FBA Actual Weight vs Dimensional Weight: Which Do You Pay On?
FBA bills the greater of actual weight and dimensional weight, every shipment. A box that actually weighs 0.9 lb therefore bills at 2.76 lb — not an Amazon error, but the rule, and the reason light, bulky products quietly bleed margin on FBA.
The decision rule, in one row
Question
Answer
Which weight does FBA use?
The greater of actual and dimensional weight — every shipment, no exceptions.
US formula
L × W × H in inches ÷ 139
International formula
L × W × H in cm ÷ 5,000
Worked case
12 × 8 × 4 in = 384 in³ ÷ 139 = 2.76 lb
Who wins that case
Dimensional weight (2.76 lb beats 0.9 lb actual)
Why the bigger number always wins
Carriers allocate truck and plane space by volume, not by scale. A feather-light box still occupies the same shelf as a heavy one, so fulfillment pricing protects the space. Read more on the dimensional weight formula itself; the practical consequence here is simpler: your packaging decision is a fee decision.
The same 10 oz home product tells the story two ways:
Packed tight in a 6 × 4 × 2 in box: dimensional weight ≈ actual, bills in the low band at $4.20.
Packed loose in a 12 × 8 × 4 in box: bills at 2.76 lb, fulfillment jumps to $6.67 — and the 3.5% fuel surcharge climbs from $0.15 to $0.23 on top.
Pillow boxes: soft goods with acres of void space — padding a 5 oz product into a 10 × 10 × 4 in box bills ~2.9 lb.
Garment mailers overboxed: a tee in 12 × 10 × 4 in is 480 ÷ 139 = 3.45 lb billable; folded tight it sits under 0.5 lb.
Retail packaging you did not design: fancy gift boxes look good on the listing and cost a band jump per unit.
The fix: shrink the carton until dimensional weight is at or below actual weight, then re-run the calculator. Downsizing packaging is the cheapest fee cut available on FBA — and it moves break-even down with it.
Check which weight you actually bill
The calculator applies the US divisor automatically and shows the billed-weight band against your dimensions:
When does dimensional weight replace actual weight on my FBA bill?
Whenever dimensional weight is the larger number. US FBA computes L×W×H in inches divided by 139 and compares it with the actual scale weight; the bigger one becomes your billing weight. A light product in a big box always bills on the box, not the contents.