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Dimensional Weight Calculator (Amazon FBA, ÷139)

Amazon dim weight = L×W×H ÷ 139, with a 2-in floor on width and height for bulky tiers. See which side governs your billable weight. Free, runs in your browser.

The two rules that produce surprising dim weights

The 2-inch floor on width and height. For Small Bulky, Large Bulky, Overmax and Extra-Large, Amazon assumes the two shorter sides are at least 2 in. Flat items — placemats, decals, fabric, anything shipped stretched — get billed on a phantom thickness. A 30 × 0.5 × 9 in banner is billed as 30 × 2 × 9. Standard tiers have no floor; the same banner at 12 × 6 × 0.5 in would use raw dimensions.

Whether the floor applies is decided by size, not by tier name. The rule switches on when any sorted side exceeds 18 / 14 / 8 in — the Large Standard limits. A light, bulky-looking item still pays standard dim weight until a side crosses that line; a compact item in a heavy tier never gets the floor.

Billable weight is a max, not a blend. Every pound above the dim weight is free; every cubic inch above the actual-weight equivalent is free. That is why shaving packaging down to the next-lowest dim-weight bracket — or splitting one dense item into its own shipment — can step the fee down in a way per-unit tweaks never will.

Divisor: 139 cubic inches per pound — Amazon's US retail divisor, verified 2026-09-28. Freight forwarders use different ratios (167–200 kg per CBM for air and express, 1000 for sea); that is a separate calculation — see the CBM and chargeable-weight calculator.

Where this number goes

Dim weight feeds the FBA fulfillment fee through billable weight, and it feeds freight quotes through volumetric weight — two different ratios applied to the same box. A unit whose dim weight exceeds its actual weight pays the dim weight to Amazon, and its carton pays volumetric weight to the forwarder. The workbench runs both, side by side, against your real carton plan.

Check your whole shipment in the workbench Size tier calculator

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