Ikea Billy Vs Kallax Which Bookshelf Holds More Books Without Sagging

When choosing a bookshelf for a home library, study, or living room, two IKEA staples consistently rise to the top: the BILLY and the KALLAX series. Both are affordable, modular, and widely available—but they serve different needs. The core question many buyers face is not just about aesthetics or space, but structural performance: Which one holds more books without sagging?

Sagging shelves aren’t just unsightly—they signal potential failure under load, especially with heavy hardcovers, textbooks, or reference materials. Over time, bowed shelves can warp permanently, damage books, or even collapse. This article compares the BILLY and KALLAX bookshelves across design, material, weight capacity, and real-world usability to determine which truly supports more books reliably.

Design and Construction: What Sets Them Apart

The BILLY and KALLAX differ fundamentally in purpose and construction, which directly affects their ability to carry weight.

BILLY is designed as a traditional vertical bookcase. It features tall, narrow units with adjustable shelves (typically five per unit), and comes in multiple heights and widths. The standard BILLY bookcase measures 79 cm wide × 28 cm deep × 202 cm high. Its primary function is storing books upright, making it ideal for libraries or media rooms.

KALLAX, on the other hand, is a cube-based shelving system. The most common configuration is a 4×4 grid (16 cubes), each measuring 39 cm × 39 cm. While often used horizontally for storage boxes or decor, it’s also marketed for books—especially when placed vertically. However, its open cube design means it lacks the depth and vertical orientation optimized for book stacks.

The key difference lies in depth: BILLY shelves are 28 cm deep, while KALLAX cubes are 39 cm deep. At first glance, this might seem like an advantage for KALLAX, but for book storage, excessive depth can be inefficient and increase leverage on unsupported spans, raising sag risk.

Tip: Shelf sag depends not only on total weight but on how that weight is distributed. Books stacked evenly across the full depth reduce point stress.

Material and Shelf Strength: Engineering Behind the Sag Resistance

Both BILLY and KALLAX use particleboard with a paper foil finish. This material is cost-effective and lightweight but inherently weaker than solid wood or plywood. Particleboard has lower tensile strength and is prone to swelling or delamination if exposed to moisture. More critically for this comparison, it tends to sag over time under sustained loads, especially in longer spans.

The critical factor in sag resistance is span length—the distance between support points. Longer spans require thicker or reinforced materials to resist bending. Here's where the two models diverge significantly.

• BILLY: Each shelf spans 79 cm (width) and is supported at both ends and the center via metal pins. The center support drastically reduces effective span to ~39.5 cm per side, greatly improving rigidity.
• KALLAX: Each horizontal board spans 79 cm (across four 39 cm cubes) with support only at the outer edges. There is no center support, meaning the full 79 cm span must bear any load applied to the middle sections.

This lack of intermediate support makes KALLAX shelves far more susceptible to sagging, especially when loaded unevenly or with heavier items like large art books or binders.

“Long unsupported spans in particleboard furniture are a recipe for deflection over time. Even within rated limits, creep deformation occurs.” — Dr. Lena Peters, Materials Engineer, Royal Institute of Technology

Weight Capacity Comparison: Numbers vs Reality

On paper, IKEA provides maximum load ratings for both systems. But these numbers don’t always reflect long-term performance or real-world conditions.

Model	Shelf Span	Max Load per Shelf	Center Support?	Realistic Safe Load (Long-Term)
BILLY Bookcase (standard)	79 cm (with center support)	30 kg per shelf	Yes	20–25 kg
KALLAX (horizontal board)	79 cm (no center support)	13 kg per shelf/board	No	8–10 kg

Note: The KALLAX “shelf” refers to the entire horizontal panel spanning all four cubes. If you place books only in the middle two cubes, the stress concentrates on the weakest part of the board—where sag begins.

A typical hardcover book weighs between 0.5 kg and 1.5 kg. A densely packed BILLY shelf can hold 40–50 average-sized hardbacks (approx. 20–25 kg) without issue. The same cannot be said for KALLAX. Even 15–20 heavy books placed across a single row can exceed safe limits and initiate visible bowing within months.

Moreover, BILLY shelves are 28 cm deep, matching the depth of most books. KALLAX cubes are deeper (39 cm), so when books are placed standing up, they occupy only the front portion of the cube, leaving the rear unsupported and creating a cantilever effect that increases downward torque on the shelf.

Real-World Performance: A Mini Case Study

In a 2022 apartment renovation project in Malmö, Sweden, a freelance researcher named Sofia installed both a BILLY bookcase and a vertical KALLAX unit in her home office to compare durability. Her collection included academic texts, photography books, and archival binders—many weighing over 1 kg each.

She loaded the BILLY with approximately 45 hardcovers per shelf (totaling ~22 kg), evenly distributed from front to back. After 18 months, no visible sag was observed. The shelves remained straight, and adjustment pins showed no signs of stress.

