How to Measure Box Liner Bags for Cartons and Pallet Boxes
To measure box liner bags, start with the container’s usable inside length, width and height. Confirm the bag type and the supplier’s dimension labels. Then allow enough material above the contents for the chosen fold or closure. A carton’s three dimensions are not automatically the three dimensions to put on a bag order.
This guide is for buyers ordering manually inserted plastic liners for cartons, pallet boxes and similar rectangular containers. It covers measurement and sample fit. Automated filling systems and liquid bag-in-box packs need their own equipment and pack specifications.
Measure the inside of the carton or pallet box
For carton liners, assemble the actual box as it will be used for packing. Measure between the inside walls. Do not use outside shipping dimensions or the footprint of the pallet underneath it. Associated Bag’s measurement guide also identifies corrugated-box dimensions as interior length, width and height.
- Inside length: measure the longer horizontal side.
- Inside width: measure the shorter horizontal side. Some liner guides call this the container’s depth.
- Inside height: measure from the internal base to the rim. Record the intended fill level separately if it differs from the rim.
Use those labels consistently on a sketch. For a tapered container, record both the top and base dimensions. Photograph any projecting rim, brace or insert. These features may block the liner or reduce the available space. A single nominal box size can hide these details.
Finally, show what the liner must do at the top: turn down over the rim during filling, fold over the contents, gather for a tie, or reach a sealing position. Include a drawing of the intended closure. These uses need different amounts of material.
Match the measurements to the bag drawing
A flat bag, a side-gusseted bag and a formed-bottom liner can all be used around rectangular contents, but their order dimensions may describe different things. The phrase “square bottom” alone does not show where the folds and seals lie.
| Construction | Dimensions to identify | The question to resolve |
|---|---|---|
| Flat, or layflat, bag | Flattened mouth width and bag length; no side gussets | Does length run to the functional bottom seal or include an unusable tail? |
| Side-gusseted bag | Face width, full expanded side depth and bag length | Is the quoted gusset the full depth or one inward fold? |
| Formed-bottom liner | Opened base dimensions, usable height and seal layout | Are the quoted dimensions opened dimensions or manufacturing dimensions? |
Associated Bag distinguishes side-gusseted dimensions from layflat dimensions and describes expanded depth as twice one gusset fold. For a symmetric side fold that measures 150 mm from the outer edge to the inward crease, the full expanded depth is 300 mm. Calling both measurements “gusset” can produce a substantial ordering error.
Ask the supplier to mark the mouth, face width, full expanded depth, bottom seal and length endpoints on one drawing. Also state the measurement condition and agreed tolerance. For a nominal size, agree on the acceptable range separately. Each bag may differ slightly from its printed size.
Keep two sets of numbers on the order record: container inside dimensions and finished bag dimensions. Label the units on both. This prevents a box measurement from being mistaken for a manufacturing instruction.
Use a sizing formula only after checking its assumptions
First, check the opening geometry
For a rectangular opening with inside sides a and b, the distance around the opening (its perimeter) is 2 × (a + b). A flat bag without gussets has two film walls across its mouth. Its opening perimeter is therefore about twice its layflat width.
For a 600 × 400 mm container, that gives a 2,000 mm opening perimeter and a 1,000 mm equivalent layflat width at zero clearance. It does not give an order-ready bag size. A 600 mm flat bag would provide only about 1,200 mm of opening perimeter; a 600 mm face-width gusseted bag is a different construction.
This simple check catches a common mismatch before sampling. It does not account for insertion ease, folds, the rim, dimensional tolerance or the bottom and closure. Do not plan to stretch the film to make up a sizing shortfall.
A worked example using one published supplier method
Great American Packaging’s box-liner sizing guide gives a width calculation of half the box perimeter plus 2 inches. Its length calculation uses the box height plus half the narrow side, with another 5–8 inches for fold-over or tie-off material.
Applied to a hypothetical box measuring 20 × 16 × 18 inches internally, that method gives:
| Check | Calculation | Result |
|---|---|---|
| Inside opening perimeter | 2 × (20 + 16) | 72 in |
| Bag width under this method | 72 ÷ 2 + 2 | 38 in layflat |
| Height and half-depth component | 18 + 16 ÷ 2 | 26 in |
| Length including the stated top allowance | 26 + 5 to 8 | 31–34 in |
The result is a 38-inch layflat width with a 31–34-inch length range under that particular method. A range still needs to become a selected sample size and an agreed specification. The extra inches are that guide’s recommendation, not a universal allowance or a TIAN FENG specification.
