Standard Mount Sizes for Picture Frames: A UK Reference

A picture mount has two relevant sizes: the outer dimension, matched to the frame, and the aperture (also called the window), matched to the artwork. Getting both right is what makes a framed piece look balanced.

This reference covers the standard mount sizes used in UK framing, including outer dimensions matched to A-series and imperial frames, mount aperture sizes for common artwork, mount thickness standards, the five common window styles, the rule of thumb for choosing margins, and what changes once a mount moves out of standard territory into bespoke on our Gunnar precision mount cutter in Bollington, Cheshire.

Standard outer mount sizes matched to frame sizes

The outer dimension of a mount is the size that determines which frame it slots into. By the standard convention, the outer mount size matches the frame inner rebate dimension, so a mount cut to A3 outer fits an A3 frame, a 16 × 20 inch outer fits a 16 × 20 inch frame, and so on. The table below sets out the common picture mount sizes for UK frames in millimetres.

A4 outer mounts are by far the most common in UK retail. Most off-the-shelf ready-made frames and pre-cut mount packs are built around the A4 outer dimension, with an A5 aperture cut as the standard configuration. For the matching frame-size reference and the question of which mount sizes for frames apply at each common dimension, see our standard picture frame sizes guide.

Standard aperture (window) sizes for common artwork

The aperture is the window cut into the mount that the artwork shows through. This dimension is matched to the artwork itself, with a small overlap, typically 3 to 5mm on each side, so the mount holds the work in place behind the window. The tables below cover the standard mount aperture sizes for A-series and imperial photo work at their actual nominal dimensions.

For photo mount sizes specifically, the imperial set is the working standard. UK photo printers, lab prints and ready-made photo frames all hold to these dimensions, so cutting a 5 × 7 inch aperture into a larger outer mount is the most common pairing for family photo work.

Mount thickness standards

The UK trade standard for mount thickness is 1.4mm conservation mount board. This is the thickness used across the majority of bespoke and retail framing, and it is what we cut as the workshop default. For higher conservation needs, 1.5mm cotton museum board is the upgrade, made from pure cotton fibre with a more inert surface for long-term preservation. Mounts can also be double-mounted (two layers, with the lower mount visible as a thin reveal around the aperture) or triple-mounted where a deeper layered look is wanted. For background on our full bespoke framing service, mount cutting is part of every standard commission.

Mount window styles

Mount window sizes are determined by the artwork, but the style of the window cut is a separate decision. Single aperture is the standard format: one window cut into the mount around a single image. This is what most people picture when they think of a mounted print, and it covers the majority of framing work.

Multi-aperture mounts have two or more windows cut into one mount, used for family photo montages, sets of prints, or sequence pieces where the images need to read as a single composition. Two-, three- and four-aperture configurations are the most common; larger grids are cut to specification.

V-groove is a V-shaped groove cut into the face of the mount as a decorative line, usually a few millimetres in from the aperture edge. The groove catches light and adds a clean architectural detail without changing the colour of the mount surface.

French line, sometimes called a French wash, is a drawn or painted decorative line around the aperture, occasionally combined with a wash of colour between two parallel lines. French line work is hand-finished on the mount face and is most often used on traditional fine art mounts.

Fillet inset is a narrow secondary moulding inset into the mount along the aperture edge, adding depth and a second material between the mount and the artwork. Fillets come in plain wood, gilded and painted finishes, and the depth they add can transform a flat mount into something closer to a shadow-box effect. V-groove and multi-aperture work is cut on the Gunnar precision mount cutter to the millimetre.

Choosing aperture margins

The rule of thumb for mount margins is equal on three sides with a slightly wider bottom margin for fine art. Typical margins are 30 to 50mm for photographs and 50 to 75mm for fine art, with larger pieces taking wider margins to feel deliberate rather than tight. The weighted bottom is a longstanding convention from gallery framing, where the wider lower margin compensates for the natural eye-line and stops the artwork looking as though it is slipping down inside the frame. For more on choosing the right mount and frame combination for a piece, our guide to choosing the right frame for your artwork covers it in more detail.

As a concrete example, an A3 print at 297 × 420mm in a fine-art mount might use 60mm equal margins on the top and sides with 75mm at the bottom, giving an outer mount of roughly 417 × 555mm.

Custom mount sizing at Harten

The standard mount sizes above cover most retail and ready-made framing, but bespoke is the default for anything outside them. We cut to the millimetre in our Bollington workshop on a Gunnar precision mount cutter, to any aperture shape, any number of windows, with V-groove, French line, fillet inset or any combination of the four. Conservation-grade 1.4mm mount board is the workshop default, with 1.5mm cotton museum board available where the work warrants it. Mount cutting is part of every bespoke framing commission, and our 5-year guarantee covers the mount and the frame as a single piece of work.

Send us the artwork dimensions and the outer mount size you want, and we will quote on the cut. If you are unsure on margins or window style, we will recommend based on the piece.