Plastic Bag Size Guide: Width, Length, Gusset & Flap

Plastic bag dimensions look simple until a purchase order uses the wrong measurement convention. A difference between layflat width, open width, gusset depth, flap length, or usable length can make thousands of bags unsuitable for the intended carton, bin, product, or packing line. This plastic bag size guide explains how wholesale buyers can specify width, length, gusset, lip, and closure dimensions clearly before requesting a quotation.

The most important rule is to define every dimension in words as well as numbers. Many suppliers use width × length for flat bags and width × gusset × length for gusseted bags, but conventions can vary by bag style and market. Include a drawing, the product dimensions, the required fit, and an approved physical sample whenever possible.

How are plastic bag dimensions written?

For a basic flat bag, dimensions are normally written as width × length. Width is measured from side to side while the bag lies flat. Length is measured from the bottom seal to the open end. For example, a 300 × 450 mm flat bag is approximately 300 mm wide and 450 mm long when laid flat.

For a side-gusseted bag, a common notation is width × gusset × length. The gusset is the folded material that expands when the bag is opened. However, one quotation may describe the total gusset while another describes the depth of each fold. State explicitly whether the number is the total expanded gusset or the depth of one side fold.

Use one unit throughout the RFQ. Millimeters are usually easier for precise manufacturing specifications, while inches are common in some markets. If both are shown, identify one as the controlling dimension and treat the conversion as a reference only.

Flat bag width and length

A flat bag has no expandable side or bottom fold. Its layflat width must be large enough for the product's width and depth, not just its front-facing width. A box that is 200 mm wide and 80 mm deep cannot fit comfortably in a 200 mm layflat bag because the film must travel around both dimensions.

Length must cover the product height plus its depth and the material required for closing, tying, heat sealing, or folding. Buyers should also allow for manufacturing tolerance and normal variation in the packed product. Do not order from a theoretical calculation alone when the fit is operationally critical. Test the proposed size with the actual product and packing method.

Side gusset dimensions

Side gussets allow a bag to expand into a more three-dimensional shape. They are often used for liners, food packaging, industrial parts, granular products, and retail bags. A specification such as 400 + 2 × 100 × 700 mm may mean a 400 mm front panel, two 100 mm side folds, and a 700 mm length. Another factory might describe the same construction as 400 × 200 × 700 mm.

To avoid ambiguity, write the specification as a sentence: “400 mm front panel width, 100 mm fold on each side, 600 mm total open circumference, and 700 mm length from bottom seal to open edge.” Add a cross-section drawing showing fold direction and measurement points.

Bottom gusset dimensions

A bottom gusset expands at the base and helps the bag accommodate bulky products or stand more effectively. Bottom-gusset notation is less consistent across bag types. Some suppliers quote the full opened depth, while others quote the folded insert. The bottom seal can also be straight, curved, K-seal, star seal, or another construction depending on the product.

Specify the flat width, total length, opened bottom depth, gusset fold, and seal style separately. If the bag must stand, carry liquid, fit a carton, or meet a load requirement, state that functional requirement rather than assuming a gusset dimension guarantees performance.

Lip, flap and wicket dimensions

Some bags have an extended lip or flap at the open end. The extension may support easy opening, adhesive closure, hanging, wicket mounting, or automatic packing. Clarify whether the quoted bag length includes or excludes the lip or flap.

For wicketed bags, provide the wicket hole diameter, distance between hole centers, distance from the holes to the bag edge, lip length, block quantity, and wicket orientation. Packing-line compatibility depends on these details. A bag can have the correct body size and still fail on the line if the wicket geometry is wrong.

Zipper and adhesive closure measurements

For zipper bags, usable internal length is normally less than the overall cut length because the closure and top lip consume space. Specify overall length, distance from the top edge to the zipper centerline, usable length below the zipper, and any tear notch or tamper-evident seal.

For self-seal bags, state the flap length, adhesive strip width, strip position, release-liner type, and whether the seal is permanent or resealable. If the adhesive must work in cold storage, dusty conditions, or repeated opening cycles, include that application information in the RFQ.

T-shirt bag and handle dimensions

T-shirt bags require more than body width and length. A complete specification can include body width, side-gusset depth, bag length, handle length, handle width, handle opening, bottom seal, and distance from the die-cut area to the top edge. The intended carrying load and product shape also matter.

Handle geometry affects comfort and strength. A larger bag with narrow handles may perform worse than a smaller bag designed around the same load. Combine dimensional approval with a representative load test and review our guide to testing plastic bag strength.

How to select a bag size for your product

1. Measure the actual product. Record maximum width, depth, height, irregular projections, and normal dimensional variation.
2. Define the packing orientation. A product loaded sideways may require a different bag from one loaded upright.
3. Add functional clearance. Allow room for loading, air removal, sealing, tying, folding, or a carton overhang.
4. Select the bag construction. Compare flat, side-gusseted, bottom-gusseted, drawstring, on-roll, and liner formats.
5. Make a prototype. Pack the real product using the real equipment and closure method.
6. Freeze an approved specification. Sign and date the drawing or sample before mass production.

Bag size should be reviewed together with material and thickness. A larger bag, deeper gusset, or longer flap increases film area and therefore affects unit weight and price. Use our plastic bag thickness guide and bag weight and cost calculator guide to build a more accurate specification.

Common sizing mistakes in wholesale orders

• Using product width as bag width without allowing for product depth.
• Failing to state whether gusset depth is per fold or total expanded depth.
• Mixing millimeters and inches without identifying the controlling unit.
• Measuring an old stretched bag instead of an unused approved sample.
• Ignoring seal width, zipper position, flap length, or closure allowance.
• Approving dimensions but not material, thickness, tolerances, print position, and packing.
• Ordering mass production before testing the bag with the actual product and packing line.

Plastic bag RFQ size checklist

A clear request for quotation should include the following information:

• Bag style and intended use.
• Flat width, open width, length, gusset depth, lip, flap, and closure dimensions.
• Measurement drawing with arrows and units.
• Product dimensions, weight, shape, and packing orientation.
• Material, recycled-content requirement, color, and target thickness.
• Seal and handle construction.
• Printing area, artwork position, and registration tolerance.
• Quantity, unit packing, carton requirement, and destination.
• Required samples, inspections, tests, and acceptance criteria.

Viet Phat Plas manufactures a broad range of custom bags, garbage bags, and PE liners. Sharing a precise size specification helps us compare structures and prepare a quotation with fewer revisions. For commercial planning, also review our MOQ, pricing, and RFQ guide.

Frequently asked questions

Is plastic bag size written as width × length?

Flat bags are commonly specified as width × length. Gusseted and closure bags require additional dimensions. Always label each number and confirm the supplier's convention.

Does bag length include the flap or lip?

It depends on the specification. State body length and flap or lip length separately so the usable internal length is clear.

What is layflat width?

Layflat width is the side-to-side measurement of an empty bag lying flat. It is not always the same as the usable open width around a three-dimensional product.

How much clearance should I add?

There is no universal allowance. It depends on product geometry, film flexibility, loading method, closure, and tolerances. A fitted sample is the safest basis for approval.

Can a supplier calculate the size from my product dimensions?

Yes, a supplier can recommend a starting size, but the buyer should test a sample with the actual product and approve a controlled drawing before production.

Request a correctly sized bag quotation

Send Viet Phat Plas your product dimensions, bag drawing, material, thickness, print requirement, annual volume, and destination. Our team can review the specification and recommend a practical structure for sampling. Contact us for a custom plastic bag quotation.