USPS Flat Rate Box Sizes, Dimensions and Prices (2026)

Updated September 27, 2026Prices effective July 12, 2026Inside and outside dimensions

USPS flat rate box sizes (inside dimensions): Small 8 5/8 x 5 3/8 x 1 5/8" · Medium 1 top-loading 11 x 8 1/2 x 5 1/2" · Medium 2 side-loading 13 5/8 x 11 7/8 x 3 3/8" · Large 12 x 11 3/4 x 5 1/2". Retail postage from July 12, 2026: Small $13.65 · Medium $24.80 · Large $34.00.

USPS flat rate envelope sizes: Flat Rate Envelope 12 1/2 x 9 1/2" · Window 12 1/2 x 9 1/2" · Small 10 x 6" · Legal 15 x 9 1/2" · Padded 12 1/2 x 9 1/2". Retail postage: $12.90 for the standard, window and small envelopes · $13.25 legal · $14.00 padded.

Updated September 27, 2026. Dimensions and prices checked against USPS Notice 123 (effective July 12, 2026), the USPS Priority Mail page and the Postal Store. Commercial prices apply to Click-N-Ship and other commercial postage. USPS supplies the packaging free; the prices are postage.

USPS Priority Mail Flat Rate sizes and prices, effective July 12, 2026
USPS flat rate packaging Inside size (in) Outside size (in) Inside cubic inches Retail Commercial
Small Flat Rate Box 8 5/8 x 5 3/8 x 1 5/8 8 11/16 x 5 7/16 x 1 3/4 75 $13.65 $12.10
Medium Flat Rate Box 1 (top-loading) 11 x 8 1/2 x 5 1/2 11 1/4 x 8 3/4 x 6 514 $24.80 $21.17
Medium Flat Rate Box 2 (side-loading) 13 5/8 x 11 7/8 x 3 3/8 14 1/8 x 12 x 3 1/2 546 $24.80 $21.17
Large Flat Rate Box 12 x 11 3/4 x 5 1/2 12 1/4 x 12 x 6 776 $34.00 $31.00
Large Flat Rate Box, APO/FPO/DPO 12 x 11 3/4 x 5 1/2 12 1/4 x 12 x 6 776 $32.55 $29.59
Flat Rate Envelope 12 1/2 x 9 1/2 or smaller envelope n/a $12.90 $11.12
Window Flat Rate Envelope 12 1/2 x 9 1/2 envelope n/a $12.90 $11.12
Small Flat Rate Envelope 10 x 6 envelope n/a $12.90 $11.12
Legal Flat Rate Envelope 15 x 9 1/2 envelope n/a $13.25 $11.66
Padded Flat Rate Envelope 12 1/2 x 9 1/2 envelope n/a $14.00 $11.99

Box rows show USPS inside and outside dimensions; envelope rows show the envelope size. Inside volume is rounded to the nearest cubic inch. Fit uses inside dimensions; allow room for cushioning. Prices are the Notice 123 base rates effective July 12, 2026; dimensions match the USPS Priority Mail page and the Postal Store.

USPS Priority Mail Flat Rate box sizes and prices

Domestic Flat Rate postage uses USPS-produced packaging for qualifying mailable contents up to 70 lb, subject to content and destination restrictions. USPS supplies the boxes free. Base prices in the table above come from Notice 123, effective July 12, 2026. Commercial eligibility includes Click-N-Ship and qualifying postage platforms; use the price offered by your account.

Standard Priority Mail has an expected delivery time of 2–3 days, depending on origin, destination, and acceptance time, without a money-back delivery guarantee. Eligible shipments include tracking and up to $100 insurance, with restrictions. USPS service details

Fit uses inside dimensions. Inside volume is rounded to the nearest cubic inch. Allow room for cushioning; do not subtract a fixed allowance from the outside size. The Medium shapes share a postage price but have different clearances.

Medium 2 source note: Notice 123 lists an outside length of 14 1/8 in; the USPS consumer page lists 14 in. This table follows Notice 123. Measure the actual carton when outside clearance matters.

Flat Rate envelope sizes and prices

Domestic Flat Rate envelopes share the 70 lb service ceiling for qualifying contents. Flaps must close along their original folds. Choose another package if the contents prevent normal closure. USPS packaging requirements

Envelope prices in the table above are the July 12, 2026 base rates in Notice 123. The window-envelope entry lists envelope dimensions, not the address-window opening.

Flat Rate or your own box: compare the finished shipment

There is no reliable weight-only cutoff. Compare quotes for the same origin, destination, mailing date, and postage account. Include carton and cushioning costs, any additional fees, and required delivery timing.

  1. Check fit: compare each inside dimension and allow for cushioning and normal closure.
  2. Measure the packout: use the sealed parcel's exterior dimensions and total weight for postage.
  3. Compare the complete cost: a different service can have a different delivery estimate.

Compare a shipment with the USPS price calculator or use your postage platform's current quote.

Nearby stock-carton sizes to check

These Packrift cartons are candidates for a fit check, not USPS Flat Rate packaging or guaranteed substitutes. Listed measurements are nominal inside dimensions. Check cushioning space, tolerances, and carton strength. Similar volume does not guarantee the same fit, and the USPS weight ceiling does not establish a carton load rating.

