Your scale says 5 pounds. Your invoice says 13.
If you have ever looked at a shipping invoice and wondered why the billable weight on the line item does not match anything you actually put on a scale, you have met dimensional weight.
Every major Canadian carrier prices parcels on the greater of two numbers: what the package weighs, and what the space it occupies is worth. A truck or a plane runs out of room long before it runs out of weight capacity, so carriers convert volume into a billable weight figure and charge on whichever number is higher.
The practical consequence for anyone shipping a physical product is uncomfortable but simple. Once your box is bigger than it needs to be, you stop paying for what you ship and start paying for air.
This is one of the most common reasons a business comes to us for a custom size after years on stock boxes. Here is how the math actually works, and how to find out what an oversized box is costing you.
How dimensional weight is calculated
The formula is the same everywhere. Only one number changes.
Billable weight = the greater of dimensional weight and actual weight
The divisor, sometimes called the DIM factor or density factor, is set by the carrier. A larger divisor produces a lower dimensional weight, so a divisor of 6,000 is friendlier to you than 5,000.
Measure in centimetres and you get kilograms. Measure in inches and you get pounds.
Divisors used by Canadian carriers
| Carrier and service | Metric (cm³/kg) | Imperial (in³/lb) |
|---|---|---|
| Canada Post Regular Parcel, Expedited Parcel | 6,000 | 166 |
| Canada Post Priority, Xpresspost, US and international | 5,000 | 139 |
| UPS domestic air within Canada | 4,100 | 115 |
| UPS all other services | 5,000 | 139 |
| FedEx, most services | 5,000 | 139 |
| DHL Express | 5,000 | 139 |
Canada Post refers to this as the volumetric equivalent of actual weight, or VE, rather than dimensional weight. Same concept, different label.
Purolator works differently. Rather than a single divisor, Purolator converts the parcel to cubic feet and multiplies by a density factor:
Cubed weight = cubic feet × density factor
The density factor is higher for air services than for ground, and it can differ under a negotiated agreement, so check the factor printed on your own rate guide before you model anything.
One more caution that applies broadly: divisors, factors and rounding rules change. Treat the table above as a starting point and confirm against your current carrier agreement before you build a pricing model on it.
A worked example
An Ontario brand ships a six-bottle pack across the country. Packed weight, product plus box plus void fill, is 5.3 lb (2.4 kg).
The product footprint needs roughly 10 × 8 × 6 inches. The company has been shipping it in a 12 × 12 × 12 stock box because that was what the local supplier stocked.
Ground service, divisor 139
| 12 × 12 × 12 | 10 × 8 × 6 | |
|---|---|---|
| Volume | 1,728 in³ | 480 in³ |
| Dimensional weight | 12.43 lb, billed at 13 lb | 3.45 lb, billed at 4 lb |
| Actual weight | 5.3 lb, billed at 6 lb | 5.3 lb, billed at 6 lb |
| Billable weight | 13 lb | 6 lb |
Canada Post Expedited Parcel, divisor 6,000
| 30 × 30 × 30 cm | 25 × 20 × 15 cm | |
|---|---|---|
| Volume | 27,000 cm³ | 7,500 cm³ |
| Volumetric equivalent | 4.5 kg | 1.25 kg |
| Actual weight | 2.4 kg | 2.4 kg |
| Billable weight | 4.5 kg | 2.4 kg |
Notice what happened in the right-hand column. Once the box fits the product, dimensional weight drops below actual weight and the scale takes over as the governing number. That is the floor. You cannot be billed for less than what you actually ship, and right-sizing is how you get there.
The oversized box was adding 7 lb of phantom billable weight to every single parcel. At 400 parcels a month, that is roughly 33,600 lb of billable weight per year that never existed. Multiply that by the per-pound difference on your own rate card and you have the real number for your business.
The oversized box charges you three separate times
Dimensional weight is the one people notice. It is not the only cost.
- More board. A 12 × 12 × 12 box uses meaningfully more corrugated per unit than a 10 × 8 × 6. You pay for that on the purchase order.
- More void fill. Every cubic inch of empty space needs paper, air pillows or foam to keep the product from moving. That is a consumable you buy over and over.
- More billable weight. The dimensional weight penalty, on every shipment, forever.
And there is a fourth cost that does not show up on any invoice as a line item: damage. Product that can travel inside a box takes impact from the box walls. Void fill mitigates that, but a snug fit prevents it. Damage claims, replacement shipments and the customer service time around them are the most expensive form of oversizing.
Measure the outside, not the inside
This trips up a lot of people, and it matters because carriers measure the parcel that moves through their network, not the cavity your product sits in.
