A US distributor quote for a "#8 SOS, 50 lb natural kraft" packs three separate pieces of information into a short line: a bag construction, a size label and a paper weight. European specifications describe the same bag as a square-bottom bag with a width, gusset and height in millimetres and a grade in gsm. If you buy for a chain operating on both sides of the Atlantic, or you are moving from distributor stock to custom printed takeaway bags, it helps to know how SOS bags are built and what those numbers do and do not tell you.
What is an SOS paper bag?
An SOS bag is a paper bag with a flat, rectangular bottom and folded side gussets that opens into a free-standing shape with a single flick of the wrist. SOS stands for "self-opening sack", and is also commonly read as "self-opening square". The name dates back to machine-made square-bottom bags of the late nineteenth century and is still the standard US term.
The defining feature is the bottom. Instead of a simple glued seam, as on a flat bag, the bottom of an SOS bag is folded and glued into a rectangle that matches the width and gusset depth of the bag. When the bag is opened, that rectangle snaps flat, the gussets push out, and the bag stands on the counter ready to fill. That is why SOS bags dominate counters, drive-thru windows and grocery checkouts: staff can open and fill them with one hand.
In German-speaking markets the same construction is usually called a Bodenbeutel or SOS-Beutel.
How is an SOS bag made?
SOS bags are made on tube-forming machines that run from a reel of kraft paper. The paper is printed, formed into a gusseted tube, cut to length and then folded and glued into a square bottom, all in one continuous line.
The sequence on a typical machine:
- Printing: the reel passes through flexographic print units, usually inline, so the artwork is printed before the bag is formed.
- Tube forming: the web is folded around a former, the side gussets are tucked in and a longitudinal seam is glued along the back panel.
- Cutting: the tube is cut to bag length. The top edge is often cut with a serrated (zigzag) blade, which gives the familiar grocery-bag top and makes the bag easier to open; straight cuts are also used.
- Bottom forming: the bottom end is opened out, folded into the rectangular base and glued.
- Handles (optional): flat paper handles with a reinforcing patch are applied inside the bag mouth, either inline or on a separate handle unit.
Because bag length is set by the cut, and width and gusset by the former, each size is effectively a separate machine set-up. Print plates also have to match the bag's print repeat. This is one reason converters and buyers like to keep a short list of standard sizes rather than a different bag for every product.
How do SOS bags compare with other paper bag constructions?
SOS bags sit between simple flat bags and heavier handled carrier bags. They stand up and hold boxes, but they are lighter and quicker to fill than a retail carrier bag.
| Construction | Bottom | Stands when open | Typical use | Notes |
|---|---|---|---|---|
| Flat bag | Single glued or folded seam | No | Bakery items, sandwiches, small portions | Lowest paper use; see custom printed paper food bags |
| Side-gusset bag | Folded seam with side gussets | No | Bread, fries, snacks | Expands, but bottom is not square |
| Pinch-bottom bag | Folded and sealed seam | No | Retail goods, heat-sealed packs | Often used where the bag is closed at both ends |
| SOS bag | Folded, glued rectangular bottom | Yes | Takeaway, grocery, pharmacy, retail counters | Fast to open and fill; handles optional |
| Block-bottom carrier bag | Rectangular bottom, often reinforced | Yes | Retail merchandise, premium takeaway | Heavier paper and handles; see custom printed paper carrier bags |
The line between an SOS bag with handles and a block-bottom carrier bag is blurred in everyday use. In practice, takeaway SOS bags are lighter and simpler, built for speed and food orders, while carrier bags are built for heavier merchandise and a more premium finish.
SOS bag sizes by number (#4 to #25)
In the US, SOS bags are sold by number, such as #4, #6, #8, #12, #20 or #25, with larger numbers indicating larger bags. This is what most people mean by "paper bag sizes by number". The number is a trade size label, not a measurement and not a legal standard, so actual dimensions vary between converters and should always be checked on the specification sheet.
The numbering is usually explained as a historical reference to how much a bag could hold by weight of a commodity, and the pattern still holds loosely: a higher number means more volume. Today, though, a #8 bag is defined by the converter's dimensions, not by a load in pounds, and two suppliers' #8 bags can differ noticeably in width, gusset or height.
US dimensions are normally written as width x gusset x height in inches, with the width being the face of the bag, the gusset its depth front to back, and the height measured from the bottom fold to the top edge.
The table below gives commonly cited catalogue dimensions for popular sizes. Treat them as a reference for comparing quotes, not as a specification.
| Size | Approx. width x gusset x height (in) | Approx. metric (mm) | Typical foodservice use |
|---|---|---|---|
| #4 | 5 x 3 1/8 x 9 3/4 | 125 x 80 x 250 | Cookies, pastries, small sides |
| #6 | 6 x 3 5/8 x 11 | 150 x 90 x 280 | Single sandwich, fries, small sides |
| #8 | 6 1/8 x 4 1/8 x 12 3/8 | 155 x 105 x 315 | Single meal, sandwich plus side |
| #10 | 6 3/8 x 4 1/8 x 13 3/8 | 160 x 105 x 340 | Single meal with a taller box |
| #12 | 7 x 4 1/2 x 13 3/4 | 180 x 115 x 350 | Meal for one to two, several items |
| #20 | 8 1/4 x 5 1/4 x 16 1/8 | 210 x 135 x 410 | Larger orders, family meals |
| #25 | varies by converter; check the spec sheet | - | Large orders, grocery-style use |
| 1/6 BBL | 12 x 7 x 17 | 305 x 180 x 430 | Grocery-size bag, large catering orders |
Two practical points follow from this. First, when moving between suppliers, compare dimensions rather than numbers, especially the gusset, because the gusset decides whether a given box fits flat on the bottom. Second, if you are having bags custom made, you are not tied to the numbered sizes at all: dimensions can be chosen around your boxes, within the limits of the converter's machines.
