Royal Mail Parcel Sizes UK (2026): Large Letter, Small Parcel & Medium Parcel Box Guide

Royal Mail parcel sizes for UK domestic post depend on the finished package, not just the product inside it. As checked on 19 September 2026, a Large Letter can measure up to 353 × 250 × 25mm. A Small Parcel reaches 450 × 350 × 160mm, while a Medium Parcel reaches 610 × 460 × 460mm. Weight matters too, and the Large Letter limit changes for some Tracked 24 and Tracked 48 purchase channels.

Royal Mail parcel sizes at a glance

For standard Royal Mail 1st Class and 2nd Class, the main domestic formats are:

These Royal Mail parcel sizes are maximum packed dimensions. Exceed one relevant measurement and the parcel no longer fits the smaller format. The other two dimensions cannot compensate for that.

There is one important exception to the table. Royal Mail currently allows a Large Letter up to 1kg when a consumer buys Tracked 24 or Tracked 48 postage online. For Tracked 24 or Tracked 48 bought at a Post Office, Royal Mail lists the Large Letter limit as 750g. The dimensions remain 353 × 250 × 25mm in both cases.

The Large Letter weight limit is not one universal number

Many Royal Mail size guides give 750g as the Large Letter limit. That is correct for standard 1st Class and 2nd Class, including Signed For. It also applies to Tracked 24 or Tracked 48 Large Letters bought at a Post Office.

Online consumer Tracked 24 and Tracked 48 are different. Royal Mail currently lists a 1kg maximum for the Large Letter format when consumers purchase those services online.

That distinction can change the classification decision. Take a fully packed item measuring 350 × 240 × 20mm and weighing 900g. It is too heavy for a standard 1st Class or 2nd Class Large Letter, but it can fall within the current online Tracked Large Letter weight allowance. The contents and other service conditions must still be acceptable.

For businesses that switch between services, designing around 750g gives more flexibility across the familiar Large Letter options. If a fulfilment workflow uses one specific service consistently, check that service’s live terms before fixing the packaging specification.

Large Letter: the 25mm external depth is the critical limit

A Royal Mail Large Letter can measure up to 353mm long, 250mm wide and 25mm deep. For boxed ecommerce products, depth is often the hardest limit to meet.

The 25mm figure applies to the finished external package. It is not an internal box depth. Corrugated or solid-board walls occupy some of that space, while inserts, folded flaps, tape and the product itself can affect the final profile. If the packed box bows above 25mm, measuring the empty carton will not tell you whether it qualifies.

Consider a slim skincare carton that appears comfortably below 25mm on its own. A protective insert and postal box can push the finished package past the Large Letter depth. Its length and width may still sit well inside their limits. In that case, the relevant comparison is with the Small Parcel band, not with the product dimensions.

For products genuinely suited to this format, shallow Large Letter postal boxes can reduce unused space. Measure and weigh the final packed sample before selecting postage.

Small Parcel: 450 × 350 × 160mm and up to 2kg

A Royal Mail Small Parcel can measure up to 450 × 350 × 160mm and weigh up to 2kg on the standard domestic services covered here. Compared with a Large Letter, the extra 135mm of permitted depth creates far more room for protective packaging, deeper products or multiple items.

Royal Mail defines the parcel’s length as its longest side. Measure the other dimensions across the remaining sides of the finished package rather than relying on a supplier’s internal box measurements.

A packed box measuring 440 × 340 × 155mm is within the Small Parcel dimensional limits. Increase that 155mm measurement to 165mm and it no longer fits the Small Parcel band. The other two measurements may remain unchanged. If it remains within the Medium Parcel limits and the selected service accepts it, Medium Parcel becomes the relevant size band.

That boundary matters when a product sits close to 160mm deep. An unnecessarily tall stock carton can move the shipment into a larger format. Excessive void fill or a lid that does not close flat can do the same, without improving protection.

Medium Parcel: 610 × 460 × 460mm and up to 20kg

For the standard consumer services in this guide, a Medium Parcel can measure up to 610 × 460 × 460mm. Its maximum weight is 20kg. This is the familiar retail ceiling for larger boxed items sent through Royal Mail’s domestic parcel services.

At this size, postal classification is only part of the packaging decision. A box also has to support the product through handling and transport. Leaving a large product loose inside an oversized carton may keep the parcel within Royal Mail’s dimensions, yet still produce poor protection.

Royal Mail advises that its consumer UK parcel maximum is 20kg and directs parcels in the 20–30kg range to Parcelforce Worldwide. Business-account products can follow different rules. Royal Mail’s business Tracked 24 and Tracked 48 page lists a parcel format up to 610 × 460 × 460mm. Its current maximum weight is 30kg.

For that reason, businesses using a negotiated or account service should check that exact specification. The retail Medium Parcel band is not a universal Royal Mail rule.

How to decide your Royal Mail size band before ordering boxes

When using Royal Mail parcel sizes to specify packaging, classify the finished parcel, not an empty box or an unpacked product. A reliable workflow is:

1. Pack the product as it will actually be sent. Include inserts, tissue, padding, documents, tape and any other materials used in fulfilment.
2. Measure the outside of the closed package. Royal Mail treats the longest side as length. Record all three external dimensions in metric units.
3. Weigh the complete package. The box and additional packaging materials count towards the postal weight.
4. Check the exact Royal Mail service and purchase channel. This matters for Large Letter Tracked 24 and Tracked 48 because current online and Post Office weight limits differ.
5. Check the contents as well as the size. Royal Mail restricts or prohibits certain items, including some batteries, aerosols, perfumes and nail products. Meeting the dimensional limit does not make a restricted item automatically acceptable.

For a repeat ecommerce SKU, keep a measured packed sample or a documented packed specification. That makes it easier to spot a packaging change that could alter the postal band later.

