Will it mail as a letter?
The USPS rules (DMM 101)
- Letter: at least 5″ long × 3½″ high × 0.007″ thick; at most 11½″ × 6⅛″ × ¼″; up to 3.5 oz (First-Class letters over 3.5 oz pay flat prices).
- Pieces over 6″ long or 4¼″ high must be at least 0.009″ thick or they are nonmachinable.
- Nonmachinable if length ÷ height is under 1.3 or over 2.5, or it is rigid, has clasps/strings/buttons, uneven contents (keys, coins, pens), or a non-paper exterior. The surcharge is listed in USPS Notice 123 ($0.49 for First-Class letters as of this check).
- Large envelope (flat): bigger than letter size up to 12″ × 15″ × ¾″, and must be flexible and uniformly thick; otherwise it pays parcel prices.
This checker covers size and shape only. It does not calculate postage.
Sources
- USPS DMM 101 – Physical standards for letters, flats, parcels
- USPS DMM Notice 123 – Price list (nonmachinable surcharge)
Checked 2026-10-21. Postal rules change; confirm with USPS before a large mailing.