USPS reference Candidate carton (inside in) Fit and postage check
Small Check 9 x 6 x 2 boxes
SKU 962; 25-pack		 More nominal clearance in each dimension; check the item with cushioning.
Medium 1, top loading Check 12 x 9 x 6 boxes
SKU 1296; 25-pack		 Compare the finished item and cushioning with all three inside dimensions.
Medium 2, side loading Check 14 x 12 x 4 boxes
SKU 14124; 25-pack		 A shallow candidate with more nominal clearance; test the complete packout.
Large Check 12 x 12 x 6 boxes
SKU 12126; 25-pack		 Length matches the nominal Large interior. Allow for tolerances and check tight fits.
Legacy Board Game Check 24 x 12 x 4 boxes
SKU 24124; 25-pack		 Measure the packed exterior and check nonstandard-length fees. Do not assume the same postage.

Your own carton needs its own postage quote. Do not use inside volume to rule out carrier fees. See the dimensional weight chart and the current rules below.

Get a quote for the cartons above

Choose a carton to prefill its SKU, then enter your box quantity and delivery ZIP. Each listed carton comes in a 25-box bundle.

This quote is for Packrift cartons. Postage is separate; these cartons do not qualify for USPS Flat Rate postage.

Packaging and postage rules to check

  • Use the designated service. USPS Priority Mail packaging cannot be used with Ground Advantage postage, even with markings covered. DMM 601.6.1
  • Check content and destination restrictions. Domestic Priority Mail generally permits qualifying contents up to 70 lb; military and diplomatic destinations can have additional restrictions. DMM 123
  • Use exterior measurements for DIM weight. Current retail rectangular Priority Mail rules round exterior dimensions up to whole inches. Above 1,728 cubic inches, compare actual weight with dimensional weight calculated using divisor 139. Being below this threshold does not eliminate other fees: packages longer than 22 inches can incur a nonstandard-length fee. DMM 123.1.3 and 123.1.6
  • Compare Cubic when eligible. Current commercial Priority Mail Cubic limits are 0.50 cubic foot or less, 20 lb or less, and no dimension over 22 inches. Pricing depends on volume tier and zone. The July 12, 2026 rule uses 22 inches; older USPS FAQ guidance still mentions 18. DMM 223.1.3
  • Regional Rate prices ended, but remaining cartons can still be used. Since January 22, 2023, remaining boxes use applicable weight-based prices; Priority Mail packaging restrictions still apply. USPS Regional Rate guidance

For other sizes, use the box size chart or Uline model cross-reference. For other carriers, see package size and weight limits.

Legacy Board Game Flat Rate box

USPS discontinued production of this shape in 2022. Its February 24, 2022 domestic notice preserves applicable Flat Rate prices for remaining USPS-made boxes, including qualifying military and diplomatic mail. This is an existing-stock reference, not a currently produced supply option.

Legacy packaging Outside dimensions Inside dimensions Rounded inside cubic inches Domestic retail Domestic commercial
Large Flat Rate Board Game Box 24 1/16 x 11 7/8 x 3 1/8 in 23 11/16 x 11 3/4 x 3 in 835 $34.00 $31.00

These legacy dimensions and domestic base prices still appear on the USPS Priority Mail page. Confirm applicable postage for the destination; military and diplomatic treatment differs.

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Frequently asked questions

What are the USPS flat rate box sizes?

Notice 123 lists outside dimensions of Small 8 11/16 x 5 7/16 x 1 3/4 in; Medium 1 11 1/4 x 8 3/4 x 6 in; Medium 2 14 1/8 x 12 x 3 1/2 in; and Large 12 1/4 x 12 x 6 in. USPS consumer guidance gives Medium 2 length as 14 in, so measure the actual carton when clearance is tight. Use the separate inside measurements for fit; do not subtract a fixed allowance.

How much does a USPS flat rate box cost in 2026?

The July 12, 2026 base retail prices are $13.65 for Small, $24.80 for either Medium, and $34.00 for Large. Corresponding commercial prices are $12.10, $21.17, and $31.00. Check your applicable postage quote before mailing.

What is the weight limit for a flat rate box?

Domestic Priority Mail Flat Rate packaging accepts qualifying mailable contents up to 70 lb, subject to content and destination restrictions. The postage-service ceiling is not a carton load rating.

Is the medium flat rate box cheaper than the large?

At the July 12, 2026 base retail rates, either Medium shape is $24.80 and Large is $34.00. Choose enough clearance for your item and cushioning; similar volume does not guarantee the same fit.

Can I use my own box at the flat rate price?

No. USPS Flat Rate postage requires USPS-produced Flat Rate packaging. Quote your own carton using its packed weight and exterior dimensions. Eligible commercial Priority Mail shipments may also qualify for Cubic pricing.

Is using my own box always cheaper for light items?

No. Compare quotes for the same origin, destination, mailing date, and postage account. Include packaging, additional fees, and required delivery timing. Weight alone does not identify the cheaper option.

Can I still use Regional Rate or Board Game boxes?

Regional Rate prices ended in January 2023, but remaining cartons can still use applicable weight-based prices. Board Game Flat Rate box production ended in 2022; USPS preserved applicable domestic and military or diplomatic Flat Rate treatment for remaining stock. Neither is a currently produced packaging option.

Official references: current prices, service and dimensions, retail rules, commercial rules, Regional Rate guidance, Board Game notice.