The maximum size for a Small Parcel with Royal Mail is 450 x 350 x 160mm, which is 45 x 35 x 16cm or roughly 17.7 x 13.7 x 6". Your packed parcel has to sit within all three of those dimensions at once, measured on the outside, and it also has to be under the Small Parcel weight limit, which you should check with Royal Mail because weight limits and prices change.
This guide lists every single wall and crash lock box we stock that fits within the Small Parcel dimensions, and the ones that look close but do not. It also covers how mailing bags are measured, how to measure a packed parcel at home, and when an item is better sent as a Large Letter or has to go as a Medium Parcel. The last section covers how many boxes to buy and what each one costs.
In This Guide
- Quick picks for Royal Mail Small Parcel boxes and bags
- What is the size for a Small Parcel at Royal Mail?
- Which small parcel boxes fit within 450 x 350 x 160mm?
- Which boxes look close but are too big for a Small Parcel?
- Do mailing bags count as a Small Parcel, and which sizes fit?
- How do you measure a packed parcel against the Small Parcel dimensions?
- When should an item go as a Large Letter or a Medium Parcel instead?
- How many small parcel boxes should you buy, and what do they cost?
- Frequently Asked Questions
Quick picks for Royal Mail Small Parcel boxes and bags
If you only want to know what to order, these three cover boxed items, quick-build postal boxes and soft items.
Best for the biggest Small Parcel
Single Wall Cardboard Boxes
Single wall corrugated boxes, including one size made to the Small Parcel maximum.
- 17.7 x 13.7 x 6" (450 x 350 x 160mm) Small Parcel max size
- Ten sizes within 450 x 350 x 160mm
- Sold per box, minimum 5 per size
From £0.35 ex VAT (£0.42 inc VAT)
Choose sizeView product
Best for fast packing
Crash Lock Boxes
Postal boxes with a base that locks as you open them, so you only tape the top.
- CL-A (100 x 100 x 70mm) to CL-J (426 x 300 x 150mm) fit
- No tape on the base
- Sold per box, minimum 5 per size
From £0.59 ex VAT (£0.71 inc VAT)
Choose sizeView product
Best for clothes and soft items
Grey Mailing Bags
Opaque self-seal polythene bags that add very little weight to a Small Parcel.
- 16 sizes from 4 x 6" (100 x 150mm)
- Packs of 100 up to 17 x 24"
- Self-seal strip, no tape needed
From £2.90 ex VAT (£3.48 inc VAT)
Choose sizeView productWhat is the size for a Small Parcel at Royal Mail?
A Royal Mail Small Parcel can be up to 450mm long, 350mm wide and 160mm deep. In centimetres that is 45 x 35 x 16cm, and in inches it is about 17.7 x 13.7 x 6". Anything bigger than that in any one direction is a Medium Parcel, however light it is.
The three limits work together. You line up the longest side of the parcel with the 450mm limit, the middle side with 350mm and the shortest side with 160mm. A parcel that is short and narrow does not earn any extra depth, so a 200 x 200 x 200mm cube is a Medium Parcel even though it looks small on the bench. The 160mm depth is the limit that catches most people, because plenty of ordinary boxes are 6" cubes or deeper.
| Dimension | Small Parcel maximum | In cm | In inches (approx.) |
|---|---|---|---|
| Length (longest side) | 450mm | 45cm | 17.7" |
| Width (middle side) | 350mm | 35cm | 13.7" |
| Depth (shortest side) | 160mm | 16cm | 6" |
Royal Mail measures the outside of the finished parcel. That includes the cardboard, the tape, any bulge in the lid and the fold of a mailing bag. It does not matter what size the item inside is, or what size is printed on the box.
Weight and price. A Small Parcel also has a weight limit, and the price depends on the weight and the service you choose. We do not quote either figure here because they change. Check Royal Mail for the current weight limit and prices before you quote postage to a customer.
This article covers the Small Parcel only. For the other Royal Mail formats and which packaging suits each one, see our guide to Royal Mail packaging and size bands.
