By Isabella Jacobs January 01, 2026 4 min read

Royal Mail Parcel Guide 2026 – Mailing Bag Sizes, Weights & Posting Rules

Thu Jan 01 • By Isabella Jacobs • 4 min read

Royal Mail is the most commonly used delivery service for UK eCommerce sellers, especially for clothing, gifts and small parcels. In 2026, millions of parcels pass through Royal Mail’s automated sorting network every week, which means size, weight and packaging rules matter more than ever. If a parcel falls outside these limits, it can be delayed, surcharged or returned. This guide explains how to use mailing bags correctly with Royal Mail, covering Large Letter vs Small Parcel rules, posting clothes, label placement and the most common mistakes UK sellers need to avoid.

This guide explains how to use mailing bags correctly with Royal Mail. It covers Large Letter vs Small Parcel rules, weight limits, posting clothes, label placement and common mistakes that cost sellers money. It is written for UK eCommerce businesses that rely on mailing bags as their main dispatch packaging.

Royal Mail Parcel Sizes Explained: Large Letter vs Small Parcel

Understanding the difference between Large Letter and Small Parcel is essential. Many sellers overpay postage simply because their packaging tips over the size limit.

Grey mailing bags assorted pack showing Royal Mail size options — Different mailing bag sizes help you stay within Royal Mail Large Letter or Small Parcel limits.

Large Letter (typical limits):

Max thickness: 25mm
Lower postage cost
Best for thin clothing, documents and small items

Small Parcel (typical limits):

Up to 2kg for standard services
More flexible shape
Most clothing parcels fall into this category

Mailing bags are ideal for Small Parcels because they are flexible and conform to Royal Mail’s automated handling systems. Choosing the correct bag size helps you stay within limits and avoid unnecessary charges.

Best Mailing Bag Sizes for Royal Mail

Royal Mail does not require a specific mailing bag size, but certain sizes work better for common parcel types.

10×14 – T-shirts, thin garments, Large Letter attempts
12×16 – Hoodies, jeans, light jumpers
13×19 – Bulkier clothing, multiple items

Two of the most used sizes by UK sellers are Grey Mailing Bags 12×16 and Grey Mailing Bags 13×19. These sizes sit comfortably within Royal Mail Small Parcel rules when packed correctly.

Royal Mail Address Labels & Posting Rules

Incorrect label placement is one of the most common reasons parcels are delayed or misrouted. Royal Mail scanners require a flat, clear label with no folds.

Royal Mail address label placement on a mailing bag — Royal Mail address labels must be flat, clear and placed away from seams.

Best practice for Royal Mail labels:

Place on the flattest surface of the mailing bag
Avoid seams, folds and the seal area
Do not wrap labels around edges

Always check the latest Royal Mail guidance here: Royal Mail size and weight guide. This is the official reference used by Royal Mail staff.

How to Send Clothes by Royal Mail Using Mailing Bags

Clothing is the most common item sent via Royal Mail using mailing bags. The key is to keep the parcel flat, balanced and securely sealed.

Sending clothes by Royal Mail using mailing bags — Mailing bags are ideal for sending clothing and soft goods with Royal Mail.

Best practice for posting clothes:

Fold garments flat to reduce bulk
Remove excess air before sealing
Keep heavy items away from the seal area
Size up if the bag feels tight

For multiple clothing items or thicker garments, always choose a larger bag rather than forcing a tight fit.

For step-by-step packing advice, see our guide on how to pack parcels properly using mailing bags.

Mailing Bag Strength & Tear Resistance for Royal Mail

Royal Mail parcels go through automated sorting, conveyors and manual handling. This means your mailing bag must resist tearing, stretching and corner pressure.

Tear resistant mailing bag for Royal Mail parcels — Strong polymer mailing bags help prevent splitting during Royal Mail handling.

Quality mailing bags use strong polymer film with a secure Peel & Seal closure. Most Royal Mail issues occur not because the bag is weak, but because it is overfilled or poorly sealed.

When Royal Mail Requires a Box Instead of a Mailing Bag

Mailing bags are suitable for soft goods only. Royal Mail may reject or surcharge parcels that are poorly protected.

Use cardboard boxes when posting:

Fragile items
Sharp-edged products
Electronics
Rigid goods that can be crushed

Some sellers place boxed items inside mailing bags for privacy and weather protection, which is acceptable when sized correctly.

