Use dimensional weight for carrier-calculated shipping rates
Many carriers now calculate shipping rates based on the dimensional or volumetric weight of your shipments. The dimensional weight is calculated according to a formula which uses your default shipment dimensions.
If your orders are all a similar physical size, your Default shipment dimensions will generate correct carrier-calculated shipping rates, but if you ship packages in a wide variety of sizes, you should use an app for greater accuracy.
How dimensional rates work
Your default package dimensions are sent to your shipping carrier along with your order's weight to calculate shipping rates for your customer. Dimensional weight is determined by the carrier based on the height, width, and depth of your packages.
If your dimensional weight is larger than your order's weight, your customer will be charged for the dimensional weight of their order. If you feel that the generated rates are too high or too low, you can adjust your real-time carrier rates to increase or decrease your shipping rates as needed.
Dimensional weight calculations vary between carriers. Consult the carrier you're using for details about their dimensional weight policies.
Carriers with dimensional weight policies:
Carriers without dimensional weight policies:
Configure your default package dimensions
From your Shopify admin, click Settings, and then click Shipping.
In the Packages section, click the title of the package that's beside the Default label:
Enter a title, the dimensions, and weight of your default shipment package: