By Andy Wang     Photo：CANVA

Have you ever run into this situation: you hand ten cartons of yoga mats to your forwarder, the scale says 120 kg, but the quote shows Chargeable Weight = 213.5 kg?

It feels like you’re being overcharged, but in most cases no one did the math wrong—that’s simply how air freight works.

Air freight always looks at two numbers: gross weight (what the scale reads) and volumetric weight (volume converted into weight), and charges whichever is higher.

In Chinese, volumetric weight is often called 體積重 in Taiwan and 材積 / 材積重 in China. Light-but-bulky cargo—yoga mats, puffer jackets, foam—are classic “puffy cargo”: they take up space without adding much actual weight. If airlines charged only by gross weight, the aircraft would be full of “air,” and the economics simply wouldn’t work.

For standard air freight arranged by a forwarder and carried by an airline, volumetric weight is usually calculated as:

L×W×H (cm) ÷ 6,000

Many express courier products instead use ÷ 5,000 (or 139 in³/lb in imperial units).
This article uses ÷ 6,000 as the main benchmark and mentions ÷ 5,000 only for comparison.

Back to our yoga-mat example:
each carton is 80×40×40 cm, 10 cartons in total, gross weight 120 kg.

Volumetric weight = (80×40×40×10) ÷ 6,000 = 213.3 kg

Most tariffs will round this up to 213.5 kg or 214 kg (depending on whether the rule is round up to the next 0.5 kg or to the next full kilogram).

If you switched to a courier product using ÷ 5,000, volumetric weight jumps to 256.0 kg, and the invoice climbs accordingly.

Why not just use what the scale says?
Because what’s truly scarce on an aircraft is cubic space in the hold. Volumetric weight translates “how much space your cargo eats up” into a weight figure, which is a fairer way to price capacity for airlines, forwarders, and shippers.

Five common mistakes that lead to unnecessary freight charges

1. Treating the divisor as something that never changes

Many people assume air freight is always ÷ 6,000 and courier is always ÷ 5,000.

As a rough rule of thumb, that’s mostly true—but different product lines, peak seasons, or regions may publish different volumetric factors in the tariff.

Before you ask for a quote, confirm how the numbers are calculated:

• Is the divisor ÷ 6,000 or ÷ 5,000 (or something else)?
• Does the rounding rule use 0.5 kg, whole kilograms, or whole pounds?

A single line in the tariff can change the pricing tier for your entire shipment.

2. Mixing units—and watching the volumetric weight swing

For air freight, the safest practice is to always quote:

• Centimeters (cm) and kilograms (kg)

If you work internally in inches (inch) and pounds (lb), make sure you label the units clearly or include conversions:

• 1 inch = 2.54 cm
• 1 lb = 0.4536 kg

A quick reality check:

• Most countries (including Taiwan and China) use cm/kg in day-to-day air-cargo paperwork.
• The U.S. and many courier price sheets still rely heavily on inch/lb.

Once units are mixed or unclear, your volumetric weight can be off by a shocking margin, and the final freight bill will look “mysteriously expensive.”

3. Forgetting to multiply by the number of pieces—or skipping piece-by-piece math

Volumetric weight is based on the total outer volume of all pieces in the shipment.

Many services calculate each piece separately and then add them up, especially when carton sizes differ.

Two safe approaches:

1. Calculate one carton and multiply by the carton count, or
2. List each carton’s outer dimensions and gross weight line by line, and let your forwarder calculate it piece by piece.

Miss one carton or one layer, and your volumetric weight is off—making later reconciliation painful for both sides.

4. Applying the wrong rounding rule: it’s usually not “round to nearest”

Plenty of tariffs use:

• Round up to the next 0.5 kg or the next full kilogram,
• while couriers often round up to whole pounds or kilograms.

In other words, it’s not the everyday “round to nearest” you might expect.

Whenever you see decimals, the right sequence is:

1. Apply the rounding rule in the tariff first, and
2. Only then compare rates or quotes.

That’s the only way to compare like-for-like numbers between different quotes.

5. Measuring the inner carton instead of the true shipping footprint

Chargeable weight is based on the actual space your shipment occupies, so you always use the outermost dimensions at their largest points.

• If your cargo is on a pallet, calculate using the pallet footprint × total height (pallet included).
• Corner boards, edge protectors, strapping, and any bulging tape all count as part of the footprint.

