If you sell clothing, accessories, or small goods on platforms like Vinted, eBay, Depop, or Etsy, postage fees directly impact your profit margins. Royal Mail’s Large Letter category offers one of the most cost-effective shipping options in the UK—provided your parcel stays within the strict size and weight thresholds.
Exceeding the depth or dimensions by just a few millimetres bumps your parcel into the Small Parcel tier, instantly doubling your postage cost per item.
In this guide, we break down the exact dimensions of the Royal Mail Large Letter format, explain which grey mailing bag sizes keep you compliant, and share practical packing tips to avoid expensive tier upgrades.
Royal Mail Large Letter Restrictions at a Glance
To qualify for Large Letter pricing, your packaged item must satisfy four specific criteria:
- Maximum Length: 353 mm (35.3 cm)
- Maximum Width: 250 mm (25.0 cm)
- Maximum Thickness/Depth: 25 mm (2.5 cm)
- Maximum Weight: 750 g
The 25mm Trap: The most common mistake online sellers make isn’t length or width—it’s depth. When a mailing bag is packed with a folded t-shirt or garment, the middle often bulges past 25mm. Royal Mail revenue protection uses a rigid wooden slot gauge; if your parcel cannot pass smoothly through the 25mm slot without forcing it, it will be billed as a Small Parcel.
Selecting the Right Grey Mailing Bag Size
To stay safely under the 353 x 250mm limit, choose mailing bags engineered specifically around A4/Large Letter dimensions.
1. Grey Mailing Bags | 9″ x 12″ (225x300mm) | Royal Mail Large Letter A4 Size
- Dimensions: 9 inches x 12 inches (225 x 300 mm)
- Best Used For: T-shirts, lightweight tops, thin knitwear, scarves, jewellery boxes, A4 documents, and flat accessories.
- Why it Works: The 225x300mm size sits comfortably inside the 250mm x 353mm outer limit, leaving sufficient length for the self-seal flap fold-over without extending past 353 mm.
2. 10″ x 14″ (254x355mm) Bags – Use with Caution
- Dimensions: 10 inches x 14 inches (254mm x 355mm)
- Best Used For: Items heading into the Small Parcel tier (e.g., hoodies, jeans, padded jackets).
- Why it Fails Large Letter: At 254mm wide, a flat 10×14 inch bag technically exceeds the 250mm Large Letter width limit before even inserting an item. Always reserve 10″ x 14″ and 12″ x 16″ bags for Small Parcel dispatches.
Size Quick-Reference Matrix
| Product / Garment Type | Recommended Bag Size | Royal Mail Category | Expected Postage Tier |
| Single T-Shirt / Summer Top | 9″ x 12″ (229x305mm) | Large Letter | Lowest Tier (<100g / <250g) |
| Thin Jeans / Leggings | 9″ x 12″ (229x305mm) | Large Letter | Mid Tier (<500g) |
| Thick Denim / Jumpers | 10″ x 14″ or 12″ x 16″ | Small Parcel | Small Parcel Tier (<1kg / <2kg) |
| Multiple Items / Bundles | 12″ x 16″ or 16″ x 21″ | Small Parcel | Small Parcel Tier |
3 Packing Hacks to Guarantee Large Letter Compliance
- Master the Flat Fold: Fold garments flat with wide, single folds rather than tight rolls. Rolling clothing creates a round cylinder shape that pushes the centre depth over the 25mm limit.
- Expel Trapped Air Before Sealing: Poly mailers trap air during folding. Flatten the bag with your hands to push out excess air before removing the peel-and-seal strip, ensuring the package stays slim.
- Invest in a 3D Printed or Wooden Size Template: Keep a standard 353 x 250 x 25mm slot template at your packing station. Test any borderline parcels through the slot before purchasing postage online.
