Table of Contents
Flat rate shipping is a pricing model where the package’s size, not its weight or destination, sets the postage: with USPS Priority Mail Flat Rate, anything that fits in the box and weighs up to 70 pounds ships for one price to any domestic address. It’s the best deal for small, heavy items going far, and usually the wrong choice for light items or short distances, where weight-based services cost less.
This guide lists every USPS Flat Rate box and envelope with its dimensions, explains how to tell when flat rate beats weight-and-zone pricing, and compares the alternatives: Priority Mail Cubic and UPS Simple Rate.
What is flat rate shipping?
Most shipping services price a package by its weight and the distance it travels, measured in zones. Flat rate removes both variables: you pay the price printed for the box or envelope, whether it holds a paperback or a set of wrenches, and whether it’s going across town or across the country.
USPS Priority Mail Flat Rate is the best-known version. The boxes and envelopes are free from the Post Office or the USPS online store, the service includes USPS Tracking, and USPS includes insurance coverage of up to $100 with most Priority Mail shipments that carry a tracking barcode. USPS also sells Priority Mail Express Flat Rate envelopes for its fastest domestic service.
Which USPS Flat Rate boxes and envelopes are there?
USPS publishes inside and outside dimensions for each box. Every option has the same 70-pound weight limit, so the choice comes down to which one fits your item most snugly.
| Box or envelope | Inside dimensions | Weight limit | Best for |
|---|---|---|---|
| Flat Rate Envelope | 12 1/2″ × 9 1/2″ | 70 lb | Documents, thin dense items |
| Legal Flat Rate Envelope | 15″ × 9 1/2″ | 70 lb | Legal-size paperwork, contracts |
| Padded Flat Rate Envelope | 12 1/2″ × 9 1/2″ | 70 lb | Books, small electronics, clothing |
| Small Flat Rate Box | 8 5/8″ × 5 3/8″ × 1 5/8″ | 70 lb | Jewelry, phone accessories, small parts |
| Medium Flat Rate Box 1 (top-loading) | 11″ × 8 1/2″ × 5 1/2″ | 70 lb | Shoes, hardware, multi-item orders |
| Medium Flat Rate Box 2 (side-loading) | 13 5/8″ × 11 7/8″ × 3 3/8″ | 70 lb | Flat dense items, binders, document sets |
| Large Flat Rate Box | 12″ × 11 3/4″ × 5 1/2″ | 70 lb | Heavy bulk orders, tools, canned goods |
| Large Flat Rate Board Game Box | 23 11/16″ × 11 3/4″ × 3″ | 70 lb | Board games, long flat items |
USPS also offers a Small Flat Rate Envelope, Window Flat Rate Envelopes and an APO/FPO/DPO Flat Rate Box for military mail, which has the same size as the Large box. If you want more detail on how the boxes are priced against each other, our guide to USPS Flat Rate boxes covers each size in depth.
When is flat rate shipping the better deal?
Flat rate wins when an item is both dense and travelling far. Because the price ignores weight and distance, a heavy item going to a distant zone gets the most out of it, while a light item to a nearby zone pays for capacity it doesn’t use.
| Shipment | Flat rate usually | Why |
|---|---|---|
| Heavy, compact item to a distant state | Wins | Weight-and-zone pricing climbs fastest on heavy, long-distance parcels |
| Heavy, compact item to a nearby zone | Depends | Short distance keeps weight-based prices low; compare both |
| Light item in a large box | Loses | You pay for box capacity you aren’t using |
| Light item under a pound or two | Loses | USPS Ground Advantage prices light parcels by weight and zone |
| Bulky item that doesn’t fit any box | Not eligible | Ships at weight-based or dimensional-weight pricing |
The only reliable way to decide is to price the same package both ways. You can compare Flat Rate, weight-based Priority Mail and Ground Advantage for your package in the ParcelPath rate calculator before you print. The trade-off between the two main USPS services is covered in our comparison of USPS Priority Mail vs Ground Advantage.
Speed matters too. Flat Rate boxes travel as Priority Mail, which USPS lists at 2 to 3 days, while Ground Advantage takes 2 to 5 business days. If you need something faster still, our guide to expedited shipping options compares the faster USPS and UPS services.
How does Priority Mail Cubic compare to flat rate?
Priority Mail Cubic is a second kind of size-based pricing. Instead of a USPS box, you use your own packaging, and the price is set by its volume in five tiers from 0.10 to 0.50 cubic feet, regardless of weight up to 20 pounds. Like Flat Rate, the price also depends on the zone, so it isn’t truly flat.
