How to use the package size helper
- Measure the packed box. Enter the outside length, width and height, rounded up to the nearest whole inch for USPS (or centimetre), and pick inches or centimetres.
- Weigh it packed. Include the product, padding and box. Choose lb, oz, kg or g.
- Pick a divisor. USPS domestic uses 139 and only applies dimensional weight to parcels over 1 cubic foot (1,728 in³). UPS and FedEx daily rates also use 139. Some international or retail accounts still show 166 — pick that option only if your label tool does. Metric couriers use 5000 or 6000 cm³ per kg.
- Read the billable weight. It is the higher of actual and dimensional weight, rounded up to the next pound. That is the weight carriers price on.
- Copy the summary into your Etsy or eBay shipping profile notes so you enter the same size and weight every time.
Dimensional weight explained
Dimensional (volumetric) weight charges for the space a parcel takes up in a truck or plane. The rectangular formula is length × width × height ÷ divisor. A 12 × 12 × 12 inch box is exactly 1,728 in³ (1 cubic foot). Under USPS rules, dimensional weight applies only when the parcel is larger than 1 cubic foot, so that exact cube is usually billed on actual weight. One inch larger on each side (13 × 13 × 13 = 2,197 in³) ÷ 139 = about 15.81 lb dimensional weight — so a 3 lb pillow in that box can be billed as 16 lb. UPS and FedEx use the same 139 divisor without the 1 ft³ threshold on many daily rates.
In metric: 40 × 30 × 20 cm = 24,000 cm³; ÷ 5000 = 4.8 kg dimensional weight.
For the full USPS formula, worked examples, Flat Rate tips and sources, read our guide: USPS dimensional weight explained (billable weight + calculator).
USPS Priority Mail Flat Rate boxes (approximate inside sizes)
| Box | Inside size (in) |
|---|---|
| Small Flat Rate Box | 8 5/8 × 5 3/8 × 1 5/8 |
| Medium Flat Rate Box (top-loading) | 11 1/4 × 8 3/4 × 6 |
| Medium Flat Rate Box (side-loading) | 14 × 12 × 3 1/2 |
| Large Flat Rate Box | 12 1/4 × 12 1/4 × 6 |
Sizes are approximate and were noted in November 2026. Check usps.com for current boxes, prices and the 70 lb limit. This tool does not show live postage prices.
When flat rate wins, and when calculated wins
- Flat rate tends to win for heavy, dense items (books, ceramics, tools) going to far zones (5–8), because the price does not rise with weight or distance.
- Calculated tends to win for light items (under about 2 lb) and short trips (zones 1–4). Ground Advantage is often cheapest for small, light parcels.
- Dimensional weight changes the picture. A big, light box may be billed far above its real weight on calculated rates, so a smaller box or a flat-rate box can save money.
Use it with your pricing
Once you know the billable weight, put the real postage into the Etsy fee calculator or the Shopify & eBay fee calculator to see profit after shipping, build it into your price with the product pricing calculator, and check your monthly target with the break-even calculator. Need to switch cm and inches or grams and ounces? Use the unit converter. For how to set Etsy shipping prices after you know the cost, see how to price Etsy shipping.
Frequently asked questions
What is dimensional weight?
A billing weight based on the size of the box: length × width × height divided by a divisor such as 139 (inches and pounds) or 5000 (centimetres and kilograms). Carriers charge the higher of this and the actual weight. Some international or retail accounts still use 166 — confirm in your label software.
What is billable weight?
The higher of the actual weight and the dimensional weight, usually rounded up to the next whole pound or half kilogram. It is the weight the postage price is based on.
Does USPS use dimensional weight?
Yes. For Priority Mail, Ground Advantage and Priority Mail Express parcels larger than one cubic foot (1,728 cubic inches), USPS uses a divisor of 139 (checked on usps.com and the DMM in November 2026). Smaller parcels are priced on actual weight. Always confirm in your label software.
What divisor do UPS and FedEx use?
139 for standard daily rates in the US. Some accounts and retail counters may show a different divisor (including 166). Check the divisor shown in your shipping account.
Is a flat rate box cheaper?
Often for heavy items going far, because the price is the same up to 70 lb anywhere in the US. For light parcels and nearby zones, calculated shipping is usually cheaper. Compare both in your label software — this tool does not show live postage prices.
Does this tool show postage prices?
No. It does the size and weight maths and gives a general hint. Rates change and depend on your account, so it is an educational estimate, not a postage quote.
How should I measure my package?
Measure the outside of the packed box at its longest points and round up to the nearest whole inch (USPS) or centimetre. Weigh it with all packing materials.
Is my data sent anywhere?
No. All the maths runs in your browser and nothing you enter is stored or sent to our server.