Which small parcel boxes fit within 450 x 350 x 160mm?
Ten sizes of our single wall cardboard boxes and nine sizes of our crash lock boxes have listed dimensions within 450 x 350 x 160mm. The table lists them. We checked each one by sorting its three dimensions from longest to shortest and comparing them with the three limits.
| Box | Listed size | Longest side | Shortest side |
|---|---|---|---|
| Single wall 5 x 5 x 5" | 127 x 127 x 127mm | 127mm | 127mm |
| Single wall 6 x 6 x 6" | 152 x 152 x 152mm | 152mm | 152mm |
| Single wall 7 x 5 x 5" | 178 x 127 x 127mm | 178mm | 127mm |
| Single wall 8 x 6 x 4" | 203 x 152 x 102mm | 203mm | 102mm |
| Single wall 8 x 6 x 5" | 203 x 152 x 127mm | 203mm | 127mm |
| Single wall 9 x 6 x 6" | 229 x 152 x 152mm | 229mm | 152mm |
| Single wall 10 x 8 x 6" | 254 x 203 x 152mm | 254mm | 152mm |
| Single wall 12 x 9 x 4" | 305 x 229 x 102mm | 305mm | 102mm |
| Single wall 12 x 9 x 6" | 305 x 229 x 152mm | 305mm | 152mm |
| Single wall 17.7 x 13.7 x 6" | 450 x 350 x 160mm | 450mm | 160mm |
| Crash lock CL-A | 100 x 100 x 70mm | 100mm | 70mm |
| Crash lock CL-B | 160 x 150 x 75mm | 160mm | 75mm |
| Crash lock CL-C | 180 x 100 x 50mm | 180mm | 50mm |
| Crash lock CL-D | 220 x 111 x 78mm | 220mm | 78mm |
| Crash lock CL-E | 252 x 136 x 88mm | 252mm | 88mm |
| Crash lock CL-F | 290 x 208 x 95mm | 290mm | 95mm |
| Crash lock CL-G | 320 x 200 x 130mm | 320mm | 130mm |
| Crash lock CL-H | 375 x 255 x 150mm | 375mm | 150mm |
| Crash lock CL-J | 426 x 300 x 150mm | 426mm | 150mm |
The last single wall row, the 17.7 x 13.7 x 6" (450 x 350 x 160mm) box, is the one made for this job. Our product listing describes it as the largest box Royal Mail accepts as a Small Parcel. Order that size if you want the most space you can get for Small Parcel postage.
Best for filling the Small Parcel size
Single Wall Cardboard Boxes
Order the 450 x 350 x 160mm size when you need every millimetre the Small Parcel allows.
- 17.7 x 13.7 x 6" (450 x 350 x 160mm) Royal Mail Small Parcel Max
- Single wall corrugated board, supplied flat
- Price per box falls as the quantity rises
From £0.35 ex VAT (£0.42 inc VAT)
Choose sizeView productListed sizes are internal. Our single wall box sizes are internal, length x width x height. The cardboard and tape add a little to the outside, so a box with a 152mm internal height sits close to the 160mm limit once it is sealed. Build one, tape it, and measure the outside before you rely on it.
The same care applies to crash lock sizes CL-H and CL-J, which are listed at 150mm high. They fit, but not with much to spare, so close the lid flat and do not let the contents push it up. Crash lock boxes cost from £0.59 ex VAT (£0.71 inc VAT) per box and single wall boxes from £0.35 ex VAT (£0.42 inc VAT) per box, both with a minimum of 5 per size.
If the item is heavy or fragile, two sizes of our double wall cardboard boxes have listed dimensions inside the Small Parcel size: 9 x 6 x 6" (229 x 152 x 152mm) and 12 x 9 x 6" (305 x 229 x 152mm). Double wall board has two layers of fluting, so it is thicker than single wall and the outside height needs checking with a tape measure before you post. Double wall boxes start from £0.59 ex VAT (£0.71 inc VAT) per box.
Which boxes look close but are too big for a Small Parcel?