The KALLAX, however, was another story. She used two horizontal rows for books, placing about 18 large-format books (totaling ~14 kg) across each 79 cm span. Within six months, a 1.5 cm dip appeared in the center of both loaded boards. By month 12, the sag had increased to nearly 2.5 cm, causing adjacent cubes to tilt slightly and making retrieval difficult.

Sofia reinforced the KALLAX shelves with wooden backing and added center brackets—modifications not included in the original design. Post-reinforcement, performance improved, but she noted the fix defeated the purpose of a simple, ready-to-assemble solution.

Her conclusion: “BILLY handled my library with ease. KALLAX needed upgrades to perform comparably—and even then, it didn’t feel as stable.”

Maximizing Load Without Sag: Practical Tips

If you’re committed to using either model for heavy book storage, consider these strategies to prevent or delay sagging:

Tip: Install additional shelf supports or braces, especially for spans over 60 cm in particleboard furniture.

• Add Center Brackets: For BILLY, ensure the central support pin is correctly installed. For KALLAX, retrofit L-brackets or wooden cleats underneath the middle of horizontal boards.
• Use Shelf Liners or Backing: Adding a thin plywood or MDF sheet (screwed into the back panel) can distribute weight more evenly and stiffen the structure.
• Distribute Weight Evenly: Avoid clustering heavy books in the center. Alternate lighter and heavier volumes across the span.
• Limit Shelf Fill Depth: On KALLAX, place books toward the front but use bookends to prevent forward tipping. Consider adding small risers to utilize vertical space without overloading the shelf.
• Upgrade Screws and Anchors: Replace included fasteners with longer, sturdier screws and wall anchors to improve overall stability and reduce torsional stress on shelves.

Step-by-Step Guide: How to Reinforce a KALLAX for Book Storage

If you prefer the KALLAX aesthetic but need it to hold more books safely, follow this reinforcement process:

1. Assemble the Unit: Build the KALLAX according to instructions, but do not tighten all screws fully yet.
2. Measure the Center: Identify the midpoint of each horizontal shelf (at 39.5 cm from either side).
3. Install Support Cleats: Cut two 30 cm strips of 1×2 pine or hardwood. Position one under each middle shelf, centered and flush with the back edge. Secure with wood screws through the back panel into the cleat.
4. Add a Backing Board: Cut a piece of 6 mm plywood or hardboard to fit the entire back. Screw it into the rear edges of all panels. This adds torsional rigidity and helps transfer load to the walls.
5. Anchor to Wall: Use IKEA’s included anti-tip device or a heavy-duty bracket to secure the top to wall studs.
6. Test Gradually: Load books slowly over several weeks, checking for movement or deflection.

This upgrade can increase KALLAX’s effective load capacity by 50–70%, bringing it closer to BILLY’s performance—but at the cost of added complexity and visibility of modifications.

FAQ: Common Questions About BILLY and KALLAX Durability

Can I use KALLAX horizontally for books?

You can, but it’s not ideal for heavy collections. The long, unsupported spans make it prone to sagging. If used horizontally, limit loads to lighter paperbacks or decorative objects, and consider reinforcing the shelves.

Does BILLY need wall anchoring?

Yes. All tall furniture, including BILLY, should be anchored to the wall to prevent tipping, especially in homes with children or pets. Anchoring also reduces forward flex during loading, indirectly helping shelf longevity.

Are there stronger versions of BILLY?

Yes. IKEA offers the BILLY with glass doors and a white stained oak veneer finish, which includes a steel frame and sturdier particleboard. This version has better structural integrity and is less prone to sagging under heavy loads.

Final Verdict: Which Holds More Books Without Sagging?

The answer is clear: the BILLY bookshelf holds significantly more books without sagging than the KALLAX.

Its deeper shelves (optimized for book depth), center-supported design, higher weight rating, and vertical orientation make it superior for dense book storage. Real-world testing and material science both confirm that BILLY maintains structural integrity under loads that would visibly deform KALLAX shelves over time.

KALLAX excels in versatility, modularity, and aesthetic flexibility—it’s perfect for bins, folded clothes, toys, or light decor. But when the priority is maximizing book capacity with minimal maintenance and long-term reliability, BILLY is the better investment.

That said, KALLAX can be adapted for heavier use with reinforcements. However, doing so adds cost, effort, and visual bulk, diminishing its appeal as a minimalist, plug-and-play solution.

Conclusion: Choose the Right Tool for the Job

Selecting between BILLY and KALLAX isn’t just about price or appearance—it’s about matching form to function. For book lovers, students, academics, or anyone building a lasting home library, the BILLY’s engineering advantages make it the smarter, more durable choice. It’s designed for books, tested under load, and proven over time to resist sagging where others fail.

If you already own a KALLAX and want to use it for books, take proactive steps to reinforce it. But if you’re starting fresh, go with BILLY for pure book-holding performance.

🚀 Ready to build a reliable, sag-free bookshelf? Choose BILLY for heavy loads, reinforce KALLAX for mixed use, and always anchor your furniture. Share your setup or ask questions in the comments—we’d love to hear how you organize your collection!