Small Flat Rate Box
Inside: 8.625 × 5.375 × 1.625 in
Does not fit by these measurements
- Retail
- $14.65
- Eligible commercial
- $12.95
- Published price difference
- $1.70
US-origin postage · Flat Rate boxes
USPS Flat Rate Box Fees: Size and Price Checker
Find a Flat Rate box for your shipment and compare published retail and commercial postage. Check the two Medium sizes separately: the price is the same, but the fit is different.
Rates effective October 4, 2026 · Verified
Check your shipment
Measure the contents together with all protective padding, before putting them in the USPS box. Use a rectangular bounding shape; rotation is allowed, diagonal packing is not tested.
Enter the final weight including the USPS box and all packing materials. Reweigh if you change boxes.
Your Flat Rate comparison
Packed weight: 5 lb. Domestic Flat Rate prices do not vary by zone.
Prices below are reference prices. Finish these checks before using a recommendation:
- Confirm the matching USPS box and normal closure.
Medium Flat Rate Box (format 1)
Inside: 11 × 8.5 × 5.5 in
Fits by dimensions; check physical closure
- Retail
- $25.80
- Eligible commercial
- $22.02
- Published price difference
- $3.78
Medium Flat Rate Box (format 2)
Inside: 13.625 × 11.875 × 3.375 in
Does not fit by these measurements
- Retail
- $25.80
- Eligible commercial
- $22.02
- Published price difference
- $3.78
Large Flat Rate Box
Inside: 12 × 11.75 × 5.5 in
Fits by dimensions; check physical closure
- Retail
- $36.10
- Eligible commercial
- $32.75
- Published price difference
- $3.35
A fitting measurement is a screening result, not a mailability certificate. Check the actual box with all cushioning in place. Commercial prices depend on your postage method; provider fees or negotiated prices may differ.
Base Priority Mail postage per box, for shipments originating in the US. Extra services, insurance upgrades, provider fees, prepaid packaging, Express, envelopes and international postage are not included.
Flat Rate is not always the lowest-cost shipping service. Compare retail Priority Mail by weight and zone if you can use your own packaging.
Published USPS box postage
Reference prices effective 2026-10-04, checked 2026-11-04. Prices are per box; the two Medium formats share a price. These are dated reference figures, not a quote for every shipment.
| Box | Retail | Commercial |
|---|---|---|
| Small Flat Rate Box | $14.65 | $12.95 |
| Medium Flat Rate Box (format 1) | $25.80 | $22.02 |
| Medium Flat Rate Box (format 2) | $25.80 | $22.02 |
| Large Flat Rate Box | $36.10 | $32.75 |
| Large to APO/FPO/DPO | $34.65 | $31.34 |
Price source: USPS Notice 123, domestic Flat Rate, Priority Mail retail and commercial sections. The calculator does not add a separate seasonal amount to these published prices.
What the size check means
The tool compares each protected-content side with the box's published inside dimensions after sorting both sets from shortest to longest. It allows a quarter-turn rotation, without rounding down your measurements. It does not test diagonal packing, compressibility or how several loose items can be rearranged. A match at the exact limit leaves no spare clearance: test the real box.
Packed pounds plus ounces divided by 16 must be positive and at most 70 lb. Flat Rate base postage does not increase with weight within that limit. A box that passes dimensions enters the recommendation only after you confirm USPS packaging and normal closure, any commercial-price eligibility and, for APO/FPO/DPO, destination restrictions.
For example, 10 × 8 × 4 in fits Medium format 1 but exceeds format 2's 3.375 in depth. Both cost $25.80 retail; commercial base postage is $22.02, a $3.78 difference. A 13 × 11 × 3 in item fits format 2, while a 12 × 11 × 5 in item requires Large among the listed boxes.
Use USPS-produced packaging for the matching service. Its normal folds must close without reconstructing or enlarging the box. A regular Priority Mail box or a box of the same size from another supplier is not Flat Rate packaging. Prepaid products are outside this calculation.
Flat Rate box questions
What does it cost to mail a USPS Medium Flat Rate Box?
The checked price list gives $25.80 retail or $22.02 for eligible commercial postage, for either Medium format. Those are base prices, not a guarantee that your contents are mailable or that extra services are included.
Can anyone use the commercial price?
DMM 223.1.2 lists qualifying methods, including USPS Click-N-Ship and approved PC Postage labels. Buying online does not by itself prove that a particular third-party quote matches this published tariff.
Does an overseas APO/FPO/DPO address use international rates?
The domestic Flat Rate rules list APO/FPO/DPO destinations. Large uses its separate reduced price. Check the destination ZIP Code restrictions before mailing; the general 70 lb ceiling is not proof that every military address permits that weight.
Can I use the domestic price for Canada or another country?
No. USPS publishes separate international price groups and weight limits. This domestic checker sends international shipments to USPS for a country-specific quote instead of applying domestic rates.
Is the displayed amount the final shipping bill?
It is base postage. Extra services, insurance upgrades and provider charges are excluded. Hazardous, restricted, live-animal and perishable shipments require a separate check and receive no recommendation here. Weight-and-zone postage or another service may be cheaper.
Sources and review dates
- USPS Notice 123: domestic Flat Rate and Priority Mail retail/commercial. Read .
- USPS DMM 123.1.4: Flat Rate packaging and destinations. Read .
- USPS DMM 223.1.2 and 1.5: commercial eligibility and packaging. Read .
USPS publication frequency is unknown. Our review interval is 30 days; the calculator stops producing shipment estimates when that interval expires. Unpublished future prices and negotiated discounts are excluded.