To measure a box for custom packaging, measure the inside cavity rather than the outside shell, and record the numbers as length x width x depth (L x W x D) in that order. Looking down at the open top of the box, length is the longer of the two opening edges, width is the shorter opening edge, and depth is the vertical distance from the inside bottom to the top opening. Add clearance so the product, any insert, and any protective material actually fit. Send those three numbers, the unit, and a note saying they are inside dimensions with your quote request.
That sounds simple, and the measurement itself is. What causes remakes is everything around it: giving outside dimensions when the supplier assumed inside, listing the numbers out of order, forgetting the insert, or approving a size without ever holding a physical sample. This guide walks through the measurement method and the judgment calls that go with it, so the box you receive fits the product you actually ship.
What L x W x D means and why the order matters
Packaging dimensions are conventionally written in a fixed sequence, and suppliers read them positionally. If you write 4 x 8 x 2, a manufacturer will generally interpret 4 as length, 8 as width, and 2 as depth — even though 8 is the larger number and was probably meant to be the length. The box gets built to what was written, not to what was intended.
Orient the box the way it will be used, with the opening facing up:
Length (L) — the longer edge of the opening.
Width (W) — the shorter edge of the opening, perpendicular to the length.
Depth (D) — measured vertically from the inside bottom panel to the top edge of the opening. Some suppliers and templates call this height (H);
L x W x HandL x W x Drefer to the same third measurement in most packaging contexts.
Two habits prevent nearly all order-related confusion. First, always label the numbers explicitly rather than sending three bare figures: write Length 8 in x Width 4 in x Depth 2 in. Second, state the unit. Inches and millimetres are both common, and a number like 120 is unambiguous only when the unit travels with it.
A tall, narrow box is where people most often second-guess themselves. A carton for a slim bottle might be 2 inches by 2 inches at the opening and 9 inches tall. That is not a 9-inch length. It is 2 x 2 x 9 — the 9 is depth, because it is the vertical dimension from the opening down to the base.
Inside dimensions versus outside dimensions
This is the distinction that matters most, and it is the one most likely to be skipped.
Inside dimensions describe the usable cavity — the empty space the product occupies. Outside dimensions describe the finished exterior of the box, which includes the thickness of the board on every wall.
Custom packaging suppliers normally build to inside dimensions. The manufacturer's job is to construct a box around your product, so the cavity is the specification that has to be right. If you supply outside dimensions but the supplier tools to them as inside dimensions, or vice versa, the finished box is off by roughly twice the board thickness on each axis. On a thin folding carton that error may be small enough to tolerate. On double-wall corrugated it can be a substantial fraction of an inch per side, which is more than enough to make a snug product impossible to insert or a printed sleeve refuse to slide on.
Outside dimensions still matter for other reasons. They determine how many boxes fit on a pallet or a retail shelf, how the carton behaves in a case pack, and how carriers assess dimensional weight for shipping. It is reasonable to track both. Just make it unmistakable which set you are quoting, and check with your supplier which they expect if the request form does not say.
A practical convention: label every dimension set you share as ID (inside dimensions) or OD (outside dimensions). Two extra characters remove the ambiguity entirely.
How to measure, step by step
The right method depends on what you have in hand.
If you have the product but no box
This is the most common starting point for a first custom run.
Place the product in the orientation it will sit inside the box. Orientation is a real decision — a bottle can lie flat or stand upright, and each produces a different box.
Measure the product's own length, width, and height at its widest points. Include caps, pumps, handles, spouts, and any protruding element. A pump dispenser is frequently the tallest part of a cosmetic bottle, not the bottle body.
Add clearance to each dimension (covered in the next section).
Record the result as the required inside dimensions.
Use a rigid ruler, a tape measure, or calipers. Calipers are worth using for small, tight-fitting products where a fraction of a millimetre changes the fit. Measure each dimension at least twice, and measure more than one unit if your product has any manufacturing variation of its own — a hand-poured candle jar or a sewn textile item will not be perfectly identical unit to unit, and the box has to accommodate the largest one.
If you have an existing box you want to replicate or adjust
Open the box fully and, if it is glued, leave it assembled rather than flattening it.
Measure from inside wall to inside wall across the opening for length and width.
Measure from the inside bottom to the top edge for depth.
