Inside dimensions are the only numbers that matter
Every custom mailer is built from three inside measurements. They are always written in the same order: length, then width, then depth.
Length is the longer side of the opening. Width is the shorter side, and depth is the height from the floor of the box to the underside of the lid.
Outside dimensions change with the board, so they cannot be used to build a dieline. A 10 x 8 x 3 inch box in B-flute is smaller inside than the same outside size in paperboard.
For tiny goods the fit is tighter still. Small mailer boxes for jewelry and minis often need measurements to the nearest 1/16 inch.
Write the three numbers down before you look at any box options. It stops you bending your product to fit a size that already exists.
How to measure a product in eight steps
Use a steel ruler or calipers rather than a cloth tape. Cloth tapes stretch and round off the small numbers that decide a good fit.
If you want to see how your logo sits on the size you measured, try the online mailer box preview before you request a proof.
- Get the item into its ready-to-post state first, retail carton, sleeve or pouch included.
- Place it on a flat table with the side you want seen first facing up.
- Measure the longest side across the top. That is your length.
- Measure the shorter side across the top. That is your width.
- Measure from the table to the highest point, including caps and handles. That is your depth.
- Add tissue, cards or inserts that sit on top or around the product.
- Add clearance from the table in the next section.
- Round up to the nearest 1/16 inch and write it as L x W x D.
How much room to leave around the product
Clearance is the gap between the product and the inside wall. Too little and the box will not close, too much and the product slides and dents.
The right gap depends on what the product is and what goes around it. Use these figures as a starting point and adjust after you test a sample.
Fitted cavities change the math. With mailer boxes with die-cut inserts, the outer box is sized to the insert, and the insert is sized to the product.
Test the gap with a quick mockup. A box cut from scrap card to your numbers shows in two minutes whether the lid will close.
Leave a little more room when packers wear gloves or work fast. A box that needs careful wiggling slows down every order.
| Product type | Clearance per side | Notes |
|---|---|---|
| Folded apparel | 1/4 to 1/2 inch | Allow for tissue wrap |
| Rigid goods in their own carton | 1/16 to 1/8 inch | Snug, no rattle |
| Glass bottles and jars | Sized to the insert | Never loose in the box |
| Books and flat prints | 1/8 inch | Tight fit protects corners |
| Sets with several items | Sized to the insert | Each piece in its own cavity |
| Soft items in a pouch | 1/4 inch | Pouch can bulge when filled |
Board thickness and the space it takes
Board thickness eats inside space, and it differs a lot between materials. Paperboard is measured in points, where 1 point equals 0.001 inch.
An 18 pt paperboard wall is 0.018 inch thick, and 24 pt is 0.024 inch. E-flute corrugated is about 1/16 inch, and B-flute is about 1/8 inch.
Roll end mailers fold their side flaps into a double wall. In B-flute, that double wall can take about 1/4 inch from each side if the dieline is not drawn for it.
This is why we draw the dieline from your inside size. Our comparison of corrugated and paperboard mailers explains which board suits which weight.
Paperboard walls barely change the inside size. That is why tight-fitting small mailers are often made in 18 or 24 pt board.
Typical size ranges by product
Every brand ends up with its own exact numbers. Still, products in the same category land in similar ranges, which helps you sanity check a measurement.
Apparel is the most forgiving group. Shirt mailer boxes for folded tees usually stay shallow, around 2 to 3 inches deep, so the shirt keeps its fold.
Treat these ranges as a check, not a target. If your number is far outside them, measure again before you send it.
- Jewelry, cards and samples: about 3 to 6 inches long, 1 to 2 inches deep.
- Skincare sets and candles: about 6 to 9 inches long, 3 to 5 inches deep.
- Folded apparel: about 10 to 14 inches long, 2 to 4 inches deep.
- Books and prints: trim size plus 1/8 inch per side, depth to page count.
- Subscription boxes: often 9 to 12 inches long and 3 to 4 inches deep.
- Shoes, kits and bundles: 12 inches long or more, often in B-flute.
Depth is where most sizing errors happen
Length and width are easy to see. Depth hides behind pumps, lids, handles and the paper you lay on top.
A pump bottle with its nozzle twisted shut can sit 1/2 inch lower than one with the nozzle raised. Measure the product in its shipping state.
Headwear is a classic example. Hat mailer boxes sized to the crown height start from the crown, not the brim, so the cap never gets crushed.
If your depth falls between two numbers, go up to the next 1/16 inch. A lid that bulges by even 1/8 inch will not stay tucked.
Lids on tuck front mailers also need a little headroom for the tuck flap. Leave at least 1/16 inch above the tallest point.
Measuring round, tapered and soft products
Round jars and bottles are measured at their widest diameter. A jar that is 3 inches across needs a 3 inch width, plus clearance, even though the corners of the box stay empty.
Tapered items, like a bottle with a wide shoulder, are measured at the widest point too. An insert can fill the gaps lower down so the item stands straight.
Soft goods change shape. Fold apparel, blankets or plush toys the way your packer will, press lightly and measure the stack at its natural height.
For mixed shapes, measure each item alone, then the full group laid out together. Send both sets of numbers so we can plan an insert if it helps.
Sizing for cheaper postage
Many carriers price parcels by size as well as weight. Each extra inch can move a parcel into a higher band, even when it weighs the same.
Check the size bands of the carrier you use most, then work back to an outer size that stays under them. Remember the outer size includes the board.
Slim products gain the most here. Low-profile postal boxes for flat goods keep depth to the minimum so parcels can fit a smaller rate.
Weigh a packed sample on a kitchen or postal scale too. Size and weight together decide the rate, so you need both numbers.
One size, or a small range of sizes
A single box size is easy to stock and reorder. It works well when you sell one product, or products that are all close in size.
Larger catalogs need a range. Most stores manage with three sizes, picked from the order combinations that ship most often.
Ecommerce boxes made as a matched size range share one artwork design. Each size still gets its own dieline and its own proof.
Look at your last 200 orders before you decide. Group them by size and you will usually see two or three clear clusters.
Fewer sizes also mean simpler packing. Staff grab the right box faster when there are only three to choose from.
Checking the size before the full run
Your free digital proof shows the dieline with every inside measurement labeled. Check each number against your notes before you approve it.
Hold your product against a ruler while you read the proof. Numbers on a screen are easy to misread, while a ruler on the real item settles any doubt.
Resizing costs nothing at proof stage, and you can revise as often as you like. Once the dimensions are signed off, the run takes 4–6 business days and the boxes travel for 2–3 more.
Plan about two weeks from first measurement to boxes on your table. When the numbers are ready, send your sizes for a free quote and we confirm the build.