Will an Unframed Print Fit an IKEA Ribba? UK Frame Sizes Explained

The short answer

An A3 print will not fit directly into a 30 x 40cm frame: A3 measures 297 x 420mm, while the frame opening is 300 x 400mm, so the print is 20mm too long. For an A3 print, choose an A3 frame, or a larger frame such as 40 x 50cm with a mount cut to an A3 aperture. If you are checking a frame associated with IKEA or another retailer, read the size printed on that particular frame rather than assuming the name of the range tells you the fit.

The confusion comes from two different sizing systems. Most prints use international A-series sizes, while many ready-made frames use round-number metric sizes. A mount can make a smaller print work in a larger frame, but it cannot make a longer print fit a shorter frame.

The two sizing systems on every UK high street

The A-series is an internationally standardised paper system. Its sizes keep the same proportions, with the longer side about 1.41 times the shorter side. That consistent shape means an A4 design can be enlarged to A3, or reduced to A5, without changing its proportions. The ISO 216 standard defines the A-series paper sizes and their 1.41 proportion.

Alongside A-series sizes, you will commonly see round-number frame sizes such as 30 x 40cm and 50 x 70cm. These are a separate system with different proportions. They may look close to A-series sizes when listed online, but they are not the same shape. That difference is the source of many apparently incorrect print-and-frame combinations.

The A-series sizes, in millimetres and centimetres

These are the standard A-series dimensions. Millimetres are the most useful unit when checking whether a print will fit an opening.

Size name	Millimetres	Centimetres	Roughly the size of
A5	148 x 210mm	14.8 x 21cm	A smaller sheet of paper
A4	210 x 297mm	21 x 29.7cm	A sheet of office paper
A3	297 x 420mm	29.7 x 42cm	Two sheets of office paper side by side
A2	420 x 594mm	42 x 59.4cm	A small poster
A1	594 x 841mm	59.4 x 84.1cm	A large poster

Each A size is made by halving the next size up along its longer side. For example, folding A3 across its longer side produces A4, and folding A4 produces A5. ISO 216 rounds each result down to whole millimetres, so the halving is not exact to the millimetre.

The round-number frame sizes you'll also see

Frames described as 30 x 40cm or 50 x 70cm use round-number dimensions rather than the A-series. Their proportions are different, so neither size has exactly the same shape as an A-series sheet.

This matters because a print is not just a collection of measurements. Its proportions determine where the edges and design sit. An A-size print cannot normally be trimmed into a differently proportioned frame without cutting into the design. The mismatch is not simply a small rounding error that can be tidied away.

Why an A-size print doesn't fit a 30x40cm frame

A3 is 297 x 420mm. A 30 x 40cm frame is 300 x 400mm. The widths are close, but the frame is 20mm shorter on its long side than the A3 print. That means the A3 print physically cannot sit inside the frame, with or without a mount.

This is why A3 is bigger than 30 x 40cm in the direction that matters: its 420mm side is longer than the frame's 400mm side. It is also a different proportion, so rotating or slightly trimming the print does not provide a clean solution. Use an A3 frame for an unmounted A3 print, or use a larger frame with a correctly sized A3 mount.

What a mount is, and why it solves this

A mount, also called a mat, is a card border with a window cut into it. It sits between the print and the frame. The window shows the image, while the border covers the outer edge of the print and holds it centred inside a larger frame.

A mount therefore makes a smaller print fit a larger frame. For example, an A4 print fits comfortably inside a 30 x 40cm frame with an A4 mount. This is why a frame can have a round-number outside size while still being suitable for an A-series print.

A mount cannot make an A3 print fit a frame that is shorter than the print. It can only fill the extra space around a smaller print. It also gives the image breathing room, which can make the finished picture look more considered.

Check two measurements when a frame includes a mount:

• The frame opening: the size of the image or mount the frame is designed to hold.
• The mount window: the visible opening that the print needs to fit behind.

The stated frame size normally refers to the image it is designed to hold, not the outer dimensions of the frame itself. A frame sold with a mount effectively has two sizes to check, which is where many sizing mistakes happen.

How to measure a frame you already own

Do not measure the outside edge of the frame. Measure the space where the print will sit.

1. Remove the backing or mount if it can be removed safely.
2. Measure the internal opening from edge to edge, both ways.
3. If there is a mount, measure the mount window as well.
4. Write the measurements in millimetres, including which measurement is the longer side.
5. Compare those measurements with the print size, not just with a product name or a rounded centimetre description.

If the frame opening is smaller than either side of the print, the print will not fit directly. If the opening is larger and the mount window matches the print, the mount is intended to solve the difference.

Buying a frame for a print you haven't received yet

Start with the print's stated size. For an A3 print, buy an A3 frame if you want the print to fill the frame directly. Alternatively, choose a larger frame whose mount window is specifically cut for A3. Do not substitute a nearby round-number size without checking the actual opening.

The same principle applies to personalised personalised map prints. A map print is centred on a location you choose in the online designer, and you can add a short line of text and optionally a date beneath the map. If you are choosing wording, roughly two to six words works best; longer text has to be set smaller to fit the print's width and is harder to read from across a room.

For a city-based design, you can explore the Custom City Map. If you are comparing the idea with another personalised design, this guide to map print vs star map explains the distinction. You can also see more city map prints before deciding on the location and format.

When to get a print framed instead

Choose a framed print when you want to avoid checking compatibility between an unframed print, a frame opening and a mount window. It can be the simpler choice when the print is a gift or when you want the recipient to hang it without sourcing a suitable frame.

If the decision is mainly about giving rather than measuring, read this guide to framed or unframed as a gift. If you are ready to choose a location-based design, Design a Print for Your Wall.

Frequently asked questions

Will an A3 print fit a 30 x 40cm frame?

No. A3 has a 420mm long side, while the frame opening is 400mm on its longer side. The frame is too short, and a mount cannot overcome that physical difference.

Is A3 bigger or smaller than 30 x 40cm?

A3 is 297 x 420mm and 30 x 40cm is 300 x 400mm. A3 is close in width but longer in the other direction, and the two formats have different proportions.

What is a mount in a picture frame?

A mount is a card border with a window cut into it. It covers the border of a smaller print, holds it centred inside a larger frame and creates space between the image and the frame.

How do I know what size frame to buy for a print?

Match the print's measurements to the frame opening in millimetres. Buy the same A-series frame for a direct fit, or choose a larger frame only when its mount window is the correct size for the print.

Looking more broadly? Browse our personalised gifts.