Natural light changes the fundamental feel of a home, yet the success of any skylight installation depends heavily on selecting the correct dimensions. When looking at Velux skylights sizes, the process involves more than just measuring a hole in the ceiling. It requires an understanding of mounting systems, structural limitations of rafters, and the specific daylighting goals of the space. As of 2026, the standard range has expanded to accommodate both traditional residential structures and modern, high-efficiency building envelopes.

Selecting the wrong size can lead to structural compromises or insufficient thermal performance. This analysis breaks down the specific measurements for the most common Velux models, ensuring a precise fit for both new constructions and replacement projects.

The Core Logic of Skylight Sizing

Before diving into specific charts, it is essential to distinguish between the two primary mounting methods used by Velux. The sizing logic for a deck-mounted unit does not apply to a curb-mounted unit.

Deck-Mounted vs. Curb-Mounted Dimensions

Deck-Mounted Skylights (such as the FS, VS, VSE, and VSS series) are designed for roofs with a pitch between 14 and 85 degrees. These units attach directly to the roof deck. For these models, the critical measurement is the Rough Opening. This is the actual hole cut into the roof's plywood and rafters. The skylight frame itself is slightly larger than the rough opening to ensure a snug, weatherproof fit.

Curb-Mounted Skylights (such as the FCM, VCM, VCE, and VCS series) are more versatile, often used for flat roofs or low-pitched roofs (0 to 60 degrees). These units sit atop a site-built or pre-fabricated wooden frame known as a curb. For these models, sizing revolves around the Outside Curb Dimensions. The skylight acts like a shoebox lid fitting over the curb.

Deck-Mounted Skylights Size Charts

Deck-mounted units use an alphanumeric coding system. The letter usually indicates the width, while the numbers indicate the height. For example, a "C" code typically indicates a width that fits between standard 24-inch on-center rafters without requiring significant structural modification.

Fixed (FS) and Manual/Electric/Solar Fresh Air (VS, VSE, VSS)

Standard sizes for deck-mounted units are engineered to work with common framing practices. Below are the dimensions for the most frequent size codes used in residential projects:

  • A06: Rough Opening 14 1/2" x 45 3/4". This narrow profile is excellent for hallways or small bathrooms where space between rafters is limited.
  • C01: Rough Opening 21" x 26 7/8". A compact square-ish option for small task lighting.
  • C04: Rough Opening 21" x 37 7/8". One of the most common sizes for kitchens.
  • C06: Rough Opening 21" x 45 3/4". A versatile mid-range size.
  • C08: Rough Opening 21" x 54 7/16". Ideal for deep shafts where more vertical light is needed.
  • C12: Rough Opening 21" x 70 1/4". A dramatic, long skylight often used in series to create a "glass roof" effect.
  • D26: Rough Opening 22 1/2" x 22 15/16". Specifically designed to fit perfectly between roof trusses spaced 24" on center.
  • D06: Rough Opening 22 1/2" x 45 3/4". Another truss-friendly option with double the length of the D26.
  • M02: Rough Opening 30 1/16" x 30". Requires cutting at least one rafter in standard framing.
  • M04: Rough Opening 30 1/16" x 37 7/8". Provides a significant increase in daylight area compared to the C-series.
  • M06: Rough Opening 30 1/16" x 45 3/4". A popular choice for master bedrooms.
  • M08: Rough Opening 30 1/16" x 54 7/16". Offers substantial sky views.
  • S01: Rough Opening 44 1/4" x 26 7/8". A wide, short unit often used over garden tubs.
  • S06: Rough Opening 44 1/4" x 45 3/4". One of the largest standard residential units, providing massive light output.

When planning for these sizes, always verify the Outside Frame Dimensions. For a C04 unit, while the hole is 21" wide, the frame is actually 21 1/2" wide. This extra half-inch is vital for the integrated flashing system to function correctly.

Curb-Mounted Skylights Size Charts

Curb-mounted units use a four-digit numeric code. The first two digits represent the nominal width, and the last two represent the nominal height. These units are the industry standard for replacements because they can accommodate slight variations in existing curb sizes.

Fixed (FCM) and Venting (VCM, VCE, VCS)

For curb-mounted models, the industry focuses on the Inside Curb Dimensions, which should match the rough opening of the roof hole.

