement, the actual unit dimension is often 1/2 inch smaller than the callout size to allow the window to actually slide into the structural framing of your house. If your rough opening is exactly 32 inches wide, you aren't buying a 32-inch window. You're buying a unit that is 31.5 inches wide. This allows for shimming, insulation, and the inevitable fact that your house isn't perfectly square. No house is.
400 Series vs. A-Series: Why the Charts Differ
You might think a casement is a casement. It isn't. The 400 Series is their bread and butter—the "Woodwright" and standard casements that you see on every suburban street. Their size chart is pretty rigid because they produce them in massive quantities. If you move over to the A-Series (Architectural), the sizing becomes way more granular.
The A-Series is designed to hit specific historical proportions. Because of this, the andersen casement size chart for A-Series allows for heights up to 8 feet. You won't find that in the standard 400 Series lineup without jumping into custom territory. The 100 Series, which is their composite Fibrex material, has a much more limited grid. It’s cheaper, sure, but if you have a weirdly tall opening in a 1920s bungalow, the 100 Series chart will let you down almost immediately.
Breaking Down the "Standard" Widths
Let's talk real numbers. For the 400 Series, you're usually looking at widths that jump in specific increments.
- CR series: These are the narrow ones, often starting around 17 1/8 inches.
- CW series: These are the "wide" versions, usually hitting around 28 1/8 inches.
- CX series: The big boys, getting you closer to 32 or 36 inches in a single sash.
Why does this matter? Because of the "washability" factor. Andersen designs their hinges so that when the window is cranked open 90 degrees, you can reach your hand through the gap to wash the outside glass from inside your house. If you pick a size that’s too narrow, that gap vanishes. You'll be back on a ladder with a squeegee like it's 1950.
How to Measure Your Existing Opening Correctly
Stop. Put down the tape measure for a second. Are you measuring the glass, the frame, or the hole in the wall?
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If you are replacing an old window (insert replacement), you need the "pocket" dimensions. If you are doing a full-frame replacement where you've ripped everything down to the studs, you need the "rough opening" (RO).
- Measure the width at the top, middle, and bottom. Use the smallest number.
- Measure the height at the left, middle, and right. Again, use the smallest number.
- Check for square. Measure diagonally from top-left to bottom-right, then top-right to bottom-left. If the two numbers are more than 1/8 inch off, you have a parallelogram, not a rectangle.
Once you have those smallest numbers, you compare them to the RO column on the andersen casement size chart. If your RO width is 24 inches, you look for the window that specifies a 24-inch RO. The actual window will be 23.5 inches.
The Custom Size Trap
Many people see the chart and realize their window is 25 inches wide, but the chart only shows 24 or 28. They panic. They think they need custom windows.
Custom sizing at Andersen is possible, but the price jump is steep. It can add 20% to 50% to the cost of the unit. Often, a clever contractor will tell you to "sister" a stud into the rough opening to make a standard 24-inch window fit a 26-inch hole. It’s almost always cheaper to pay a carpenter for two hours of framing work than it is to order a single custom-sized casement.
Venting vs. Picture Casements
Not every window on the chart opens. The "Stationary" or "Picture" casement charts look identical to the "Venting" charts, but the glass area is actually different. On a venting window, the frame has to be thicker to house the operating hardware and the weatherstripping. If you put a venting casement right next to a stationary one of the same "size," the glass lines might not match up perfectly. It looks janky.
To avoid this, pros use "Picture" units designed to match the sash profile of the venting units. Always check the "Sash Profile" notes on the technical documents.
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Real-World Example: The 2835 Casement
Let's look at a common size: the 2835.
On paper, it sounds like 28" x 35".
In reality, for a 400 Series:
- The Actual Unit Size is 28 1/8" x 34 7/8".
- The Rough Opening required is 28 7/8" x 35 3/8".
- The Glass Size is actually much smaller because of the heavy wood frame, usually around 22" x 29".
If you bought that window thinking you’d have 28 inches of glass, you’d be disappointed. If you bought it thinking it would fit in a 28-inch hole, it wouldn't even fit through the frame.
The Role of Joining (Mulling)
If you see a massive row of casements in a fancy living room, that isn't one giant window. It’s a series of individual units "mulled" together. The andersen casement size chart typically only shows single units. When you join them, you have to add the thickness of the mullion (usually 1/2 inch to 2 inches depending on if it's reinforced) to your total width.
If you're DIY-ing this, don't try to calculate the math for a triple-mulled unit yourself. Use the Andersen "Price and Quote" tool or talk to a dealer. They have software that calculates the exact exterior-to-exterior dimension so you don't end up with a window that's too wide for your house.
Next Steps for Your Project
The most practical thing you can do right now is get the technical product guide for the specific series you want. Don't rely on the simplified charts you see on Pinterest or image search results.
- Step 1: Determine if you are doing "Insert" (keeping the old frame) or "Full-Frame" replacement.
- Step 2: Measure your rough opening in three places for both width and height.
- Step 3: Visit the Andersen website and search for the "Technical Documents" section. Look for the "Product Guide" PDF for the 400, A-Series, or 100 Series.
- Step 4: Find the "Opening Specifications" table. This is the "true" size chart that shows the Rough Opening, Unit Dimension, and Unobstructed Glass area.
- Step 5: Match your smallest RO measurement to the closest standard size that is smaller than your hole. Never go bigger. You can always shim a gap, but you can't easily stretch a 2x4.
Avoid the temptation to guess based on the model number of your old windows, even if they are Andersen. Sizing standards have changed slightly over the decades, and a "2840" from 1985 might have slightly different tolerances than a "2840" manufactured in 2026. Get the tape measure out. Trust the RO column, not the model name.