Five of our single wall sizes and one crash lock size are over the Small Parcel size on at least one dimension, and three of them are near misses that catch people out. In every case the fault is a single measurement, and it is usually the depth.
| Box | Listed size | Why it is over |
|---|---|---|
| Single wall 12 x 9 x 9" | 305 x 229 x 229mm | Shortest side is 229mm, over the 160mm depth |
| Single wall 12 x 12 x 12" | 305 x 305 x 305mm | Shortest side is 305mm, over the 160mm depth |
| Single wall 15.35 x 11.6 x 7.3" Extra Strong | 390 x 295 x 185mm | Shortest side is 185mm, over the 160mm depth |
| Single wall 18 x 10 x 5.5" | 457 x 252 x 143mm | Longest side is 457mm, over the 450mm length |
| Single wall 24 x 18 x 18" | 610 x 457 x 457mm | Over on all three dimensions |
| Crash lock CL-K | 426 x 300 x 300mm | Shortest side is 300mm, over the 160mm depth |
The 18 x 10 x 5.5" box is the one to watch. It is slim and well inside the width and depth, but at 457mm its length is 7mm over the limit before you add the board. The 390 x 295 x 185mm Extra Strong box fails in a similar way: the footprint is fine and the height is 25mm too much. Both are useful boxes, but they are Medium Parcel boxes, so price the postage that way.
Do not cut a box down or crush it to get under a limit unless you are confident doing it. A box with a scored and folded-down top can work, but a squashed box with a loose lid protects the item badly and can still measure over. Picking a size from the first table is easier. Our guide on what size box you need shows how to work from the item outwards if none of these look right.
Do mailing bags count as a Small Parcel, and which sizes fit?
Yes, a mailing bag can go as a Small Parcel as long as the packed bag measures within 450 x 350 x 160mm. With a bag, the size on the pack is the flat, empty bag. What gets measured is the bag after you have filled it, sealed it and folded any spare material over.
Our grey mailing bags are polythene bags with a self-seal strip, sized width x length. Five sizes are within the 450 x 350mm footprint even when they are lying flat and empty, so the only thing left to check is how thick the contents make them.
| Grey mailing bag | Flat size | Against the 450 x 350mm footprint | Typical contents |
|---|---|---|---|
| 4 x 6" | 100 x 150mm | Inside when flat | Small accessories |
| 6 x 9" | 165 x 230mm | Inside when flat | A5 items |
| 9 x 12" | 230 x 300mm | Inside when flat | A4 documents, thin tops |
| 10 x 14" | 250 x 350mm | Inside when flat | Folded T-shirts, paperbacks |
| 12 x 16" | 305 x 405mm | Inside when flat | Hoodies, jumpers, jeans |
| 13 x 19" | 330 x 485mm | Length is over when flat. Fold the spare end over and measure | Knitwear, dresses |
| 14 x 21" | 350 x 525mm | Length is over when flat. Fold the spare end over and measure | Two or three items together |
| 15 x 18" and larger | 381 x 457mm upwards | Over on width and length when flat. Measure the packed bag | Coats, bedding, bundles |
A filled bag is narrower and shorter than its flat size, because some of the material goes round the sides of the contents. That is why a hoodie in a 13 x 19" bag can still come in under 450mm long once the seal end is folded down and stuck. It also means the pack size alone will not tell you whether a filled bag passes. Bulky soft items such as coats and duvets are more likely to fail on the 160mm depth than on the footprint, and pressing the air out before you seal the bag helps.
Grey mailing bags cost from £2.90 ex VAT (£3.48 inc VAT) per pack. Sizes up to 17 x 24" come in packs of 100, and the outer is water resistant, so they suit clothing and other soft, non-fragile items. For a size-by-size breakdown of what fits in each bag, use our grey mailing bags size chart. We also stock white and coloured bags in the bags collection.
Bag or box? If the item cannot break and cannot crush, a bag is usually the cheaper choice and it adds less weight. Use a box when the item has corners that would poke through, or when it could be damaged by another parcel landing on it.
