No ruler to hand? An A4 sheet of paper is 297 × 210 mm, so you can use one as a rough guide.
Need to resize your shipping label? Once you know your format, use the Label Resizer to get your label print-ready.
Royal Mail prices post by format, not by exact dimensions. There are four formats: Letter, Large Letter, Small Parcel and Medium Parcel. Your item is whichever format its largest dimension and its weight both still fit inside. Getting this right matters, because the jump from Large Letter to Small Parcel can roughly double your postage for an item that is only a few millimetres too thick.
| Format | Max length | Max width | Max thickness / depth | Max weight |
|---|---|---|---|---|
| Letter | 240 mm | 165 mm | 5 mm | 100 g |
| Large Letter | 353 mm | 250 mm | 25 mm | 750 g |
| Small Parcel | 450 mm | 350 mm | 160 mm | 2 kg |
| Medium Parcel | 610 mm | 460 mm | 460 mm | 20 kg |
An item only needs to exceed one limit to move up a format. A 4 mm thick item that is 250 mm long is still a Letter. The same item at 6 mm thick becomes a Large Letter, even though nothing else changed.
Source: these limits follow Royal Mail's official format and size specifications, published at royalmail.com. Verified May 2026.
A Large Letter is anything up to 353 mm long, 250 mm wide and 25 mm thick, weighing no more than 750 g. That covers most A4 documents, paperback books and slim padded mailers, as long as they stay under 25 mm thick. The thickness limit is the one people miss most often: a well-stuffed A4 envelope frequently tips over 25 mm and becomes a Small Parcel, which costs noticeably more. If you are posting through a letterbox, our letterbox size checker shows exactly what fits.
A Royal Mail Medium Parcel can be up to 610 mm long, 460 mm wide and 460 mm deep, up to a maximum weight of 20 kg. Anything larger than that falls outside the standard Royal Mail formats and needs Parcelforce or another courier. The tier below it, Small Parcel, is capped at 450 mm by 350 mm by 160 mm and 2 kg, so there is a big gap between the two and it is worth checking which one your box actually falls into.
Royal Mail charges by format first and then by weight band within that format, with 1st Class costing more than 2nd Class. Prices usually change each April, so rather than print a table that goes out of date, the checker above gives an indicative 1st and 2nd Class price for your exact item. For the live rate before you post, check royalmail.com. As a rule of thumb, a Letter is the cheapest option, a Large Letter is a small step up from that, and the jump to Small Parcel is the single biggest increase, which is exactly why the Large Letter trap below is worth knowing.
Many sellers overpay by sending items as Small Parcels when they'd fit in a Large Letter. If your item fits the Large Letter dimensions, use that instead, you'll save significantly on postage costs and often get faster delivery.
You'll need Royal Mail Special Delivery or a specialist courier like DPD or FedEx. Special Delivery is tracked and guaranteed next-working-day.
Yes, if it's up to 25mm thick. Royal Mail will attempt letterbox delivery for items in Large Letter format up to that thickness.
1st Class is faster (usually next working day) and more expensive. 2nd Class is slower (2–3 working days) but significantly cheaper. Choose 2nd Class for non-urgent items to reduce postage costs.
Standard Royal Mail compensation is limited. For valuable items, consider buying additional insurance or using a tracked service like Special Delivery Guaranteed.
A Royal Mail Medium Parcel can be up to 610mm long, 460mm wide and 460mm deep, with a maximum weight of 20kg. Anything larger falls outside the standard Royal Mail formats and needs Parcelforce or another courier.
A Large Letter is up to 353mm long, 250mm wide and 25mm thick, weighing no more than 750g. The 25mm thickness limit is the one most items fail, which pushes them up into Small Parcel pricing.
A Small Parcel can be up to 450mm long, 350mm wide and 160mm deep, weighing up to 2kg. If your item is larger or heavier than this it becomes a Medium Parcel.
This tool uses the official Royal Mail format specifications. However, Royal Mail may occasionally update requirements or apply edge-case rules, so always verify with royalmail.com before sending high-value items.