Flat rate vs weight-based
USPS international flat rate boxes: sizes, weight limits, prices, and when they beat weight-based pricing
Last reviewed on October 31, 2026.
USPS Priority Mail International offers flat rate envelopes and boxes. Pack your contents, and the price is fixed by the container and the destination's price group, regardless of weight up to the container's limit. UPS, FedEx and DHL, and other USPS services, price by weight, dimensions and zone.
Flat rate looks like a clear win, but the answer depends on the box. Small containers beat weight-based prices at almost any weight but have no tracking. Medium and Large boxes only pay off above a certain weight. This page covers the sizes, current reference prices, break-even weights, and when to avoid flat rate.
International flat rate sizes and limits
Figures from the USPS Priority Mail International page, inside dimensions unless noted:
| Container | Size (inches) | Max weight | USPS Tracking |
|---|---|---|---|
| Flat Rate Envelope (also Small, Window, Padded, Legal) | 12½ × 9½ outside; Legal 15 × 9½; Small 10 × 6 | 4 lb | No |
| Small Flat Rate Box | 8⅝ × 5⅜ × 1⅝ | 4 lb | No |
| Medium Flat Rate Box 1 (top-loading) | 11 × 8½ × 5½ | 20 lb | Yes |
| Medium Flat Rate Box 2 (side-loading) | 13⅝ × 11⅞ × 3⅜ | 20 lb | Yes |
| Large Flat Rate Box | 12 × 11¾ × 5½ | 20 lb | Yes |
| Large Flat Rate Board Game Box | 2311⁄16 × 11¾ × 3 | 20 lb | Yes |
USPS also lists DVD and Large Video Flat Rate Boxes, priced as Small boxes. Packaging is free from the Post Office or by online order. The flaps must close within the normal folds, so don't bulge the box. Some countries set lower limits, shown in the Individual Country Listing.
Reference prices
Retail prices from USPS Notice 123 as of October 2026. Commercial prices are a little lower, and USPS changes these periodically, so treat them as estimates and check the current figure before mailing.
| Container | Group 1 (Canada) | Groups 2 to 8: lowest | Groups 2 to 8: highest |
|---|---|---|---|
| Flat Rate Envelope | $32.65 | $48.80 | $62.35 |
| Small Flat Rate Box | $33.75 | $50.45 | $64.45 |
| Medium Flat Rate Box | $61.80 | $90.85 | $133.10 |
| Large Flat Rate Box | $75.15 | $102.55 | $155.00 |
Find your destination's price group in its country listing. Group numbers are not in geographic order, and First-Class Package International Service uses its own groups.
The decision rule
Comparing the same Notice 123 retail prices gives approximate break-even weights, above which the flat rate box is cheaper than Priority Mail International by weight:
| Price group | Small box | Medium box | Large box |
|---|---|---|---|
| Group 1 (Canada) | Any weight | About 6 lb | About 8 lb |
| Group 3 | Any weight | About 6 lb | About 9 to 10 lb |
| Group 5 | Any weight | About 6 lb | About 7 to 8 lb |
| Group 7 | Any weight, by a smaller margin | About 10 lb | About 12 to 13 lb |
- Up to 4 lb and it fits a Small box or envelope: the flat rate price beats Priority Mail International by weight. Compare it with First-Class Package International Service, and decide whether you can live without tracking.
- Under about 6 lb in a Medium box (about 10 lb in Group 7): your own box priced by weight is usually cheaper.
- Above the break-even and up to 20 lb, if it fits: flat rate usually wins, and the gap grows quickly with weight. At 20 lb to Group 5, weight-based is about $187, against $95.95 for a Medium box.
- Over 20 lb: flat rate is not available. Use weight-based pricing or a courier.
Worked comparison
Sample retail prices to a Group 5 destination (Notice 123, estimates only):
| Shipment | By weight | Flat rate option | Cheaper |
|---|---|---|---|
| 2 lb item in a Medium box | $77.05 | $95.95 | By weight |
| 3 lb book in a Small box | $83.15 | $54.00 | Flat rate, no tracking |
| 10 lb of tools in a Medium box | $125.80 | $95.95 | Flat rate |
| 15 lb in a Large box | $156.70 | $108.85 | Flat rate |
A courier may still beat these for some weights under a discounted account, and it will be faster. Compare totals, including the fuel surcharge, using the rate calculator and how rates work.
Where flat rate is the right call
- Heavy, small items. Books, hardware, tools and ceramics that fill a box to well over the break-even weight.
- Several items consolidated. Everything in one Medium box is a single fixed price.
- Light but bulky contents that fit. Flat rate is unaffected by dimensional weight.
- Destinations with high weight-based prices, once you are over the break-even weight.
Where flat rate is wrong
- Light, small items. A 4-ounce gift is usually cheaper by First-Class Package International Service, from about $19 to $26 at retail. Letters and flat documents can be cheaper still by First-Class Mail International.
- Anything you need to track. Flat Rate Envelopes and Small boxes have no USPS Tracking. Medium and Large boxes do, though it may be limited in some destinations.
- Medium or Large boxes holding little weight. You pay for capacity you are not using.
- Destinations with weak last-mile delivery. The price may win, but check the delivery times and whether USPS has suspended the destination.
- Anything over 20 pounds, or over 4 pounds in a Small box or envelope.
Practical packing tips
- Use the official container. The flat rate price applies only to USPS-supplied flat rate packaging.
- Don't overstuff. Flaps must close within the normal folds.
- Tape the seams. International transit is rougher than domestic.
- Check the insurance. Priority Mail International normally includes up to $200 for merchandise, with some exceptions. See shipping insurance.
- Check what's allowed. The usual USPS rules apply. Ammunition, perfume and loose batteries can't go in any box.
- Sort the customs form first. USPS requires customs data to be sent electronically. Use Click-N-Ship or approved software, or fill in PS Form 2976-R and present it at the counter. The older handwritten forms are obsolete. See the customs forms guide.
Common mistakes
- Assuming every flat rate box goes to 20 lb. Small boxes and envelopes stop at 4 lb.
- Assuming flat rate means tracked. Small boxes and envelopes are not.
- Using flat rate for ordinary documents. First-Class Mail International is usually much cheaper.
- Forgetting that flat rate is USPS-only here. UPS, FedEx and DHL don't offer the same model on ordinary international service, so you are comparing carriers, not just services.
- Not checking the side-loading Medium. The two Medium shapes are very different. The side-loading version suits flat items that won't fit the top-loading one.
Where to go next
- Dimensional weight: the concept flat rate sidesteps.
- International shipping rates: fuel, discounts and list prices.
- USPS vs UPS: the choice is often a flat rate box against UPS Worldwide Expedited.
- Customs forms: what you need for a flat rate box.
- Rate calculator: reference prices for your shipment.
- Shipping to Canada: the lowest flat rate group.