"1/6 BBL" is a grocery term, read as "one-sixth barrel", from the era when a standard grocery bag was described as holding a sixth of a barrel. It is a size label in the same way as the numbers.
Choosing a size for your menu
The table tells you roughly how big each numbered bag is, not which one your orders need. The right size comes from the footprint of your boxes, drinks carriers and typical order baskets. That method is set out in the takeaway bag sizes guide.
Basis weight and gsm: how heavy is an SOS bag?
US SOS bags are specified by basis weight, the weight in pounds of a 3,000 square foot ream of the paper. European suppliers specify grammage in grams per square metre (gsm). One pound of basis weight equals about 1.63 gsm.
| US basis weight | Approx. gsm | Where it is commonly used |
|---|---|---|
| 30 lb | about 49 gsm | Light lunch bags, small items |
| 35 lb | about 57 gsm | Light-duty small SOS bags |
| 40 lb | about 65 gsm | Standard small and mid-size SOS bags |
| 50 lb | about 81 gsm | Heavier duty mid-size bags, takeaway meals |
| 57 lb | about 93 gsm | Grocery and heavier SOS bags |
| 60 lb | about 98 gsm | Large SOS and handled bags |
| 70 lb | about 114 gsm | Heavy grocery and large delivery bags |
The heavier the order and the longer it spends in a bag, the more the paper weight matters. Hot, steaming food is a particular stress because wet kraft loses strength quickly; that is covered in grease-resistant takeaway bags. For a general explanation of paper weights across packaging, see the packaging paper grades guide.
Recycled content also affects the choice. Some US state laws require minimum post-consumer recycled content in paper carryout bags at covered stores, and recycled fibre is typically somewhat weaker than virgin kraft at the same basis weight, so a recycled SOS bag may need a heavier grade to perform the same.
SOS bags with and without handles
Handle-less SOS bags are the fastest format to pack and the cheapest per bag, which is why they dominate drive-thru and counter service. Adding flat paper handles turns the same body into a bag that riders and walk-in customers can carry comfortably.
Without handles, the bag is closed by folding or rolling the top, sometimes held with a seal label. This works for single meals and short trips. With handles, the bag is easier to carry over distance and is common for delivery and click-and-collect orders, but the top cannot be folded down as tightly because the handles are glued inside the mouth. That changes how you close and seal the bag. The available closures and their trade-offs are covered in tamper-evident takeaway bags.
For orders built around several meal boxes and a drinks carrier, a wider-gusset delivery bag usually performs better than simply scaling up an SOS size, because a standard SOS gusset is proportioned for counter orders rather than side-by-side boxes.
Where SOS bags are used outside foodservice
The same construction is used in grocery checkouts, pharmacies, hardware stores and general retail counters. In those settings the bag is valued for the same reason as in takeaway: staff can open it one-handed and it stands while being filled.
A retail SOS bag and a takeaway SOS bag may look identical, but the takeaway version is exposed to heat, steam and grease, has to keep boxes level and often needs a closure method for delivery. That is why takeaway specifications often move beyond commodity SOS sizes and grades.
Printing on SOS bags
SOS bags are typically printed by flexography with water-based inks, in one to four colours, directly on the reel before the bag is formed. The front and back panels give the main print areas; gussets and the bottom fold are harder to use reliably.
Design points specific to the format:
- Front and back panels: keep logos, messages and QR codes here.
- Gussets: printable, but the fold lines move slightly from bag to bag, so avoid critical detail across them.
- Bottom area: the lower part of the printed web becomes the folded bottom, so artwork near the base disappears into the fold.
- Serrated top: the zigzag edge cuts through anything printed right at the top, so leave a margin.
- Brown kraft: colours print darker on natural kraft; white kraft gives brighter, more accurate colours.
The packaging printing methods guide compares flexo with other processes.
Specifying a custom printed SOS bag
When you ask for an SOS bag quotation, describe the bag in a way that works for both US and European converters:
- Dimensions as width x gusset x height, in millimetres or inches (and the numbered size you are replacing, if any).
- Paper: brown or white kraft, and grade in gsm or basis weight; recycled content if required by your market.
- Handles: none, or flat paper handles.
- Top: serrated or straight cut; any extended flap for closing or sealing.
- Contents: the boxes or items the bag must hold, ideally with samples or dimensions.
- Print: number of colours, coverage and brand colour references.
- Quantities per size and destination market.
Your answers feed directly into the overall takeaway bag specification on the custom printed takeaway bags hub. When you have the details together, use the contact page to request a quotation.