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USPS Postcard Size: Minimum 3.5×5 in, Maximum 4.25×6 in
The two size limits compared
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USPS DMM 201.1.2.1
USPS postcard minimum (3.5×5 in)
5 × 0.007 in (127 × 0.18 mm)
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USPS DMM 201.1.2.1
USPS postcard max, postcard rate (4.25×6 in)
6 × 0.016 in (152.4 × 0.41 mm)
The short answer
- Minimum3.5×5 in (88.9×127 mm), 0.007 in (0.18 mm) thick
- Maximum (postcard rate)4.25×6 in (107.95×152.4 mm), 0.016 in (0.41 mm) thick
- DifferenceMaximum is 0.75 in (19.05 mm) taller and 1 in (25.4 mm) longer than minimum
See the sizes in AR (shown as boxes)
The two sizes as boxes at their published dimensions
Preparing 3D…
1 Postcard min2 Postcard max
On a phone, the "View in your room (AR)" button places the boxes in your room. The boxes are locked to the published dimensions and cannot be resized. They show size only and are not 3D replicas of the products.
Drawn to the same scale
Dimensions side by side
| Dimension | Minimum | Maximum (postcard rate) | Difference |
|---|---|---|---|
| Height | 3.5 in (3-1/2 in) / 88.9 mm | 4.25 in (4-1/4 in) / 107.95 mm | 0.75 in (19.05 mm) taller |
| Length (width) | 5 in / 127 mm | 6 in / 152.4 mm | 1 in (25.4 mm) longer |
| Thickness | 0.007 in / 0.18 mm | 0.016 in / 0.41 mm | 0.009 in range |
| Area | 17.5 in² (112.9 cm²) | 25.5 in² (164.5 cm²) | Maximum is 45.7% larger |
Millimeter values converted at 1 in = 25.4 mm and rounded to two decimal places. Thickness range is the caliper span, not a difference between cards.
What the numbers mean
USPS defines two thresholds. The first is the minimum: no mailpiece shorter than 3-1/2 inches, narrower than 5 inches, or thinner than 0.007 inch can be mailed at all. The second is the postcard rate ceiling: 4-1/4 × 6 inches. A card that fits inside both ends gets the postcard rate, which is cheaper than Letter.
A card larger than 4-1/4 × 6 inches — say, a 5 × 7 inch card — is still a card and not a flat as long as it stays within 6 × 9 inches and 0.016 inch thick. But the postcard rate no longer applies: the piece is charged the First-Class Mail Letter rate. Cards thicker than 0.016 inch, or larger than 6 × 9 inches, are classified as flats and charged flat rates.
Aspect ratio also matters: a card must be rectangular, uniformly thick, and have a length-to-height ratio between 1.3 : 1 and 2.5 : 1 (DMM 601.1.1). A 3.5 × 5 inch card has a ratio of 1.43 : 1, within range. A 4.25 × 6 inch card has a ratio of 1.41 : 1, also within range.
Product links
FAQ
What is the minimum size for a USPS postcard?
A USPS postcard must be at least 3-1/2 inches high, 5 inches long, and 0.007 inch thick (88.9 × 127 mm, 0.18 mm). Pieces smaller than this cannot be mailed.
What is the maximum size for a USPS postcard?
To qualify for First-Class Mail postcard pricing, a card must be no more than 4-1/4 inches high, 6 inches long, and 0.016 inch thick (107.95 × 152.4 mm, 0.41 mm). Cards larger than this are charged the Letter rate.
What happens if my postcard is larger than 4.25 × 6 inches?
A card that exceeds 4-1/4 inches in height or 6 inches in length still qualifies as a card (not a flat) if it is no more than 6 inches high, 9 inches long, and 0.016 inch thick — but it is charged the First-Class Mail Letter rate, not the postcard rate.
See also: USPS Flat Rate Box Sizes — Small, Medium, and Large Priority Mail boxes compared. US Envelope Sizes — #10, #9, #6¾, A2, and A7 drawn to the same scale. 9×12 vs 10×13 Catalog Envelopes — the two standard catalog sizes compared.
Sources
- USPS Domestic Mail Manual, Section 201.1.2.1, "Physical Standards for Cards": minimum 3-1/2 × 5 in, 0.007 in thick; maximum (postcard rate) 4-1/4 × 6 in, 0.016 in thick. pe.usps.com/text/dmm300/201.htm, checked 2026-10-03.
- Credit card reference: ISO/IEC 7810 ID-1, 85.60 × 53.98 mm (shown as 85.6 × 54 mm).