Picture frame sizes explained: mounts, aspect ratios and hanging

This is a desk-based guide using current official UK frame specifications and a repeatable measuring method. Recommended Today did not buy, hang or stress-test a frame or fixing. Wall construction, artwork value, frame build and fitting instructions vary, so the responsible manufacturer’s limits take priority.

The four measurements that prevent most mistakes

Use one unit throughout. Millimetres reduce rounding errors when a print is close to the opening. Measure the artwork in at least two places because paper can be cut slightly out of square. Record width first and height second, but remember that rotating a rectangular frame simply exchanges them.

Common UK and metric frame sizes

Popular metric options include 10 × 15cm, 13 × 18cm, 21 × 30cm, 30 × 40cm, 40 × 50cm, 50 × 70cm and 61 × 91cm. The list is useful for shopping, but it is not a promise that a print produced to a paper standard will fit with the intended border. A4 is 210 × 297mm, for example, while a frame described as 21 × 30cm provides a nominal 210 × 300mm picture size. That small difference may be harmless behind a mount but should be verified for edge-to-edge display.

Traditional photo ratios create another family: 6 × 4in, 7 × 5in, 8 × 6in and 10 × 8in. Converting inches to centimetres does not change the composition. A 6 × 4in image is roughly 15.2 × 10.2cm and has a 3:2 ratio; a 10 × 8in print has a 5:4 ratio. Putting one into the other’s opening requires a crop, a border or a custom mount.

Mounts: the opening is deliberately smaller

A mount separates the artwork visually from the moulding and can keep the image away from glazing. Its cut opening is normally a little smaller than the artwork, creating an overlap that prevents the sheet falling through. Therefore, do not order a mount whose aperture exactly equals a print that has essential detail at the edge. Decide how many millimetres can be hidden on every side.

An official IKEA example checked on 19 August 2026 states that its 30 × 40cm PLOMMONTRÄD frame fits a 21 × 30cm picture with the included mount or a 30 × 40cm picture without it. That is a clear illustration of the two-size convention. The same retailer says the mount is acid-free, but archival suitability is broader than one component: backing, hinges, tape, glazing, humidity and light exposure all matter for valuable work.

For signatures, edition numbers or deckled paper edges, consider a larger aperture or float mounting by a competent framer. Do not attach irreplaceable work with ordinary pressure-sensitive tape. A replaceable poster and an original drawing justify different materials and risk controls.

Aspect ratio: crop, border or custom size

Aspect ratio is width divided by height. If the artwork and aperture ratios differ, scaling alone cannot make both edges fit. Filling the aperture crops part of the image; fitting the whole image leaves a border on two sides. Neither is automatically wrong, but the choice should be deliberate.

If a standard frame is only slightly larger, a custom-cut mount can bridge the gap economically. When the artwork is unusually long, square or panoramic, a made-to-measure frame may avoid an ungainly border. Compare the complete delivered cost and cancellation terms before commissioning; personalised dimensions are less forgiving than standard stock.

Glazing, backing and depth

Glass is usually more scratch-resistant than plastic but heavier and breakable. Acrylic or polystyrene front protection is lighter and can be safer in busy areas, yet it may scratch more readily and attract dust through static. Product names such as “glass look” are not material specifications. Check the exact front-protection material and cleaning instructions.

Conventional shallow frames are intended for flat work. A folded textile, medal, dried flower or other object needs enough internal depth and a method that does not crush or chemically damage it. A shadow-box frame can be more suitable, but its greater projection and loaded weight change the hanging decision.

Light causes fading even when a product mentions UV reduction. Avoid treating glazing as complete protection. Limit direct sunlight, maintain a stable indoor environment and consider conservation advice for valuable or irreplaceable pieces.

Plan the wall with true outer dimensions

The retailer’s picture size is not the wall footprint. A broad moulding makes a 30 × 40cm frame substantially larger; two products accepting the same print can occupy different areas. Use the listed outer dimensions where available, or measure the assembled frame. Then cut paper rectangles, tape them lightly to the wall with a surface-appropriate method, and live with the arrangement before making holes.

Anchor the layout to a real reference: the centreline, furniture width, top edge or a consistent gap. For a group, the spaces between frames influence the result as much as the frames. Treat the collection as one overall rectangle and check that doors, curtains, switches and lamps still work. Our furniture measuring guide offers the same route-and-clearance discipline, while the product-shortlisting method helps reduce a sprawling choice of finishes.

A staircase or uneven wall needs extra care because visual level and physical level can conflict. Mark a reference line with removable tape, verify it with a spirit level, and calculate each hanging point from the frame hardware rather than assuming every cord or bracket sits at the same distance from the top.

Fixings: match wall, weight and hardware

A hook is only one part of a load path. Identify whether the wall is masonry, timber stud, plasterboard or another construction; weigh the complete frame with glazing and artwork; inspect the supplied hanger; and follow both fixing and frame instructions. Do not use an adhesive product above its stated load or on an unsuitable surface. Paint condition, texture, humidity and removal method can all affect performance.

Price and value: pay for the specification you need

On 19 August 2026, IKEA’s official UK 30 × 40cm category showed a black RÖDALM at £7, an oak-effect version at £9 and a white FISKBO at £4; a 30 × 40cm aluminium LOMVIKEN page showed £15. These are dated samples, not permanent prices or recommendations. They demonstrate that identical nominal picture capacity does not mean identical moulding, front protection, finish, mount, weight or longevity.

Compare a written specification, not just colour thumbnails. Include mount, replacement backing, specialist fixing, delivery and any custom cutting. For a multiple-frame wall, test one unit before committing to the whole set. Our total-cost guide keeps accessories and replacement risk visible, while the discount-checking guide prevents a sale badge substituting for fit.

If the frame is a present, record the recipient’s print dimensions or choose a reversible standard size rather than guessing from a social-media crop. Our practical gift guide explains how to preserve choice, and the Style & Gifts section covers related sizing, delivery and presentation decisions.

Who should choose which route?

Our verdict

The best frame is the one whose aperture protects the intended composition, whose rebate accepts the complete artwork package and whose outside dimensions genuinely suit the wall. A familiar number in the product title is not enough. Measure in millimetres, record both with-mount and without-mount capacities, and make a full-size paper template.

Standard frames are excellent when the ratios cooperate; a custom mount often solves a modest mismatch. Use custom framing where trimming, ordinary materials or a shallow rebate would compromise the work. Finally, treat hanging as a structural choice: loaded weight, wall type and correct fixings matter more than visual neatness.