Keep the tape flat and taut against the panel. A tape that bows across the cavity reads long.
If the existing box is one you want to change, note what is wrong before quoting: too much void space, product too tight, lid does not close cleanly, insert does not seat. That description is often more useful to a packaging specialist than a bare set of new numbers.
If you have a CAD file, a drawing, or a dieline
Dimensions can be pulled directly from the file, but confirm which dimension set the file expresses. Engineering drawings sometimes describe the flat blank, sometimes the assembled outer size, and sometimes the cavity. Ask, or check whether the drawing calls out board caliper separately. If you already have a dieline from a previous supplier, sending it is usually faster and safer than re-measuring by hand.
Clearance: the allowance most people forget
A box built to the exact dimensions of the product will not work. Real packaging needs a small amount of extra space, and how much depends on what is going inside.
Consider each of these before finalising the cavity:
Product tolerance. Products vary slightly. The cavity should suit the largest unit you realistically produce, not the average one.
Inserts. A foam, paperboard, or corrugated insert consumes space on the axes it occupies. Size the cavity around the product plus the insert, not the product alone. If you are still deciding on insert type, resolve that before locking dimensions.
Void fill and protective material. Tissue, crinkle paper, air pillows, or a wrap layer all need room. Unboxing-focused brands often want visibly generous space; a shipper wants the minimum that still protects.
Insertion and removal. A cavity that matches the product exactly can be very hard to load on a packing line, and hard for a customer to open without damaging the box. Some working room is functional, not wasteful.
Retail and display needs. Shelf hanging, a window panel, a sleeve, or a display orientation can impose their own dimensional constraints.
There is no universal clearance figure that applies to every product and structure — a rigid electronics box with a die-cut foam insert and a soft apparel mailer behave very differently. This is exactly the kind of question worth putting to a packaging specialist, who can recommend an allowance for your specific material, structure, and product, and then confirm it with a sample.
How material and structure change the finished size
The board itself is part of the box, and different materials occupy different amounts of space.
Caliper and board thickness. Paperboard is measured in caliper (points or millimetres). Corrugated is described by flute profile and wall count. Thicker board means the outside dimensions grow further beyond the inside dimensions, and it also affects how tightly panels fold and how much a corner radius encroaches on the cavity. If you are still deciding between substrates, our comparison of paperboard versus corrugated packaging covers how each behaves.
Structure. A tuck-end carton, a glued mailer, and a shipping box each convert a flat blank into a cavity differently. Glue laps, tuck depth, dust flaps, locking tabs, and folded side walls all consume material and can subtly affect the usable interior. This is one more reason to specify the cavity you need and let the manufacturer engineer the blank to deliver it.
Manufacturing tolerance. No converting process is dimensionally perfect. Every material and process carries some variation, and it is worth asking your supplier what tolerance applies to your specification and material — particularly for a tight-fitting product, a matched lid-and-base set, or a sleeve that has to slide over a base box. Confirm the answer for your job rather than assuming a figure you read elsewhere applies.
Structure-specific measuring notes
Mailer boxes. Measure the cavity with the lid open and flat. Mailers are usually specified by inside dimensions and are frequently made from corrugated, so the outside footprint will be noticeably larger. If you are shipping the mailer on its own without an outer carton, check the outside dimensions against your carrier's size bands too. See custom mailer boxes.
Folding cartons. Depth on a folding carton is measured from the inside bottom to the opening, excluding the tuck flap. The flap is not usable cavity. See custom folding carton boxes.
Tuck boxes. The same rule applies, and the tuck-end style affects panel layout. If you have not settled the closure style, our guide on straight tuck versus reverse tuck boxes explains the difference.
Shipping boxes. These usually hold a primary package rather than a bare product, so measure the primary package and add clearance for cushioning. Outside dimensions matter more here because of dimensional weight and pallet fit. See custom design printed shipping boxes.
Sleeves and two-piece sets. Any structure where one component slides over or into another needs the fit relationship specified, not just two independent dimension sets. Say explicitly which part is the base and which slides.
Common measurement mistakes
Giving outside dimensions when the supplier expects inside. Label every set
IDorOD.Writing the numbers out of order. Label them
Length,Width,Depth.Omitting the unit. Always state inches or millimetres.
