Choosing the right box for Royal Mail comes down to matching your packed item to one of four postal formats: Letter, Large Letter, Small Parcel and Medium Parcel. The cheapest format your item fits within, by every dimension and by weight, sets your postage price. Get a box that keeps you inside Large Letter or Small Parcel limits and you can cut your shipping costs significantly without changing your product.
If you sell online, packaging size is one of the easiest levers you have on margin. This guide explains the current Royal Mail size and weight limits, how the pricing tiers actually work, and how to pick a box that protects your product while keeping you in the lowest possible bracket. For custom sizes made to your exact specification, explore our custom printed boxes at Fast Printed Boxes.
Royal Mail parcel sizes at a glance
Royal Mail sorts every item into one of four formats. The rule is simple but strict: your item is charged at the smallest format that its largest dimension and its weight both fit inside. Exceed just one limit, on either size or weight, and the item jumps to the next tier up. You can confirm the latest figures on the Royal Mail parcel sending page.
- Letter: up to 24cm long, 16.5cm wide and 0.5cm thick, weighing no more than 100g.
- Large Letter: up to 35.3cm long, 25cm wide and 2.5cm thick, weighing no more than 750g.
- Small Parcel: up to 45cm long, 35cm wide and 16cm deep, weighing no more than 2kg.
- Medium Parcel: up to 61cm long, 46cm wide and 46cm deep, weighing no more than 20kg.
The thickness limit is where most sellers get caught out. A Large Letter must stay under 2.5cm thick, which sounds generous until you add a padded mailer and some void fill. If your packed item is even a few millimetres over, it is priced as a Small Parcel instead.
Why the format tiers matter for your postage
The gap between formats is real money. Posting as a Large Letter is typically far cheaper than a Small Parcel, and a Small Parcel is cheaper than a Medium Parcel. For a high-volume shop, shaving a centimetre off your box height to stay within Large Letter can save hundreds of pounds a year. It pays to design your packaging around the format boundary rather than reaching for the nearest box on the shelf.
How to choose the right box size
Start with the packed dimensions of your product, not the bare item. Measure it wrapped in whatever protection it needs, then add a small tolerance so the box closes cleanly. The goal is the snuggest box that still fits the format you are aiming for.
Match the box to the format you want to hit
If your product is flat and light, such as prints, jewellery, cosmetics or apparel accessories, a shallow box or a printed mailer box designed to stay under 2.5cm thick keeps you in Large Letter territory. For bulkier goods, aim for a Small Parcel box that uses the full 16cm depth only if you genuinely need it, because a taller box invites more void fill and more weight.
Do not forget the weight limit
Size and weight are separate tests and both must pass. A box can fit the Small Parcel dimensions comfortably yet tip over 2kg once packed, pushing it into Medium Parcel pricing. Heavier products benefit from stronger double wall board, but that board also adds weight, so balance protection against the bracket you are targeting.
Protect fragile items without oversizing
Fragile goods need cushioning, but oversized boxes filled with loose fill are both wasteful and expensive. Choose a box that leaves just enough room for a consistent layer of protection on every side. Right-sizing reduces damage in transit and keeps your parcel in the cheaper tier. The same logic applies to recurring shipments such as pet subscription boxes, where a repeatable, well-fitted box protects both your products and your margins every single month.
Cardboard grades and box strength
The board you choose affects both protection and postage weight. Single wall corrugated board suits most lightweight to mid-weight products and is the standard choice for mailer boxes and small parcels. Double wall board offers greater crush resistance for heavier or more fragile items, at the cost of extra weight and thickness. For very light Large Letter items, a lighter folding boxboard or thin corrugated grade helps you stay under both the thickness and weight caps.
Custom sizing is the real advantage here. A box built to your product means less wasted space, less void fill, lower weight and a tidier unboxing experience, all of which support your brand. Well-fitted branded packaging is also proven to lift the customer experience, as we covered in our guide to boosting your Etsy store with branded packaging.
A quick checklist before you order boxes
- Measure your product packed and protected, not bare.
- Decide which Royal Mail format you want to hit, then design within it.
- Check the thickness limit carefully for Large Letter items.
- Weigh a fully packed sample to confirm you are under the weight cap.
- Choose the lightest board grade that still protects your goods.
- Order a sample and test post it before committing to a bulk run.
Frequently asked questions
What size is a Royal Mail Small Parcel?
A Royal Mail Small Parcel can measure up to 45cm long, 35cm wide and 16cm deep, and weigh up to 2kg. Your item is only classed as a Small Parcel if it fits within all of those dimensions and the weight limit. Exceed any single measurement and it becomes a Medium Parcel.
What is the difference between a Large Letter and a Small Parcel?
A Large Letter must fit within 35.3cm by 25cm by 2.5cm and weigh no more than 750g. A Small Parcel is bigger and heavier, up to 45cm by 35cm by 16cm and 2kg. The thickness limit of 2.5cm is usually the deciding factor, so anything thicker than that is charged as a Small Parcel.
How do I stop my parcel being charged at a higher price?
Design your packaging to stay inside the format you are targeting on every dimension and on weight. Keep boxes as shallow and light as possible, avoid over-thick padding, and weigh a packed sample before you order in bulk. A custom-sized box is the most reliable way to hold a tight format boundary.
Does the box thickness really affect the price?
Yes. Thickness is one of the measured dimensions, and it is the limit most sellers breach. A Large Letter must stay under 2.5cm thick. If your packed item is even slightly over, Royal Mail prices it as a Small Parcel, which usually costs more.
Which cardboard grade should I choose for posting?
Single wall corrugated board suits most lightweight and mid-weight products and keeps parcels light. Choose double wall board for heavier or more fragile goods that need extra crush resistance. For Large Letter items where every gram counts, a lighter board helps you stay under both the thickness and weight limits.
Get the right box for every parcel
The quickest way to control your shipping costs is to post in the smallest format your product allows, and that starts with the right box. At Fast Printed Boxes we make custom printed boxes and mailer boxes in the exact sizes and board grades you need, so you can hit your target Royal Mail format, protect your goods and put your brand front and centre. Browse our range or get in touch for a bespoke size, and post smarter from your very next order.