How do you measure a packed parcel against the Small Parcel dimensions?
Measure the parcel after it is packed and sealed, on the outside, at its longest, widest and deepest points. A tape measure and a flat table are all you need. These steps take under a minute and they are worth doing once for every box size you use.
- Pack and seal the parcel fully. Tape the box or close the bag exactly as you will send it. An open box measures smaller than a sealed one.
- Put it on a flat surface. Stand it on its largest face so the shortest side points up. That shortest side is the depth.
- Measure the length. Take the longest side at its longest point, including any tape tails or bag folds. It must be 450mm or less.
- Measure the width. Take the side at right angles to the length. It must be 350mm or less.
- Measure the depth at the highest point. Lay a ruler or a book flat across the top and measure from the table up to it. This catches a domed lid or a lump in a bag. It must be 160mm or less.
- Weigh it. Use kitchen or postal scales, then compare the weight with the current Small Parcel weight limit on the Royal Mail website.
The depth is the measurement that goes wrong, because box flaps that do not meet, void fill piled above the rim and bubble wrap under the lid all lift the top of the box by a few millimetres. If your box is one of the 152mm or 150mm high sizes, pack so the flaps close flat without pressure, and run the tape along the centre seam without building up layers. One strip of brown packing tape along the seam and one across each end is enough for most small boxes. The standard 48mm x 66m rolls come in packs of 6 from £4.50 ex VAT (£5.40 inc VAT).
Inside the box, wrap the item and fill the gaps at the sides, not the top. Small bubble wrap with 10mm bubbles comes on 100m rolls in 300mm or 500mm widths, from £9.00 ex VAT (£10.80 inc VAT) per roll, and kraft paper scrunches into the corners. Our step-by-step guide on how to pack a box for shipping covers the packing itself in more detail.
Measure once, write it down. When you have measured a sealed box of each size you use, note the outside dimensions on the shelf where you keep them. After that you only need to weigh each parcel.
When should an item go as a Large Letter or a Medium Parcel instead?
Send an item as a Large Letter when it packs down to 25mm thick or less, and as a Medium Parcel when any side of the packed parcel is over 450 x 350 x 160mm. The Small Parcel covers everything in between. A Large Letter generally costs less to post than a Small Parcel, so it is worth checking the thin items in your range first.
Our Royal Mail PIP large letter boxes are die cut cardboard boxes 20mm deep, made to sit inside the 25mm Large Letter thickness limit. They come in five sizes, in packs of 50:
- Mini (101 x 101 x 20mm): rings, earrings, badges and small cosmetics.
- C6 (163 x 112 x 20mm): greeting cards, phone accessories and A6 prints.
- DL (218 x 108 x 20mm): pens, straps and other long, thin items.
- C5 (229 x 159 x 20mm): an A5 item lies flat, so slim books and DVDs fit.
- C4 (325 x 225 x 20mm): an A4 item lies flat, for prints, documents and thin clothing.
Best for flat items under 20mm
Royal Mail PIP Large Letter Boxes
If the item and its wrapping are no more than 20mm thick, this is the box to try before a Small Parcel box.
- Mini, C6, DL, C5 and C4 sizes, all 20mm deep
- Packs of 50, supplied flat
- Die cut, folds together by hand
From £5.00 ex VAT (£6.00 inc VAT)
Choose sizeView productA Large Letter has length, width and weight limits as well as the thickness limit, so check Royal Mail for the current figures before you move a product over. The item also has to fit without forcing the lid. Our large letter boxes guide explains what fits in each PIP size and how to stay inside the limits.
Going the other way, an item needs a Medium Parcel when you cannot get it under one of the three Small Parcel limits without risking damage. Common examples are shoe boxes for adult sizes, anything packed in a cube bigger than 6", and long items over 450mm. Do not strip out protection to save on postage. A breakage costs you the item, the postage and a refund. For Medium Parcel sizes, look at the larger single wall and double wall boxes in our range. Our postal boxes guide sets out which box types suit each Royal Mail band.