  • 1430: Inside Curb 14 1/2" x 30 1/2" (Outside Curb 17 1/2" x 33 1/2").
  • 2222: Inside Curb 22 1/2" x 22 1/2" (Outside Curb 25 1/2" x 25 1/2"). This is the most common size for replacement projects.
  • 2230: Inside Curb 22 1/2" x 30 1/2" (Outside Curb 25 1/2" x 33 1/2").
  • 2234: Inside Curb 22 1/2" x 34 1/2" (Outside Curb 25 1/2" x 37 1/2").
  • 2246: Inside Curb 22 1/2" x 46 1/2" (Outside Curb 25 1/2" x 49 1/2"). Often used in pairs.
  • 2270: Inside Curb 22 1/2" x 70 1/2" (Outside Curb 25 1/2" x 73 1/2"). A massive fixed unit for dramatic lighting.
  • 3030: Inside Curb 30 1/2" x 30 1/2" (Outside Curb 33 1/2" x 33 1/2").
  • 3046: Inside Curb 30 1/2" x 46 1/2" (Outside Curb 33 1/2" x 49 1/2").
  • 3434: Inside Curb 34 1/2" x 34 1/2" (Outside Curb 37 1/2" x 37 1/2").
  • 4622: Inside Curb 46 1/2" x 22 1/2" (Outside Curb 49 1/2" x 25 1/2"). A horizontal orientation.
  • 4646: Inside Curb 46 1/2" x 46 1/2" (Outside Curb 49 1/2" x 49 1/2"). Maximum standard square footprint.

One unique feature of the curb-mounted FCM series is that there is no designated top or bottom. This allows for flexible installation—a 2246 can be installed as a 4622 if the roof pitch allows. However, venting models like the VCM must follow specific orientation rules to ensure proper drainage and operation.

International and European Size Codes

For projects outside North America, particularly in Europe and Oceania, Velux uses a different coding system based on two letters and two numbers (e.g., CK02, MK04). These are measured in millimeters and are standard across the international market.

  • CK02: 550 x 780 mm. Compact, often used in bathrooms.
  • CK04: 550 x 980 mm. A common choice for standard rafters.
  • FK06: 660 x 1180 mm. Mid-sized with balanced proportions.
  • MK04: 780 x 980 mm. Wider profile for better views.
  • MK06: 780 x 1180 mm. A versatile bedroom or living room size.
  • MK08: 780 x 1400 mm. Large vertical area for deep light penetration.
  • PK08: 940 x 1400 mm. Broad coverage for larger rooms.
  • SK06: 1140 x 1180 mm. Near-square, large format.
  • UK04: 1340 x 980 mm. Extra-wide for maximum horizontal impact.

In these international markets, the terminology often shifts toward "Roof Windows." While the sizing logic remains consistent regarding frame vs. opening, these units often include different opening mechanisms, such as center-pivot or top-hung, which might require slightly more clearance for the sash movement.

How to Choose the Right Size for Your Room

The most common mistake in skylight selection is choosing a unit that is too small for the room's volume. While individual preference varies, architectural standards suggest a "daylight-to-floor-area" ratio.

The 10% Rule

A general recommendation is that the total glass area of skylights should be at least 10% of the floor area of the room they are illuminating. For a room that is 150 square feet, you would ideally want about 15 square feet of skylight glass.

Looking at the standard charts:

  • An M08 unit provides approximately 7.1 square feet of daylight area.
  • An S06 unit provides approximately 10.1 square feet of daylight area.

Therefore, for a 150-square-foot room, two M08 units or one S06 and one smaller unit would be appropriate. In rooms with fewer vertical windows, increasing this to 15% can significantly reduce the need for artificial lighting during the day.

Room-Specific Sizing Considerations

  • Bathrooms and Small Areas: Privacy is often a concern, so smaller units like the A06 or C01 are preferred. If moisture is high, choosing a size that supports the "Fresh Air" venting models is more important than sheer dimensions.
  • Kitchens: This is where task lighting is critical. Positioning multiple smaller units (like three C04s) over an island is often more effective than one large central unit.
  • Stairwells: These are usually deep shafts. Selecting a long, narrow unit like the C08 or C12 allows light to reach the lower levels more effectively.
  • Living Rooms and Master Bedrooms: These spaces benefit from "sky views." Larger units like the M08 or 4646 create a sense of openness and luxury.

