Priority Mail Flat Rate Calculator

Select Your Flat Rate Packaging: Flat Rate Envelope (12-1/2″ x 9-1/2″) Legal Flat Rate Envelope (15″ x 9-1/2″) Padded Flat Rate Envelope (12-1/2″ x 9-1/2″) Small Flat Rate Box (8-5/8″ x 5-3/8″ x 1-5/8″) Medium Flat Rate Box – 1 (11-1/4″ x 8-3/4″ x 6″) Medium Flat Rate Box – 2 (14″ x 12″ x 3-1/2″) Large Flat Rate Box (12-1/4″ x 12-1/4″ x 6″) Large Flat Rate Box (APO/FPO/DPO)

Quantity:

Weight (lbs) – Max 70:

Shipping Summary

function calculateShipping() { var rate = parseFloat(document.getElementById("boxType").value); var qty = parseInt(document.getElementById("packageQuantity").value); var weight = parseFloat(document.getElementById("packageWeight").value); var resultDiv = document.getElementById("shipResult"); var totalPriceP = document.getElementById("totalPrice"); var warningP = document.getElementById("weightWarning"); if (isNaN(qty) || qty 70) { totalPriceP.innerHTML = "Weight Exceeds Limit"; warningP.innerHTML = "Priority Mail Flat Rate has a weight limit of 70 lbs for domestic shipping. Please reduce weight or use a different service."; resultDiv.style.display = "block"; return; } var total = rate * qty; totalPriceP.innerHTML = "Total Cost: $" + total.toFixed(2); warningP.innerHTML = "Estimated retail rate based on USPS Flat Rate pricing. Delivery time typically 1-3 business days."; resultDiv.style.display = "block"; }

Understanding Priority Mail Flat Rate Shipping

Priority Mail Flat Rate is one of the most popular shipping methods offered by the United States Postal Service (USPS). The primary advantage is its simplicity: "If it fits, it ships." Regardless of the destination's distance or the weight of the package (up to 70 lbs), the price remains the same for a specific box or envelope size.

How the Flat Rate Calculator Works

Our calculator uses current retail pricing to estimate your shipping expenses. To get an accurate quote, follow these steps:

Select Packaging: Choose the specific USPS-branded Flat Rate envelope or box you intend to use. Note that you cannot use your own packaging for Flat Rate pricing.
Quantity: If you are shipping multiple identical boxes, enter the quantity to see the total bulk cost.
Weight Check: While the price doesn't change based on weight, USPS enforces a 70 lb limit for domestic Flat Rate shipments and a 4 lb limit for certain international Flat Rate envelopes.

Priority Mail Flat Rate Box Sizes and Dimensions

Box Type	Inside Dimensions
Small Flat Rate Box	8-5/8″ x 5-3/8″ x 1-5/8″
Medium Flat Rate Box (Top Loading)	11″ x 8-1/2″ x 5-1/2″
Medium Flat Rate Box (Side Loading)	13-5/8″ x 11-7/8″ x 3-3/8″
Large Flat Rate Box	12″ x 12″ x 5-1/2″

When Should You Use Flat Rate?

Flat Rate shipping is most cost-effective when you are shipping heavy items over long distances. For example, a 20 lb box sent from New York to California will almost always be cheaper in a Medium Flat Rate Box than using standard weight-and-zone-based Priority Mail. However, for very light items (under 1 lb) or short distances (Zone 1 or 2), standard Priority Mail or Ground Advantage may be more economical.

Key Benefits

No Weighing Required: As long as the package is under 70 lbs, you don't need a scale to determine the price.
Free Packaging: USPS provides Flat Rate boxes and envelopes for free at local post offices or via their website.
Predictable Costs: Businesses can calculate shipping margins easily because the price doesn't fluctuate based on the customer's location.
Insurance Included: Most Priority Mail shipments include up to $100 of insurance and basic tracking services.