Privacy, Returns & Secure Posting with Royal Mail

Privacy is important when sending clothing and personal items. Opaque mailing bags protect contents from view and reduce theft risk.

Opaque mailing bag for privacy protection with Royal Mail — Opaque mailing bags protect customer privacy during Royal Mail delivery.

Mailing bags are also popular for returns because they are lightweight and easy for customers to reseal or reuse.

Common Royal Mail Posting Mistakes to Avoid

Overfilling mailing bags
Incorrect label placement
Trying to force items into Large Letter size
Using mailing bags for fragile items
Ignoring weight limits

A consistent packing process prevents most issues and keeps postage costs predictable.

Trade & Business Posting with Royal Mail

For trade and wholesale sellers, Royal Mail posting needs to be fast and repeatable. Most businesses standardise around two mailing bag sizes and use Tracked services for better visibility.

If you ship in volume, building your dispatch setup around the Mailing Bags collection ensures consistency and cost control.

Useful Downloads

Our Trust & Service Guarantee

We provide fast UK delivery, same-day dispatch on qualifying orders, and reliable packaging trusted by thousands of businesses. Our customers rate us 5★ on Trustpilot for quality and service. Bulk and trade support is always available.

Final Summary: Using Mailing Bags with Royal Mail in 2026

Royal Mail remains a reliable choice for UK sellers when parcels are packed correctly. By choosing the right mailing bag size, sealing properly and following Royal Mail rules, you can avoid surcharges and delays. For a dependable dispatch setup, explore the full Mailing Bags collection and build your packaging around proven sizes and quality materials.

FAQs

Can I send clothes in a mailing bag with Royal Mail?

Yes. Mailing bags are ideal for clothing because they are lightweight, water-resistant and flexible. Fold garments flat, remove excess air and choose a bag size that is not tight so the seal closes cleanly.

Is a mailing bag classed as a Royal Mail Small Parcel?

A mailing bag can be sent as a Large Letter or Small Parcel depending on the final packed size and thickness. Most clothing orders usually fall under Small Parcel once packed.

What is the difference between Royal Mail Large Letter and Small Parcel?

Large Letter has stricter thickness limits and is cheaper, while Small Parcel allows more depth and suits most clothing parcels. Your price changes if the parcel goes over the Large Letter size rules.

What mailing bag size is best for Royal Mail clothing parcels?

For most UK clothing orders, 12×16 works well for hoodies and jeans, while 13×19 is better for bulkier clothing or multi-item orders. If the bag feels tight near the seal, size up.

Where should I put the address label on a mailing bag for Royal Mail?

Place the label on the flattest front surface, away from seams and the seal area. Keep it flat with no wrinkles so Royal Mail scanners can read it clearly.

Can Royal Mail reject a parcel if the mailing bag is overfilled?

Yes, overfilled parcels can cause sealing issues and damage during handling. This may lead to delays or extra charges. The best fix is to use a larger mailing bag so the seal area is not under pressure.

Do I need tape if my mailing bag has Peel and Seal?

Usually no. Peel and Seal is designed to be secure. Tape is only useful as extra reassurance on heavier parcels, but it should never replace correct sizing and proper sealing.

When should I use a box instead of a mailing bag for Royal Mail?

Use a box for fragile, rigid or sharp-edged items, and anything that could be crushed. Mailing bags are best for soft goods like clothing and textiles.

How do I avoid Royal Mail surcharges with mailing bags?

Avoid forcing parcels into Large Letter size, use the correct postage band, keep labels clear and flat, and stay within the final packed size and weight limits. Most surcharges come from size or weight being outside the paid service.

Are opaque mailing bags better for Royal Mail deliveries?

Opaque mailing bags help protect customer privacy and reduce the chance of contents being seen during delivery. This is especially useful for clothing, personal items and returns.

Can customers reuse mailing bags for Royal Mail returns?

Yes. Many customers reuse mailing bags for returns if the bag opens cleanly and the item fits back inside. For easier returns, choose a bag with enough spare space and include clear return instructions.

Article By

Isabella Jacobs

Isabella is a packaging specialist with over a decade of experience helping UK retailers find sustainable solutions for their business needs.

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