Internal box size and Royal Mail external size are not interchangeable

Packaging suppliers commonly quote dimensions for the usable space inside a carton. A carrier, by contrast, classifies the outside of the finished parcel. Royal Mail parcel sizes must therefore be checked against external packed measurements, not the usable internal space.

Do not assume that a box with a 25mm internal depth qualifies as a 25mm Large Letter; its external depth will be greater. Likewise, a box sold under a name such as “Large Letter box” is not a substitute for checking the finished parcel.

If the distinction is unfamiliar, our guide to how to measure a box correctly explains internal, external and panel dimensions in more detail.

Which type of box suits each Royal Mail size band?

The postal band tells you the maximum outer envelope. It does not prescribe one box construction.

Large Letter: shallow postal cartons or mailer-style boxes can work for thin products where the packed depth remains within 25mm. Here, every millimetre of external depth matters, so consider the product, board, closure and any insert together.

Small Parcel: mailer boxes and compact shipping cartons have more room for cushioning or multiple items while staying within 450 × 350 × 160mm. Right-sizing is still useful because unused volume adds packing material without changing the product.

Medium Parcel: corrugated shipping boxes are more typical for larger or heavier items. Select the box style and material around product weight, fragility, stacking and handling requirements rather than postal dimensions alone.

After identifying the Royal Mail band, buyers can compare standard footprints in the Postal Boxes by Size range. For larger applications, compare the available custom shipping boxes.

Common mistakes that push a parcel into a higher size band

Because Royal Mail parcel sizes set maximum limits, small measurement errors matter most near a threshold. The following are worth checking before committing to a box specification:

• Measuring the product instead of the posted package. Protective materials and the carton add both size and weight.
• Using internal dimensions for a carrier limit. Royal Mail classifies the finished exterior.
• Treating 750g as the only Large Letter limit. Online consumer Tracked 24 and 48 currently allow up to 1kg. Other Large Letter services can remain at 750g.
• Ignoring a bulging lid or closure. A nominally shallow box can exceed its intended external depth once packed.
• Assuming the box name guarantees compliance. “Large Letter” or “Small Parcel” in a product title describes intended use. It does not guarantee every finished shipment will qualify.
• Forgetting packaging weight. Royal Mail tells senders to include the box and additional packing materials when weighing a parcel.

A final check with the actual packed product is more dependable than designing from nominal dimensions alone.

Can custom box sizing help keep a parcel in a lower Royal Mail band?

Yes, when the product and the protection it needs already fit within that band’s limits. Treat Royal Mail parcel sizes as external constraints. Custom sizing can remove unnecessary space around a product and may keep the finished parcel below a dimensional threshold that an oversized stock carton would exceed.

It cannot make an inherently larger product qualify for a smaller band. Nor should reducing box size remove padding that the product needs for transit. A good specification works backwards from the product, required protection and target external limit.

This is particularly useful for repeat shipments near 25mm or 160mm depth. Rather than specifying a carton exactly at the Royal Mail maximum, allow practical room for packing and closure, then verify the finished sample.

Royal Mail parcel sizes: frequently asked questions

What are the Royal Mail Large Letter dimensions in 2026?

The maximum Large Letter dimensions are 353 × 250 × 25mm. Standard 1st Class and 2nd Class Large Letters have a 750g maximum. Royal Mail currently allows up to 1kg for consumer Tracked 24 and Tracked 48 Large Letters bought online. For Post Office purchases of those Tracked services, the limit is 750g.

What is the maximum Royal Mail Small Parcel size?

A Small Parcel can measure up to 450 × 350 × 160mm on the standard domestic services covered here. Its maximum weight is 2kg. Measure the closed parcel externally, with the longest side treated as length, and include all packaging materials when checking its weight.

What is the maximum Royal Mail Medium Parcel size?

The standard consumer Medium Parcel limit is 610 × 460 × 460mm and up to 20kg. Royal Mail directs heavier consumer parcels in the 20–30kg range to Parcelforce Worldwide. Business-account services can have different weight limits, so regular commercial shippers should verify the terms of their specific account product.

Is the 25mm Large Letter limit an internal or external measurement?

It is an external limit for the finished package. The product, box walls, inserts, folded closures and any bulging all sit within the final 25mm profile. If a supplier quotes a 25mm internal depth, do not assume the box will pass as a Large Letter once packed.

Does Royal Mail’s weight limit include the box?

Yes. Royal Mail tells senders to include the weight of the box and any additional packaging materials. Weigh the parcel in its final posted form rather than using the product’s listed weight on its own.

What happens if one dimension is over the Small Parcel limit?

The parcel no longer fits the Small Parcel dimensional band. If its finished dimensions and weight remain within the relevant limits, check it against the Medium Parcel format for a service that accepts that size. If it exceeds those limits too, check another suitable Royal Mail or Parcelforce service before posting.

Are international Royal Mail parcel sizes the same as UK sizes?

No. Royal Mail publishes separate size and weight guidance for domestic UK and international post. This guide covers UK domestic formats. Before sending abroad, check the current international rules for the destination and service. Do not apply the UK Large Letter, Small Parcel or Medium Parcel limits automatically.

Plan the box around the finished parcel

Royal Mail parcel sizes are easiest to use when the packaging decision starts with a fully packed sample. Measure its external length, width and depth, then weigh the whole shipment. Match those figures to the exact service and purchase channel you intend to use. That approach catches the 25mm Large Letter threshold, the 160mm Small Parcel depth limit and service-specific weight differences before they become fulfilment problems.

If you need a custom box built around a target Royal Mail format, send Custom Postal Boxes the packed product dimensions and intended postal service. Include the required quantity as well. Free design support can then work from the real postal constraint, with in-house manufacturing and UK delivery available for suitable orders.