Measure first when the box looks empty
If you can shake the contents around inside the carton, the tape measure is going to decide your price, not the scale. Do the volume arithmetic before you tape it shut, while downsizing is still an option.
Answered with two quotes
Does UPS charge by weight or by size?
Both, and the larger of the two numbers is the one you pay on. UPS compares the scale reading against a size-based weight worked out from the outside of the carton, then rates the parcel on whichever comes out bigger, so a light roomy box and a heavy compact one can land on exactly the same price.
Two parcels, one laneEleven pounds apartSame quoted price
The scale is not the referee
Whichever number is higher becomes the billable weight, and on bulky boxes it is almost never the one you weighed.
Eleven pounds apart, priced the same
Two parcels went down the same lane on the same morning. One was heavy and compact, the other light and roomy. Watch what the scale column does, then watch what the price columns do not do.
| Parcel | Scale weight | Carton | Size weight at 139 | UPS Ground | UPS Ground Saver |
|---|---|---|---|---|---|
| Heavy and compact | 16 lb | 16 x 14 x 10 in | 17 lb | $23.70 | $20.50 |
| Light and roomy | 5 lb | 18 x 14 x 10 in | 19 lb | $23.70 | $20.50 |
Eleven pounds of difference on the scale, zero difference in the price, on both services. The heavy parcel is dense enough that its size weight barely clears its actual weight. The light one is roomy enough that volume takes over completely. Once the billable weights land close together, the rate card cannot tell the two parcels apart.
When size wins outright
Push the shape difference further and the lighter parcel becomes the more expensive one. On the same lane and the same day, 5 lb inside a 24 x 18 x 14 in carton quoted $36.75 on UPS Ground, while 32 lb inside a 20 x 16 x 12 in carton quoted $32.09. The parcel that weighed 27 lb less cost $4.66 more.
Padding is cheap, headroom is not
Bubble wrap and paper add ounces. A carton three inches larger on every side adds thousands of cubic inches. Protect the item tightly inside the smallest box that closes square rather than buying room you then have to fill.
The divisor is published
UPS states 139 for Daily Rates and 166 for Retail Rates on its shipping dimensions and weight page, checked 2026-07-28, and rounds any fraction up to the next whole pound. Nothing about the comparison is hidden, it just happens after you have already packed.
Weight against size, questioned
Does UPS charge by weight or by size?
By both, taking whichever produces the larger number. The scale weight is compared against a size-based weight from the carton dimensions, and the parcel is rated on the greater of the two.
Can two parcels of different weights cost the same at UPS?
Yes, and it happens often. On 2026-07-28, Newark to Atlanta, a 16 lb parcel in a 16 x 14 x 10 in carton and a 5 lb parcel in an 18 x 14 x 10 in carton both quoted $23.70 on UPS Ground and $20.50 on UPS Ground Saver.
When does a lighter parcel cost more than a heavier one?
When it is bulkier. The same capture put 5 lb in a 24 x 18 x 14 in carton at $36.75 on UPS Ground and 32 lb in a 20 x 16 x 12 in carton at $32.09, so the lighter parcel cost $4.66 more.
How do I stop paying for size?
Shrink the carton rather than the padding. Every inch you take off all three sides removes hundreds of cubic inches, and once the size-based weight drops below the scale weight you are back to paying for what you actually shipped.