Cubic is a commercial price, so it’s available through online postage and label services rather than at the retail counter. It suits small, heavy parcels in tight packaging, and USPS points former users of the discontinued Regional Rate boxes to it. Our page on Priority Mail Cubic explains the tiers and how to measure for them.
Does UPS offer flat rate shipping?
Yes. UPS Simple Rate lets you use any box and pay a set price based on its size tier rather than weight and distance. The tiers run from Extra Small (up to 100 cubic inches) to Extra Large (1,051 to 1,728 cubic inches), and packages can weigh up to 50 pounds.
| Feature | USPS Priority Mail Flat Rate | UPS Simple Rate |
|---|---|---|
| Packaging | Free USPS-branded boxes and envelopes | Your own box, sorted into five size tiers |
| Weight limit | 70 lb | 50 lb |
| Size limit | Must fit the USPS box or envelope | Up to 1,728 cubic inches (one cubic foot) |
| Distance | Same price to any domestic address | Set price within the 50 states |
| Delivery speed | 2 to 3 days (Priority Mail) | Depends on the UPS service selected |
FedEx has a similar program called FedEx One Rate, which uses FedEx-branded packaging. If you’re open to UPS or USPS, ParcelPath offers up to 89% off USPS and up to 85% off UPS, free to use with no subscription or per-label fee. Simple Rate tends to compete best on heavier items in the smaller tiers, while USPS Flat Rate keeps the edge when the free box fits your product closely.
What are the rules for packing a Flat Rate box?
The box has to close along its original folds without bulging. USPS lets you reinforce the box with tape, but a Flat Rate box that’s been reshaped or stretched to hold more can be refused or charged at a different price.
- Use the box as supplied. Don’t cut, fold or extend a Flat Rate box to make it bigger.
- Keep the weight under 70 pounds, and spread heavy contents so no single wall carries the load.
- Add cushioning freely. Packing material adds weight but doesn’t change a Flat Rate price.
- Check the contents are mailable. Hazardous materials and restricted items follow the same USPS rules as any other Priority Mail package.
- International is different. Priority Mail International Flat Rate boxes have lower weight limits and require a customs form.
For items too big for any Flat Rate box, our guide to shipping large boxes explains dimensional weight and how carriers price oversized parcels.
Should an online store offer flat rate shipping?
For online sellers, “flat rate shipping” can also mean charging every customer the same shipping fee at checkout, whatever the order. That works well when your products are similar in size and weight, because the fee you charge stays close to what you actually pay.
With a mixed catalog, it helps to split the approach: ship dense, heavy products in USPS Flat Rate boxes and send light or nearby orders by Ground Advantage. Reviewing a sample of recent orders against both methods shows quickly which products belong in a Flat Rate box. Our overview of shipping methods and options covers the other service levels you can offer at checkout, and the delivery services hub compares same-day, overnight, ground and freight services. ParcelPath’s shipping calculator guide explains how to estimate costs before you set a checkout fee.
Frequently Asked Questions
Is flat rate shipping cheaper than regular shipping?
Flat rate shipping is cheaper for small, heavy items going a long distance, because the price doesn’t rise with weight or zone. For light items, or packages travelling to nearby zones, weight-based services such as USPS Ground Advantage usually cost less. Price the same package both ways before choosing, since the answer changes with each shipment.
What is the weight limit for USPS flat rate shipping?
Every USPS Priority Mail Flat Rate box and envelope has a 70-pound weight limit, the same as regular Priority Mail. The real limit is usually the box itself: contents must fit with the box closed along its original folds. Priority Mail International Flat Rate boxes have lower weight limits that vary by destination.
Are USPS flat rate boxes free?
Yes. USPS provides Priority Mail Flat Rate boxes and envelopes free of charge at Post Office locations and through its online store, where orders are delivered with your regular mail. You pay only the postage when you ship. The boxes must be used for Priority Mail Flat Rate, not for other services such as Ground Advantage.
How long does flat rate shipping take?
USPS Priority Mail Flat Rate packages are typically delivered in 2 to 3 days, depending on the distance, and USPS Tracking is included. Priority Mail Express Flat Rate envelopes are faster, with 1 to 3 day delivery backed by a money-back guarantee. Delivery dates are expected dates, not guarantees, for regular Priority Mail.
Sources
- USPS: Priority Mail and Flat Rate box dimensions
- USPS Domestic Mail Manual 223: Priority Mail commercial prices and Cubic eligibility
- USPS Postal Bulletin: Priority Mail Regional Rate Boxes A and B discontinued
- USPS: Ground Advantage
- UPS: Flat Rate Shipping (Simple Rate)
Part of our Shipping Methods and Options guide. Related: Delivery Services, Shipping Large Boxes.
