About BoxFit & our method

BoxFit is a free, no-sign-up tool for online sellers and anyone mailing a package in the U.S. It answers two questions: which USPS Flat Rate box or envelope will my item fit in, and is that actually the cheapest way to ship it? Everything runs in your browser.

How we check the fit

• Boxes: we sort your item's three sides and the box's inside sides from longest to shortest and compare them one by one, so the item may be turned any way. If you add padding, it is added to every side.
• Envelopes: an envelope wraps around an item, so the item's thickness is subtracted from both the length and the width. We allow up to 1 inch of thickness in the paper and Legal envelopes and 1.5 inches in the Padded envelope — the flap must close normally.
• Weight: every Flat Rate option takes up to 70 lb.

How we price Ground Advantage

• Prices are by weight and zone from the USPS Ground Advantage table. Under 1 lb the steps are 4, 8, 12 and 15.99 oz; above that, weight rounds up to the next pound.
• Dimensional weight: length × width × height ÷ 166, rounded up. USPS uses it only when the parcel is larger than one cubic foot (1,728 cubic inches); you pay the higher of actual and dimensional weight.
• Parcels longer than 22 inches add $4.50 (over 30 inches, $10.00), parcels over 2 cubic feet add $21.00, and over 108 inches length + girth use the oversized price.

Price source

All prices come from the official USPS price list, USPS Notice 123, effective October 4, 2026. “Online label” uses Commercial prices — what you pay buying a label on USPS.com or through most shipping apps. “Post Office counter” uses Retail prices. Prices were last reviewed on 2026-11-10; USPS changes them each January and July and sometimes for the holiday season.

What we don't include

Insurance, signature confirmation, packaging cost, hazmat or live-animal fees, and private carriers are not included. Priority Mail by weight and cubic pricing are not compared. Results are estimates, not a postage quote.

How we're funded

The site is free and supported by Google AdSense ads. Ads never change any result.

Contact

Found an outdated price? Visit our contact page.