Corrugated box dimensions are quoted as inside dimensions, because the inside is what has to fit your product. The board itself has thickness, so the outside is always larger:
- Single wall (C flute, for example) adds roughly 3/8 inch total to length and width
- Double wall (BC flute) adds closer to 5/8 inch
On a 10 × 8 × 6 single wall box, the carrier will measure something nearer 10.4 × 8.4 × 6.2. That is usually harmless. It stops being harmless when a dimension is sitting just under a threshold, or when you have added a flap, a bulging poly mailer, or a protruding label.
Carriers measure to the extreme points. Anything that sticks out counts.
If you are not sure which dimension is which, our box sizing guide walks through length, width and depth and how to specify them so the box you receive is the box you expected.
Rounding, re-measurement and the surcharge cliffs
Three details that quietly inflate the number.
Rounding up. Each dimension is rounded up before the multiplication, then the final figure is rounded up again to the next whole pound or kilogram. A box measured at 12.1 inches is calculated as 13. Two of those on one parcel and your dimensional weight jumps noticeably. FedEx and UPS both moved to per-dimension ceiling rounding as of August 2025.
Automated re-measurement. Carriers scan and dimension parcels in the network. If the values you declared on the label are lower than what the scanner reads, you get a billing adjustment after the fact, sometimes with a fee attached. Declaring accurately is not optional. Declaring generously is also a mistake, because carriers will not revise a number downward for you.
Oversize thresholds. Beyond dimensional weight there are hard cliffs where a flat surcharge lands: a single dimension past roughly 100 cm or 48 inches depending on carrier, or length plus girth past about 300 cm. These are step functions, not gradual. A box that is one inch over a threshold can cost more than one that is six inches under. If any of your parcels are near a limit, that is the first place to look for savings.
What to actually do about it
- Audit your worst offender first. Pull your top five SKUs by shipping volume. For each, calculate dimensional weight on the box you currently use, then on the smallest box the product would genuinely fit in. Rank by the gap, multiplied by annual parcel count. The list will not be evenly distributed. One or two SKUs usually account for most of the waste.
- Right-size, but do not shrink-wrap. A box that is too tight is its own problem: closing flaps that bulge, corners that fail, staff fighting the pack line. Aim for the smallest size that still lets one person pack it comfortably with minimal void fill.
- Consolidate into a size family. You do not need a unique box per SKU. Three or four well-chosen sizes covering your range beats one large stock size for everything, and beats twelve bespoke sizes you have to warehouse and forecast separately.
- Check whether custom pays. Custom sizes carry a die charge and a minimum quantity. Stock boxes do not. The question is whether the per-shipment savings clear that up-front cost inside your ordering horizon. When a size mismatch is costing several pounds of billable weight on every parcel, the answer is usually yes, and sooner than people expect. Our pricing structure explainer covers how tooling, quantity and board grade interact.
- Do not forget board grade. A smaller box may let you drop to a lighter grade, since a shorter panel span and lower stack height reduce the strength requirement. That compounds the saving. Our guide to 32 ECT explains what the strength rating on the box maker's certificate actually means.
The short version
Carriers bill on volume, not just weight. An oversized box costs you in board, in void fill, in billable weight and in damage claims, and the dimensional weight penalty applies to every parcel you ship for as long as you ship it that way.
Right-sizing is not a cosmetic improvement. For most shippers it is the single highest-leverage change available on freight spend, and unlike a rate negotiation it does not require you to ship more to get it.
Not sure what the right size is for your product? Send us your dimensions and we will spec it. If you can tell us the product footprint, packed weight and monthly volume, we will come back with a size, a board grade and a quote.
Frequently asked questions
What is dimensional weight?
Dimensional weight, also called volumetric weight, converts a parcel's size into a billable weight figure. Carriers charge on whichever is greater, dimensional weight or actual scale weight.
How do I calculate dimensional weight in Canada?
Multiply length by width by height, then divide by your carrier's divisor. Canada Post Regular and Expedited Parcel use 6,000 cm³/kg or 166 in³/lb. Canada Post Priority, Xpresspost and cross-border services use 5,000 cm³/kg or 139 in³/lb. UPS and FedEx generally use 5,000 cm³/kg or 139 in³/lb, with UPS applying 4,100 cm³/kg for domestic air within Canada.
Does Canada Post use dimensional weight?
Yes. Canada Post calls it the volumetric equivalent of actual weight, or VE, and applies a density factor of 6,000 for Regular Parcel and Expedited Parcel and 5,000 for Priority, Xpresspost and US and international services.
Should I measure inside or outside dimensions for shipping?
Carriers measure outside dimensions at the widest points. Corrugated boxes are specified by inside dimensions, so add roughly 3/8 inch for single wall or 5/8 inch for double wall when estimating what the carrier will measure.
Will a smaller box always cost less to ship?
It will never cost more, and it stops costing extra once dimensional weight falls below actual weight. At that point actual weight governs the rate and further shrinking the box saves nothing on freight, although it may still save on board and void fill.