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Key fact: Amazon bills FBA on the greater of unit weight and dimensional weight, so a light but bulky item is charged as though it were heavy. That is why shrinking a carton by an inch can cut a fulfilment fee even when the product itself has not changed. The free DIM calculator shows billable weight and the shipping charge it drives, before packaging is committed.
An example box is filled in (12 × 10 × 8 in, 4 lb) so you can see how it works — overwrite the fields with your own measurements.
Arithmetic only: dimensional weight = length x width x height / divisor. Divisors, rounding and minimum billable weights differ by carrier, marketplace and service level - confirm against your rate card before billing anyone.
Carriers bill the higher of the two. If the DIM bar towers over the scale weight, you are paying for air.
A smaller divisor means a heavier bill for an identical parcel. Divider used by UPS/FedEx in inches (139) and USPS (166); DHL and FedEx international use 5000 in centimetres, with 6000 seen on economy air freight.
Most surprise freight bills come from a box that was never measured against its divisor. Pass this calculator on - it is free, needs no signup.
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DIM weight is only as accurate as the numbers you type in, and size-tier boundaries are often decided by a fraction of an inch. These are the measuring tools sellers keep on the packing table:
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You pay for space, not weight — check your billable weight before the carrier does.
Calculate dimensional (DIM) weight for Amazon FBA shipments to estimate shipping costs and avoid surprise charges from carriers.
Amazon FBA sellers, cross-border ecommerce sellers, freight forwarders, and anyone shipping products that are light but bulky.
Based on Amazon's published DIM weight divisor (139 for domestic US) and standard carrier formulas. Actual charges may vary by carrier and service level.
❓ Amazon Dimensional Weight Calculator — Frequently Asked Questions → fees, limits, and worked examples