Anyone new to selling online or buying packaging for the first time runs into a wall of jargon fairly quickly. Microns, GSM, H-tape, large letter, tracked versus signed for. None of it is complicated once explained, but most of it is never actually explained anywhere, just assumed knowledge. This guide works through the terms that come up most often, in plain language, so you can read a product listing or a courier's pricing page and actually know what it means.
Packaging material terms
Micron
A micron is a unit of thickness, one thousandth of a millimetre, used to describe how thick a plastic film is. Mailing bags and packaging tape are both commonly specified in microns. A standard mailing bag for clothing is typically 50 to 60 microns. Higher numbers mean thicker, more robust material. Lower numbers mean thinner and lighter, but more prone to tearing or splitting under stress.
GSM
GSM stands for grams per square metre and is used to describe the weight, and indirectly the thickness and sturdiness, of paper and card. A higher GSM means heavier, stiffer material. Standard printer paper is around 80 GSM. A sturdy kraft paper bag might be 100 GSM or more. Cardboard box walls are measured differently again, in terms of flute type and board grade rather than GSM alone.
LDPE
Low-density polyethylene, the plastic most mailing bags and many grip seal bags are made from. It's flexible, lightweight and naturally water-resistant, which is why it's the standard material for postal packaging. LDPE is identified by recycling code 4 and is recyclable, though typically not via standard kerbside collections in most of the UK currently, usually via supermarket soft plastics collection points instead.
BOPP
Biaxially oriented polypropylene, the plastic film most standard packaging tape is made from. It's a different plastic to LDPE, stretched in two directions during manufacturing to give it strength and clarity, which is why most tape is glossy and tear-resistant in a way that mailing bag film generally isn't.
Kraft paper
A strong, typically brown paper made using a specific pulping process that produces longer, stronger fibres than standard paper. Used for paper bags, paper-based tape, and as a general term for sturdy brown packaging paper. Kraft paper tape and kraft paper bags are both fully recyclable with standard paper and cardboard.
Gusset
A folded or expandable section in a bag, usually at the base or sides, that allows the bag to expand to accommodate bulkier contents. Bags with a gusset have more depth and capacity than a flat bag of the same width and length measurement.
Sealing and tape terms
Self-seal
A closure method using a strip of adhesive under a peelable backing, common on mailing bags. You remove the backing and press the flap down to seal. Most self-seal closures are designed to be tamper-evident and single use, meaning they don't reliably reseal once opened.
Tamper-evident
A seal or closure designed to show clear visible signs if it's been opened, used to confirm a parcel hasn't been accessed in transit. Standard mailing bag seals are tamper-evident by design, which is why they generally can't be cleanly resealed for a second use.
H-tape method
A way of sealing a cardboard box using three strips of tape per face: one along the centre join of the flaps, and one down each side seam, with each strip extending onto the box wall. Named for the H shape the strips form. Significantly more secure than a single strip of tape down the centre.
Acrylic adhesive
One of the two main adhesive types used in packaging tape, known for consistent performance across a wide temperature range and good resistance to UV light and ageing over time.
Hot melt adhesive
The other main adhesive type used in packaging tape, known for very fast initial tack and high holding strength, particularly suited to heavier boxes, though generally less stable in cold conditions than acrylic.
Water-activated tape
A type of kraft paper tape that requires moisture, usually applied via a damp sponge or dispenser, to activate the adhesive before sealing. Forms a fibre bond with the cardboard surface, making it very difficult to remove without visibly damaging the box.
Courier and postal terms
Large letter
A Royal Mail format category based on specific size and weight limits, generally cheaper than the small parcel rate but more restrictive on dimensions. A thin item in a mailing bag may qualify; anything with any real thickness usually doesn't.
Small parcel
The next format category up from large letter, with more generous size and weight limits but a higher price point. Most clothing items sent in a mailing bag fall into this category once the garment's thickness is accounted for.
Signed for
A Royal Mail add-on service that requires a signature on delivery and provides limited compensation cover for loss, though notably excluded for jewellery and other valuables regardless of this add-on.
Special Delivery Guaranteed
A premium Royal Mail service offering a guaranteed delivery timeframe and meaningfully higher compensation cover, the only standard Royal Mail service that provides compensation for jewellery and valuables if something goes wrong.
Tracked
A service level, available from various couriers, that provides visibility of a parcel's journey and scan points without necessarily requiring a signature. Different from Signed For, which specifically requires a signature on delivery.
Dimensional weight
Also called volumetric weight, a pricing method some couriers use that charges based on a parcel's size rather than its actual weight, on the basis that bulky lightweight parcels still take up space in a vehicle. Relevant if you're using oversized boxes or bags for lightweight items, as it can result in a higher price than the actual weight alone would suggest.
Selling platform and label terms
Thermal label
A label printed using heat rather than ink, the standard format for courier shipping labels in the UK. Direct thermal is the common type for shipping, requiring no ink or ribbon.
4x6 / 100x150mm
The standard UK courier shipping label size, used by Royal Mail, Evri, DPD and most other services. Written interchangeably as 4x6 inches or 100x150mm; both describe the same label dimensions.
QR code (for shipping)
A scannable code used by some couriers, such as InPost and Evri, in place of a printed label. The recipient point of drop-off scans the code and generates or attaches the label themselves, removing the need for the sender to print anything.
Sustainability terms
Recyclable
Capable of being processed and reused as raw material for new products, though the specific recycling route matters: kerbside, supermarket soft plastics collection, and specialist facilities all handle different materials, and a product being "recyclable" doesn't always mean your local kerbside bin will accept it.
Biodegradable
Capable of breaking down naturally through the action of microorganisms over time. Doesn't specify a timeframe or the conditions required, which is why "biodegradable" alone is a vaguer and less useful term than "compostable" for understanding what actually happens to a product at end of life.
Compostable
Capable of breaking down into usable compost under specific conditions, usually within a defined timeframe. Often requires industrial composting facilities reaching higher temperatures than a typical home compost heap achieves, which matters for anyone assuming a compostable mailing bag can go straight in a garden compost bin.
Soft plastics collection
A UK recycling route, typically found at larger supermarkets, specifically for flexible plastic films such as mailing bags, bread bags and bubble wrap, which most kerbside collections currently don't accept directly.
Why understanding this terminology matters
Beyond just making product listings easier to read, understanding these terms helps you make better decisions: knowing the difference between micron thickness ratings means you can judge whether a cheap bag is actually adequate for what you're sending. Knowing what Signed For does and doesn't cover means you choose the right Royal Mail service for valuable items rather than assuming you're covered when you're not. Knowing the difference between biodegradable and compostable means you dispose of eco packaging correctly rather than assuming it'll break down in your garden compost bin when it actually needs an industrial facility.
If you're working through packaging or postage decisions for your business and a term comes up that isn't covered here, it's worth looking it up properly rather than guessing, since a surprising number of these terms get used loosely or incorrectly even on supplier websites, which is part of why the confusion persists in the first place.
You can browse our full range of mailing bags, grip seal bags and packaging tape, all listed with clear specifications so you know exactly what you're buying.
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