One line: FBA bills the greater of actual weight and dimensional weight — the larger number becomes your shipping weight. US uses (L × W × H in) ÷ 139; international marketplaces use (L × W × H in cm) ÷ 5,000, so a light, bulky box always bills on its size, not its contents.
The two formulas side by side
Marketplace
Units
Formula
Example (12 × 8 × 4 in box)
US (2026)
Inches / lb
L × W × H ÷ 139
384 ÷ 139 = 2.76 lb
International
cm / kg
L × W × H ÷ 5,000
30.5 × 20.3 × 10.2 ÷ 5,000 ≈ 1.26 kg
A box that actually weighs 0.9 lb therefore bills at 2.76 lb in the US — three times the real weight, because the box is "light for its size."
Carriers and Amazon allocate truck and plane space by volume as much as by weight. A feather-light but huge box occupies the same shelf as a heavy one, so fulfillment pricing protects the space, not the scale reading. The 2026 US schedule prices by band: small-standard tiers up to 15 oz and large-standard bands beyond, with a 3–20 lb per-4-oz formula for heavier units.
Actual weight = what the scale says, in lb (US) or kg (international).
Dimensional weight = box volume ÷ divisor, rounded per the schedule.
Billed weight = the greater of the two — this is what the fee table sees.
When it quietly changes your margin
Three situations where dimensional weight decides the band:
Oversized packaging: padding a 5 oz product with a 10 × 10 × 4 in box bills ~2.9 lb — a fee jump of several dollars per unit.
Odd-ounce rounding: US small-standard rounds odd ounces up to the next even tier, so 7.2 oz can bill as 8 oz.
Category + dimension stacking: the higher billing weight can push a beauty or home product into a different band, moving the whole fee line — and with it the break-even price.
Fix that actually works: shrink the box until the dimensional weight is at or below the actual weight, and re-run the calculator — packaging downsizing is the cheapest fee cut in FBA.
Check your billing weight free
The calculator applies the US divisor automatically and shows the billed-weight band in the fee line:
Do I pay the actual weight or the dimensional weight?
You pay the greater of the two. FBA computes actual weight and dimensional weight from the box dimensions, and the billing weight is whichever is higher — that is why oversized packaging on a light product raises the fee.