Most households need a standard-size curbside mailbox: roughly 6–8" wide, 6–8" high and 18–21" long, which USPS defines as any box that accepts an 18½" × 5" × 6" test block. Choose a large-capacity box (accepts a 16" × 13" × 7" block) only if you routinely receive padded mailers and small parcels at the curb and don't want them on the porch. For wall-mounted boxes there's no USPS size class; a box around 15" × 6" × 4½" holds a day's letters and a magazine, and taller vertical designs add capacity for a few days' mail.
Key facts
- USPS-STD-7C standard test gauge: 18.50" L × 5.00" W × 6.00" H must fit inside.
- Large-capacity (locked large capacity) test gauge: 16.00" L × 13.00" W × 7.00" H.
- Since 2001 USPS uses a flexible dimension range rather than fixed sizes: length 18 9⁄16"–22 13⁄16", width 6¼"–11", height 6"–15".
- The old fixed sizes were T1/C1 (18½ × 5 × 6), T2/C2 (19½ × 6 × 7) and T3/C3 (22½ × 8 × 11½), set in 1992.
- A locking mail-slot box must have a slot at least 1¾" × 10" for USPS approval.
How does USPS define mailbox sizes?
Since 2001, by performance rather than by fixed dimensions. A curbside box is acceptable if it accepts the standard test gauge — a rectangular block 18½" long, 5" wide and 6" high — and its overall dimensions fall inside the allowed range. "Traditional" boxes are the dome-top form; "Contemporary" boxes are any other shape meeting the same requirement. Large-capacity boxes accept a wider, 13"-wide block and are mostly locking parcel boxes.
Before 2001, USPS published fixed sizes, and you'll still see the names on product pages:
| Size (1992–2001) | Interior dimensions (L × W × H) | Notes |
|---|---|---|
| T1 / C1 | 18½" × 5" × 6" | Small; the same as today's test gauge |
| T2 / C2 | 19½" × 6" × 7" | The common "standard" |
| T3 / C3 | 22½" × 8" × 11½" | Large; parcels |
"T" was traditional, "C" contemporary. Older still: No. 1 (about 19" × 6½" × 8½") and No. 2 (23½" × 11½" × 13½") from the 1950s, and Joroleman's original 1915 No. 1 (18½" × 6¼" × 7½"). USPS's own size chronology is in the sources.
What size do most homes need?
Standard. Letters, cards, magazines, catalogs and most padded mailers fit a standard box. Amazon-style parcels usually go to the porch regardless of your box, because carriers deliver parcels to the door when the box is full or the item won't fit. A standard box on a slim post is also the right proportion for most houses; a T3-size box on a residential lot looks like a rural route.
Size up when: you receive parcels at the curb several times a week and your driveway is long; you have a rural route where parcels are otherwise left on the ground; you travel and mail accumulates; or you want a locking parcel box.
Wall-mounted mailbox sizes
There is no USPS size class for wall boxes. Practical guidance:
| Wall box type | Typical size | Holds |
|---|---|---|
| Horizontal / low profile | 14–16" W × 5–7" H × 4–5" D | A day's letters plus a magazine flat |
| With letter tray / newspaper holder | Adds ~3" height | Newspapers, rolled catalogs |
| Vertical / upright | 9–10" W × 13–18" H × 5–6" D | Several days' mail; locking versions hold flats upright |
Check the door opening, not just the exterior. A box with a 12" × 1½" door opening won't take a magazine flat.
How do I measure my mail?
Take a week's mail and stack it. If it's under 2" thick and nothing is wider than 9", a standard curbside box or a low-profile wall box is right. If it includes padded mailers wider than 9" or more than 2" thick, consider a large-capacity curbside box or an upright wall box. If you get parcels that would fill a shoebox, you're looking at a parcel box or accepting porch delivery.
Does size affect USPS approval?
Only at the extremes. Anything within the flexible range that accepts the test gauge is fine. A very small decorative box that won't take an 18½" × 5" × 6" block can be refused. On the large end, boxes over the maximum dimensions are considered parcel lockers and have separate rules.
Where Modbox fits
The Modbox curbside is 7" W × 7¾" H × 21¼" L — a standard-size contemporary box, sized to the proportions of the 1960s originals. The Aerobox locking curbside is 10¼" W × 7¾" H × 20.4" L, larger inside for flats and small parcels. Wall-mounted: the Vista is 15" L × 6" H × 4½" D (9" high with the optional letter tray); the Courier is an upright 9½" W × 13½" H × 5¾" D (18" with tray).
Frequently asked questions
What is a standard mailbox size? A curbside box that accepts USPS's 18½" × 5" × 6" test gauge; in practice about 6–8" wide, 6–8" high and 18–21" long.
What's the difference between T1, T2 and T3? Pre-2001 USPS size classes: small, standard and large. T2 is the common residential size. Today's standard is defined by the test gauge instead.
Can I use a large mailbox at a suburban house? Yes, as long as it's within USPS dimensions and mounted correctly. It will look oversized on a small lot.
Do I need a bigger mailbox for packages? Only if you want them in the box rather than on the porch. Carriers deliver parcels to the door when they don't fit.
Sources
- USPS, "Curbside Mailboxes: A Chronology of Size Changes" — https://about.usps.com/who-we-are/postal-history/curbside-mailboxes-size-chronology.pdf
- USPS-STD-7C, Standard Mailboxes, Curbside — https://about.usps.com/publications/engineering-standards-specifications/spusps-std-7c.pdf
- Modbox USA product pages — https://modboxusa.com/collections/mailboxes