US domestic postage · Retail rates
USPS Priority Mail Price Calculator: Flat Rate vs. by Weight
Compare USPS retail postage for your domestic parcel with Flat Rate packaging. Enter packed weight, outside dimensions and the USPS zone to see which option costs less.
Rates effective October 4, 2026 · Verified
Your parcel
Weigh the packed parcel. Measure its outside dimensions at the widest points.
Find your USPS zone from the origin and destination ZIP Codes.
Retail price comparison
Lowest compared price: $22.85 · By weight and zone
Among by-weight postage, fitting boxes and any envelopes you confirmed. Flat Rate prices require the corresponding USPS-produced packaging.
By weight and zone 5
Billable weight: 4 lb. Volume does not exceed 1,728 in³; actual weight is rounded up to the next pound.
Actual: 3.2 lb → 4 lb rounded up.
DIM dimensions: 10 × 8 × 4 in. Volume: 320 in³. No dimensional weight charge.
- Weight/zone price
- $22.85
- Length surcharge
- $0.00
- Volume surcharge (320 in³)
- $0.00
- Total by weight
- $22.85
Flat Rate boxes that fit
Box fit compares your three measurements with the sorted inside dimensions, allowing rotation. Leave room for cushioning. If your measurements include an outer carton, remeasure the protected contents before repacking into USPS Flat Rate packaging.
- Medium Flat Rate Boxes (format 1): $25.80
Inside: 11 × 8.5 × 5.5 in - Large Flat Rate Box: $36.10
Inside: 12 × 11.75 × 5.5 in
Flat Rate envelopes: check closure
Envelopes are conditional: use only if the flap closes normally, without altering the USPS packaging. Dimensions alone do not establish fit. Confirm each envelope below to include it in the cheapest-option comparison.
Retail prices only, effective October 4, 2026. Commercial and online label discounts are not included. This estimate includes dimensional weight and length/volume fees; contents-related fees, extra services and special military rates are excluded. Confirm mailability and size limits with USPS.
How it's calculated
Prices come from USPS Notice 123, effective October 4, 2026. Weight and packaging rules come from DMM 123.1. Both sources were read on ; the review interval is 30 days.
Add pounds and ounces ÷ 16, then round any fraction up to a full pound. The minimum price is the 1 lb row. For dimensional weight, round each outside dimension up to a whole inch and multiply. For nonrectangular parcels, multiply that volume by 0.785. Only if the result exceeds 1,728 in³, divide by 139 and round up to pounds. Compare with rounded actual weight and charge the greater, with dimensional pricing capped at 70 lb. Actual weight over 70 lb is ineligible.
The selected zone gives the column in the 1–70 lb retail table. A longest side over 22 in and up to 30 in adds $4.50; over 30 in adds $21.00 instead. Volume over 3,456 in³ adds $35.00 and can stack with the length fee. For this volume fee, DMM 123.1.6 rounds dimensions to the nearest whole inch, then applies 0.785 for nonrectangular parcels. This differs from the upward rounding used for dimensional weight.
Flat Rate packaging has no dimensional, length or volume charge and accepts up to 70 lb. The box comparison sorts both sets of dimensions and checks every side against the published interior. The two Medium formats are evaluated separately. Envelope fit requires your confirmation that the flap closes normally.
Contents-related nonstandard fees (such as tubes, cans or some liquid containers), hazardous-material and live-animal/perishable fees, insurance upgrades and other extra services are outside this comparison. A nonrectangular shape alone does not determine those fees. The result estimates postage; it does not certify that an item meets every USPS mailing or size restriction.
Frequently asked questions
Is Flat Rate always cheaper than shipping by weight?
No. At 3.2 lb, 10 × 8 × 4 in and zone 5, by-weight retail postage is $22.85, below the $25.80 Medium Box. At 9 lb, 11 × 8 × 5 in and zone 8, the $25.80 Medium Box beats $58.55 by weight. USPS packaging must fit the protected contents.
Are these retail or commercial prices?
Retail only, from Notice 123 effective October 4, 2026. Online postage providers and commercial accounts can have different rates. USPS Priority Mail Express and international postage are separate services.
How do I find my USPS zone?
Use the USPS Domestic Zone Chart with your origin and destination ZIP Codes, then select zone 1–9 here. Flat Rate prices do not change by domestic zone.
Can I use my own box or stretch a Flat Rate envelope?
Use your own packaging for the by-weight option. Flat Rate prices require the matching USPS-produced packaging. An envelope is only a candidate if its flap closes normally; its two published dimensions do not prove that a bulky item fits.
Why is billable weight higher than my scale weight?
USPS rounds fractional pounds up. A large, light parcel over 1,728 in³ can also be priced using dimensional weight. The calculator shows rounded measurements, volume, the 139 divisor and the selected billable weight so you can check each step.