Structural Limitations and Sizing

The physical dimensions of your roof's structure are the most significant limiting factor for skylight size. Most modern homes are built with either rafters or trusses.

Rafter Framing

In older homes, rafters are typically spaced 16 inches or 24 inches apart.

  • If your rafters are 16" on center, an A06 (14 1/2" wide) fits between them perfectly without cutting wood.
  • If your rafters are 24" on center, a C-series unit (21" wide) fits between them.

If you want a wider skylight, such as an M-series or S-series, you must cut one or more rafters and install headers. This is a common procedure but adds to the labor cost and requires careful structural reinforcement to maintain roof integrity.

Truss Framing

Trusses are engineered components that should generally never be cut without a structural engineer's approval. If you have trusses spaced 24" on center, you are largely limited to units designed to fit that 22 1/2" gap. The D26 and D06 models were created specifically for this scenario. They provide the maximum possible light without compromising the engineered strength of the truss system.

Advanced Sizing: Specialty Units

Beyond standard rectangles, Velux offers specialty sizes that serve specific architectural needs.

Sun Tunnels (TGR, TMR, TLR)

When a standard skylight isn't feasible—perhaps due to a long attic space between the roof and the ceiling—Sun Tunnels provide a solution. These are sized by diameter:

  • 10-inch: Ideal for hallways, closets, or small bathrooms.
  • 14-inch: The standard for larger rooms or kitchens. A single 14-inch tunnel can illuminate up to 300 square feet under optimal conditions.

Flat Roof Systems

For flat roofs, Velux offers units with specialized covers, such as the ISU Curvetech or polycarbonate domes. The base units for these are typically sized like the curb-mounted models (e.g., 60x60 cm, 90x90 cm, 120x120 cm). The "clearance" for these units is crucial because the drainage on a flat roof is slower, requiring a higher curb height and precise sizing to prevent pooling.

Roof Terrace and Balcony Windows (GDL)

The GDL CABRIO is a unique unit that transforms into a balcony. This requires a specific rough opening—typically around 940 x 2520 mm. Because this unit carries the weight of a person when open, the structural requirements and the precision of the rough opening are much higher than standard fixed units.

Measuring for Replacement: The Data Plate Trick

If you are replacing an existing Velux skylight, you rarely need to climb on the roof with a tape measure. Every Velux unit manufactured since the late 1960s has a small ID plate, usually located on the top or side of the inner frame (visible when the unit is open, or through the glass for fixed units).

This plate contains a code that tells you the exact size. For example:

  • An old code like "1" or "102" often corresponds to the modern C01 size.
  • A code like "6" or "606" corresponds to the modern M06.
  • Newer units will have the size code explicitly listed (e.g., FS M08 2004).

When ordering a replacement, providing this code is more accurate than manual measurements, as it accounts for the subtle frame changes made over the decades.

Environmental Impact of Sizing

Larger skylights bring in more light, but they also increase the potential for heat gain in the summer and heat loss in the winter. In 2026, building codes emphasize the "U-factor" and "Solar Heat Gain Coefficient" (SHGC).

  • Large South-Facing Skylights: In warmer climates, a massive S06 or 4646 might lead to overheating. In these cases, it is often better to use two smaller units (like C04s) with high-performance heat-shielding glass rather than one giant pane.
  • Glazing Options: Larger sizes benefit significantly from triple-pane glass or white laminated glass. Laminated glass, in particular, diffuses the light, preventing "hot spots" on the floor and reducing UV fading on furniture, which is a common issue with large-format skylights.

Final Considerations for Installation

Once the size is chosen, the final step in ensuring a successful fit is the flashing. Velux offers specific flashing kits for different roofing materials (shingles, tile, metal). These kits are size-specific. An EDW C06 kit will only fit a C06 skylight.

If you are using a curb-mounted unit on a very thick tile roof, you may need to choose a slightly larger curb height to ensure the skylight frame clears the tiles. This doesn't change the skylight size itself but affects the total vertical footprint on your roofline.

Choosing the right Velux skylights sizes is a balance of architectural desire and structural reality. By respecting the rough opening requirements and matching the glass area to your room's needs, you ensure that the addition of natural light is a permanent improvement to your home's value and comfort. Always consult the latest technical data sheets for your specific model, as frame profiles can vary slightly by region and year of manufacture.