How many small parcel boxes should you buy, and what do they cost?
Buy two or three sizes that cover most of your orders, starting with the minimum of 5 boxes per size, and reorder in larger quantities once you know which sizes you use. Single wall and crash lock boxes are sold per box, and the price per box falls as the quantity rises. The tiers are shown on each product page.
| Product | Sold as | Price from |
|---|---|---|
| Single wall cardboard boxes | Per box, minimum 5 per size | £0.35 ex VAT (£0.42 inc VAT) |
| Crash lock boxes | Per box, minimum 5 per size | £0.59 ex VAT (£0.71 inc VAT) |
| Royal Mail PIP large letter boxes | Pack of 50 | £5.00 ex VAT (£6.00 inc VAT) |
| Grey mailing bags | Packs of 100, 50 or 25 depending on size | £2.90 ex VAT (£3.48 inc VAT) |
The "from" prices are for the smallest size in each range, and larger sizes cost more. A crash lock box costs more than a single wall box of similar size. The extra pays for a base that locks without tape, which saves time on every parcel. If you pack a handful of orders a week, single wall boxes and a roll of tape will do the job for less. If you pack dozens a day, the time saved on crash lock boxes adds up.
Royal Mail Tracked 48 delivery is free on every order with no minimum, so a first order of 5 boxes costs nothing extra to deliver. Orders placed by 2pm Monday to Friday leave our Manchester unit the same working day. If you are local, click and collect from Cheetham Hill is free and usually ready within 2 hours, and we run same-day van delivery within 10 miles.
Regular buyers can apply for a trade account, and every order comes with a VAT invoice. Our guide to buying boxes in bulk covers quantities and storage. You will also need tape, which is in the tapes collection. If you are not sure which size suits a product, call 0161 706 1246 or use the contact page and tell us the item's dimensions.
Frequently Asked Questions
What is the maximum size for a Royal Mail Small Parcel?
The maximum size for a Royal Mail Small Parcel is 450 x 350 x 160mm, which is 45 x 35 x 16cm. The packed parcel has to be within all three dimensions, measured on the outside. A Small Parcel also has a weight limit, so check Royal Mail for the current weight limit and prices.
What is the biggest box I can send as a Royal Mail Small Parcel?
The biggest box you can send as a Royal Mail Small Parcel is one that measures 450 x 350 x 160mm. Pack Solution sells a single wall cardboard box in that size, listed as 17.7 x 13.7 x 6" (450 x 350 x 160mm) Royal Mail Small Parcel Max. Close the flaps flat when you pack the box so the lid does not bulge above 160mm.
Does a 6 x 6 x 6 inch box count as a Small Parcel?
A 6 x 6 x 6" box has a listed size of 152 x 152 x 152mm, which is inside the 160mm Small Parcel depth. The listed size of our single wall boxes is internal, so the outside of a sealed 6 x 6 x 6" box is close to the limit. Build and tape one box, then measure the outside before you rely on that size for Small Parcel postage.
Can I send a mailing bag as a Royal Mail Small Parcel?
A mailing bag can go as a Royal Mail Small Parcel if the packed bag is within 450 x 350 x 160mm. Royal Mail measures the filled and sealed bag, not the flat size printed on the pack. Grey mailing bags up to 12 x 16" (305 x 405mm) are within the 450 x 350mm footprint even when flat, and larger bags need the spare end folded over and measured.
What happens if my parcel is over the Small Parcel size?
A parcel that is over 450 x 350 x 160mm on any one side is a Medium Parcel and should be paid for as one. Check Royal Mail for how underpaid items are currently handled. Measuring one sealed box of each size you use avoids the problem.
When is a Large Letter box better than a Small Parcel box?
A Large Letter box is the better choice when the item and its wrapping are no more than 20mm thick and fit flat. Pack Solution PIP large letter boxes are 20mm deep and come in Mini, C6, DL, C5 and C4 sizes in packs of 50. Check Royal Mail for the current Large Letter length, width and weight limits before you switch a product over.