Measuring the product only, with no clearance. The box has to be loadable and openable.
Forgetting the insert. Insert decisions must precede final dimensions. Our packaging inserts guide covers the options.
Ignoring protrusions. Caps, pumps, and handles are part of the measurement.
Measuring one unit of a variable product. Size for the largest realistic unit.
Measuring a flattened glued box. Assemble it first; a flat blank does not read as a cavity.
Approving from a screen only. Digital mockups do not reveal fit.
Changing dimensions after artwork is complete. Any dimension change invalidates the dieline and forces the artwork to be re-laid out. Lock the size first, then produce artwork — see our guide to preparing artwork files for custom packaging.
Confirm with a dieline and a physical sample
Once dimensions are agreed, the supplier produces a dieline: the flat template showing every cut, crease, and glue area for your specific size, material, and structure. Review it before artwork is finalised. Check that the panel labelled as the front is the one you intend to be the front, that depth is where you expect it, and that any window, hang hole, or insert cutout sits correctly.
Then request a physical sample or prototype and load your actual product into it. A sample answers questions a drawing cannot: whether the product slides in easily, whether the closure stays shut when handled, whether the insert seats flat, whether the pack feels solid or flimsy in the hand. Fixing a dimension at sample stage is straightforward. Fixing it after a full production run is not.
Quote-ready dimension checklist
Send this with your request and most sizing back-and-forth disappears:
[ ] Inside dimensions as
Length x Width x Depth, each labelled[ ] Unit stated (inches or millimetres)
[ ] Marked clearly as inside dimensions (
ID)[ ] Outside dimensions too, if pallet, shelf, or carrier constraints apply
[ ] Product dimensions and weight, including any protrusions
[ ] Whether an insert is required, and which type if known
[ ] Box structure or style, if you have a preference
[ ] Material or substrate preference, if any
[ ] Any existing dieline, drawing, or reference sample
[ ] Intended use: retail shelf, ecommerce shipping, subscription, gifting
[ ] Whether the box ships on its own or inside an outer carton
[ ] Photos of the product, ideally with a ruler in frame for scale
FAQ
Should I give inside or outside dimensions when ordering custom boxes?
Inside dimensions, in most cases. Custom packaging is built around your product, so the cavity is what the manufacturer needs. Provide outside dimensions as well if pallet fit, shelf space, or carrier size limits are a constraint — and label each set clearly so there is no ambiguity.
What order do box dimensions go in?
Length x width x depth. With the box opening facing up, length is the longer opening edge, width is the shorter opening edge, and depth is the vertical measurement from the inside bottom to the opening. Some suppliers write height instead of depth for that third number.
Is depth the same as height?
In practice, yes — both usually refer to the third, vertical dimension measured from the inside bottom to the top opening. Because terminology varies between suppliers, label the number explicitly rather than relying on the abbreviation.
How much extra space should I add around my product?
It depends on the product, the structure, the material, and whether an insert or void fill is used. A snug rigid box with a die-cut insert and a soft-goods mailer need very different allowances. Describe your product and protection needs to a packaging specialist and confirm the recommendation with a physical sample rather than applying a generic figure.
Do I need exact dimensions before requesting a quote?
Approximate dimensions are enough to start a conversation, and a specialist can often suggest a size from your product measurements and photos. Exact, confirmed inside dimensions are needed before tooling and artwork, because a dimension change at that point means a new dieline and re-laid-out artwork.
Will my finished boxes match the specified dimensions exactly?
Every converting process carries some dimensional variation, which differs by material, structure, and process. Ask your supplier what tolerance applies to your specific job, and validate the fit with a physical sample before committing to a full run — especially for tight-fitting products, matched lid-and-base sets, and sleeves.
Get your dimensions confirmed before you print
Getting dimensions right is the cheapest quality control step in the entire packaging process, and the most expensive one to skip. Measure the cavity, label the numbers, account for inserts and clearance, then confirm with a dieline and a real sample.
If you are not certain your numbers are right, send what you have. Share your product measurements, photos, and intended use, and a Brisk packaging specialist can review the specification, recommend a structure and material, prepare a dieline for your approval, and quote the job on confirmed inside dimensions. Request a packaging quote and dieline review, or ask about a physical sample before you commit to a full production run.