Other guides use a different combined length allowance for the liner and its closure. For example, Colony’s gusseted box-liner method uses the full container depth plus 6 inches in its length calculation for covering and tying. Before choosing a method, compare the bag type and planned closure. Check whether the length already allows for the bottom, top coverage and tie-off. Adding these again can leave too much spare film.
For a liner with a separately formed base, request the manufacturer’s drawing and usable-height definition. The shared name “box liner” does not make a flat-bag formula suitable for a formed-base liner.
Measure an existing liner without losing its construction details
A liner that already fits is useful evidence. If possible, send an unused bag with the container measurements. Include a photo of its fold or closure in use.
- Lay it flat without pulling it tight. Identify the mouth, bottom seal and any extra film beyond the seal.
- Record the flattened face width. If there are side folds, photograph them before opening them.
- Open each gusset fully. Record the full expanded depth and show whether the two sides are symmetric. Do not report only the inward tuck.
- Mark both length endpoints. Measure the usable length from the mouth to the inside edge of the bottom seal, then record the overall cut length and any tail separately.
- Record its condition. A used liner may have stretched, creased or been trimmed. Identify it as a used sample rather than treating it as a reliable production master.
Supplier conventions vary. US Poly Pack’s measurement policy, for example, changes whether seal space is included according to bag length. Label the actual endpoints rather than relying on the word “length” alone.
Keep thickness separate from size. A folded section or gusset contains multiple film layers, so measuring across the stack does not directly give single-wall thickness. Request an agreed single-wall measurement and tolerance rather than trying to infer thickness from how heavy or stiff the old bag feels.
Check the filled sample, then freeze the order details
Start by inserting the sample into the actual container. Seat the base and corners gently, then arrange the rim fold as intended. Then fill and close it using the packing team’s normal procedure and representative contents. An empty liner that drops into the box can still be too short to close after filling.
| Observation | Check before changing the size |
|---|---|
| The mouth will not pass over the rim | Opening perimeter, widest rim, gusset interpretation and insertion clearance |
| The bottom hangs above the container base | Whether the liner is fully opened, usable length, bottom construction and any trapped folds |
| The liner reaches the rim but will not close | Actual fill level, content shape, top allowance and closure position |
| Excess film bunches in the corners or closure | Whether container dimensions, bag construction or duplicated allowances created the excess |
If the sample tears or its seal fails, record where and when it happened. Possible causes include a tight fit, a sharp box edge, the film itself or the seal. Simply increasing thickness does not resolve a misread gusset or missing closure length.
After acceptance, keep the approved drawing and sample ID. Record the material, single-wall thickness, size tolerances, closure and packing format. Link these to the quotation revision. Repeat the fit check when the box construction, contents, fill level or closure changes.
What a useful sizing inquiry looks like
The following is a hypothetical inquiry example, not a supplied product specification or a completed factory trial:
We need a manually inserted PE liner for a rectangular carton with inside dimensions of 600 × 400 × 500 mm. The contents are dry plastic components. Our target fill level is 50 mm below the rim. The liner will turn over the rim while filling, then fold over the contents before the carton closes.
Please propose a bag type and dimensions on a drawing. Show the full expanded gusset, bottom seal and usable length. We can provide an existing liner for comparison.
Initial requirement: 5,000 pieces, subject to the supplier’s confirmed minimum; destination: United States. Please quote the proposed material, single-wall thickness and tolerances separately, and confirm sample arrangements.
For carton and pallet-box lining, review our custom PE square-bottom bag options or send your container measurements for a specification review. The bag drawing, sample terms and order requirements need confirmation for the proposed construction. If a particular contact or storage requirement applies, identify it when requesting the quotation and ask which supporting documents are available.
Common questions about box liner bag sizes
Should I measure the inside or outside of a pallet box?
Measure the usable inside space for a liner. Also show any rim or other feature the mouth must pass over. The pallet’s outside footprint is useful shipping information, but it does not establish the liner size. Apply the same check to a rectangular Gaylord box: its trade name does not define one universal size.
Does “600 × 400 × 800 mm” identify one definite liner?
No. Those numbers need labels and a drawing. They might describe face width, expanded gusset and flat length, or opened base dimensions and usable height. Confirm the order, measurement endpoints and units before comparing quotations.
Can a flat liner and a gusseted liner have the same opening size?
They can have a similar opening perimeter while folding differently inside the box. Compare the equivalent opening size, bottom arrangement and usable height, then test the proposed construction. Matching one number does not prove equivalent fit.
How much extra length should I add for tying or heat sealing?
There is no single addition that covers every box, fill level and closure. Show the filled profile and intended closure location to the supplier. Confirm whether the proposed bag length already includes bottom formation and closure material, then check it with a filled sample.