Any time you change cartons, adjust packaging, or switch to a new pallet type, you should re-measure both dimensions and gross weight—don’t reuse old numbers.

In Chinese this idea is often summed up as “measure the dimensional volume as-is”: in Taiwan 體積要實量, in China/Hong Kong 材積要照實量. Always rate against the actual outer dimensions, not what you remember from a previous run.

Dimensions & units cheat sheet (include these when you request a quote)

If you provide the following in one go, your quote will usually be faster and more accurate:

• Per-carton outer dimensions (L×W×H in cm) + per-carton gross weight (kg) + carton count
• If you have to use inch/lb, clearly label the units or include:
  • 1 inch = 2.54 cm
  • 1 lb = 0.4536 kg
• Volumetric weight (standard air freight): L×W×H (cm) ÷ 6,000
• Courier note: many products use ÷ 5,000 (or 139 in³/lb)
• Rounding rule: check the tariff—most air freight rounds up to the next 0.5 kg or whole kg; couriers typically round up to whole lb or whole kg

Height limits (always confirm with the actual airline / aircraft / ULD)

The numbers below are typical reference points, but real-world limits depend on the airline, aircraft type, and ULD specification at the time:

• Passenger bellyhold: commonly up to around 160 cm
• Full freighter main deck: often around 200–220 cm

A few packaging tips for shippers and exporters:

• If your shipment is close to height limits or requires unusual packaging, lock in the total palletized height (including pallet, corner boards, and straps) at the packaging design stage.
• Leave some safety margin—don’t design everything to exactly “just fit.”
• Before booking, double-check airline and ULD specs with your forwarder.

This helps you avoid last-minute surprises where cargo reaches the terminal only to be rejected as over-height or moved to a different flight.

Three plug-and-play examples (drop your own dimensions in and compare)

1. Light but bulky: yoga mats (sporting goods)

• 10 cartons, each 80×40×40 cm, gross weight 120 kg
• Volumetric weight (÷ 6,000) = 213.3 kg → rounded up to 213.5 kg or 214 kg (per tariff)
• If you move it by courier (÷ 5,000) → 256.0 kg

his is classic “puffy cargo” where volumetric weight is far higher than gross weight.

2. Dense and compact: screws & nuts (hardware)

• 4 cartons, each 50×30×20 cm → total volume 120,000 cm³
• Volumetric weight (÷ 6,000) = 20.0 kg
• Gross weight totals 45 kg → charged on 45 kg

This is a typical high-density shipment—what some Chinese speakers call 重貨 (“heavy cargo”).

In such cases, gross weight almost always governs the price, and volumetric weight is more of a formality.

3. Compressible: outerwear (apparel)

Before compression:

• 5 cartons at 60×40×40 cm, gross weight 48 kg
• Volumetric weight (÷ 6,000) = 80.0 kg → billed at 80 kg

After compression:

• 5 cartons at 60×40×30 cm, total volume 360,000 cm³
• Volumetric weight (÷ 6,000) = 60.0 kg → billed at 60 kg

Simply by optimizing the packaging, you drop an entire pricing tier.

That’s why many apparel and bedding shippers care so much about packaging design:
they want to understand the volumetric impact first, then decide whether extra packaging cost is worth the freight savings.

Wrap-up: do you look at gross weight first, or volumetric weight?

• For cargo that is light, puffy, and highly space-consuming, start by calculating:
  • Volumetric weight (÷ 6,000) and
  • The relationship between gross and volumetric weight (often called a “density ratio” in English, sometimes 重泡比 in Chinese).
• For shipments that are dense, heavy, and compact, gross weight usually decides the price.

Before you request quotes, make sure you:

1. List dimensions and gross weight for every single piece, and
2. Confirm:
   • Is the divisor ÷ 6,000 or ÷ 5,000?
   • Is the rounding rule 0.5 kg or whole-kg (or lb) upward rounding?

Most price gaps that don’t feel reasonable can be avoided up front. Before your next quote, list dimensions and gross weight for each piece, align on the calculation method and the rounding rule, then compare. If you’re weighing standard air freight against courier, or your dimensions sit near a height threshold, first review the cargo type (puffy/space-hungry vs. dense “heavy” cargo) and whether packaging can be safely compressed, then recheck the outer dimensions and the applicable height limits; in most cases you’ll land on an option that better matches transit time and cost.