USPS Flat Rate Box Calculator
Prices are based on Priority Mail® Flat Rate Retail Pricing (Domestic). Commercial rates may differ.
How to Use the USPS Flat Rate Calculator
Shipping with the United States Postal Service (USPS) using Flat Rate boxes is one of the most cost-effective ways to send heavy items domestically. The rule is simple: "If it fits, it ships."® Regardless of weight (up to 70 lbs) or destination within the U.S., the price remains fixed based on the size of the box you choose.
This calculator helps you determine exactly which Flat Rate box fits your specific item dimensions. Instead of guessing if your item will fit in a Small or Medium box, simply enter your item's length, width, and height above.
Steps to Calculate:
- Measure your item: Measure the Length, Width, and Height of the object you intend to ship. Be accurate to the nearest 0.1 inch.
- Enter Dimensions: Input these numbers into the calculator fields. Orientation doesn't matter (the calculator automatically rotates your item to find the best fit).
- Check Weight: Ensure your item is under 70 lbs. If it exceeds 70 lbs, it does not qualify for Priority Mail Flat Rate.
- View Results: Click "Find Compatible Boxes" to see a list of boxes that will hold your item, sorted by price.
2024/2025 Priority Mail® Flat Rate Box Sizes & Pricing
Understanding the inner dimensions of the boxes is crucial for ensuring a proper fit. The calculator uses these inner dimensions to process your request:
- Small Flat Rate Box:
Dimensions: 8 5/8″ x 5 3/8″ x 1 5/8″
Retail Price: ~$10.40
Best for: Small electronics, jewelry, brochures. - Medium Flat Rate Box (Top Loading):
Dimensions: 11″ x 8 1/2″ x 5 1/2″
Retail Price: ~$18.40
Best for: Shoes, office supplies, small appliances. - Medium Flat Rate Box (Side Loading):
Dimensions: 13 5/8″ x 11 7/8″ x 3 3/8″
Retail Price: ~$18.40
Best for: Flat items, clothing, binders. - Large Flat Rate Box:
Dimensions: 12″ x 12″ x 5 1/2″
Retail Price: ~$24.75
Best for: Laptops, board games, larger gifts.
Is Flat Rate Shipping Worth It?
Flat Rate shipping is excellent for heavy, small items traveling long distances (e.g., Zone 8). Because the price doesn't change based on distance or weight, shipping a 50lb brick from New York to California costs the same as shipping a 2lb book.
However, if your item is very light (under 2 lbs) and shipping a short distance, standard Priority Mail by weight/zone might actually be cheaper than the Flat Rate.
Please enter valid positive dimensions for Length, Width, and Height.
";
return;
}
if (weight > 70) {
resultDiv.style.display = "block";
resultDiv.innerHTML = "The item exceeds the USPS Flat Rate weight limit of 70 lbs. You must use standard Priority Mail or Parcel Select.
";
return;
}
// Define Box Data (Inner Dimensions in inches, Price in USD)
// Prices are approximate Retail rates (subject to change)
var boxes = [
{
name: "Small Flat Rate Box",
dims: [8.625, 5.375, 1.625], // 8-5/8 x 5-3/8 x 1-5/8
price: 10.40,
id: "small"
},
{
name: "Medium Flat Rate Box – 1 (Top Loading)",
dims: [11.0, 8.5, 5.5],
price: 18.40,
id: "med1"
},
{
name: "Medium Flat Rate Box – 2 (Side Loading)",
dims: [13.625, 11.875, 3.375], // 13-5/8 x 11-7/8 x 3-3/8
price: 18.40,
id: "med2"
},
{
name: "Large Flat Rate Box",
dims: [12.0, 12.0, 5.5],
price: 24.75,
id: "large"
},
{
name: "Flat Rate Envelope",
dims: [12.5, 9.5, 0.5], // Approx flexible thickness
price: 9.85,
id: "envelope"
}
];
// Sort item dimensions (smallest to largest) to handle rotation
var itemDims = [l, w, h].sort(function(a, b) { return a – b; });
var compatibleBoxes = [];
// Loop through boxes to check fit
for (var i = 0; i < boxes.length; i++) {
var box = boxes[i];
// Sort box dimensions (smallest to largest)
var boxDims = box.dims.slice().sort(function(a, b) { return a – b; });
// Check if item fits inside box (using sorted dimensions handles 3D rotation)
// Allow a tiny margin of error (0.05) or strict fit. Using strict <= here.
if (itemDims[0] <= boxDims[0] && itemDims[1] <= boxDims[1] && itemDims[2] <= boxDims[2]) {
compatibleBoxes.push(box);
}
}
// Output Logic
if (compatibleBoxes.length === 0) {
resultDiv.style.display = "block";
resultDiv.innerHTML = "Your item is too large for any standard USPS Flat Rate Box. Consider Standard Priority Mail (pricing by weight/zone).
";
} else {
// Sort compatible boxes by price (cheapest first)
compatibleBoxes.sort(function(a, b) { return a.price – b.price; });
var html = "Compatible Options:
"; for (var j = 0; j < compatibleBoxes.length; j++) { var box = compatibleBoxes[j]; var isBest = (j === 0); // First one is cheapest due to sort var bestClass = isBest ? "best-value" : ""; var badge = isBest ? "Cheapest Option" : ""; html += "";
html += "
";
}
resultDiv.style.display = "block";
resultDiv.innerHTML = html;
}
}
";
html += "
";
html += "" + box.name + badge + "
"; html += "Max Inner Dims: " + box.dims[0] + "\" x " + box.dims[1] + "\" x " + box.dims[2] + "\"
";
html += "$" + box.price.toFixed(2) + "
";
html += "