Estimate domestic Priority Mail Flat Rate postage costs based on package size and pricing tier.
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 (Top or Side Loading)
Large Flat Rate Box (12″ x 12″ x 5-1/2″)
Retail (Post Office Counter Price)
Commercial Base (Online Shipping Software)
Priority Mail includes $50 (Retail) or $100 (Commercial) insurance.
Base Postage:$0.00
Signature Confirmation:$0.00
Extra Insurance:$0.00
Total Cost:$0.00
How to Calculate USPS Flat Rate Shipping
USPS Priority Mail Flat Rate is one of the most popular shipping options for small businesses and individuals in the United States. The philosophy is simple: "If it fits, it ships." Regardless of the weight (up to 70 lbs) or the domestic destination, the price remains fixed based on the box size.
Using the calculator above allows you to compare the differences between Retail pricing (what you pay at the Post Office counter) and Commercial pricing (rates available through online shipping platforms like Stamps.com, Pirate Ship, or eBay labels).
Understanding the Box Sizes
Choosing the right box is critical for Flat Rate shipping. You must use the official USPS-branded Flat Rate packaging to qualify for these rates.
Flat Rate Envelope: Ideal for documents, passports, and manuscripts.
Legal Flat Rate Envelope: Slightly larger than the standard envelope, designed for legal-sized documents.
Padded Flat Rate Envelope: Features bubble padding inside. Great for clothing, books, or small electronics.
Small Flat Rate Box: Roughly the size of a VHS tape. Good for jewelry, small electronics, or vitamins.
Medium Flat Rate Box: The most versatile size (Top-loading or Side-loading). Fits shoes, office supplies, or care packages.
Large Flat Rate Box: The largest option (12″ x 12″ x 5.5″). Best for laptops, board games, or heavier bulk items.
Important Weight Limit: While Flat Rate pricing is not determined by weight, there is a strict limit. The maximum weight for any domestic Flat Rate package is 70 lbs.
Retail vs. Commercial Pricing
One of the biggest money-saving tips for shippers is avoiding the Post Office counter. As demonstrated in the calculator, Commercial Base Pricing is significantly cheaper than Retail pricing.
For example, a Medium Flat Rate Box might cost around $18.40 at the counter, but approximately $14.75 if you purchase the label online. This discount adds up quickly if you ship multiple packages a month.
Additional Services Breakdown
When calculating your shipping, keep in mind these common add-ons:
Insurance: Priority Mail typically includes $50 of insurance for Retail and $100 for Commercial shipments. If you are shipping a high-value item, you should purchase additional coverage.
Signature Confirmation: If you need proof that the recipient received the package, this service costs extra (approx. $3.50) but provides peace of mind and protection against "item not received" claims.
Frequently Asked Questions
Do I need to weigh my Flat Rate package?
Technically, no, strictly for pricing purposes (as long as it is under 70 lbs). However, you will still need to enter a weight on the label for safety compliance, even if it doesn't alter the price.
Can I use my own box for Flat Rate pricing?
No. To get the Flat Rate price, you must use the specific USPS Flat Rate branded box or envelope. If you use your own brown box, you will be charged based on weight and distance (Zone-based pricing).
How fast is Priority Mail Flat Rate?
USPS Priority Mail typically delivers within 1 to 3 business days, depending on the distance between the origin and destination.
function calculateShipping() {
// 1. Get input values
var type = document.getElementById('packageType').value;
var tier = document.getElementById('priceType'); // Mistake in mental check, ensuring ID matches HTML
// Correcting variable retrieval based on actual HTML ID
var tierValue = document.getElementById('priceTier').value;
var sigReq = document.getElementById('signatureReq').checked;
var decValueInput = document.getElementById('declaredValue').value;
// 2. Define Rate Table (Approximate 2024 Rates for Logic Demonstration)
// Structure: { type: { retail: price, commercial: price } }
var rateTable = {
'envelope': { 'retail': 9.85, 'commercial': 8.05 },
'legal': { 'retail': 10.15, 'commercial': 8.35 },
'padded': { 'retail': 10.60, 'commercial': 8.80 },
'small': { 'retail': 10.40, 'commercial': 8.55 },
'medium': { 'retail': 18.40, 'commercial': 14.75 },
'large': { 'retail': 24.75, 'commercial': 19.90 }
};
// 3. Determine Base Price
var basePrice = 0;
if (rateTable[type] && rateTable[type][tierValue]) {
basePrice = rateTable[type][tierValue];
}
// 4. Calculate Signature Cost
var sigCost = 0;
if (sigReq) {
sigCost = 3.50;
}
// 5. Calculate Insurance Cost
// Logic: Included amount varies. Retail includes $50, Commercial includes $100.
// Cost is approx $2.60 for first extra $100 increment, simplified here to $2.00 per $100 or fraction thereof for estimation.
var insuranceCost = 0;
var declaredValue = parseFloat(decValueInput);
if (!isNaN(declaredValue) && declaredValue > 0) {
var includedCoverage = (tierValue === 'commercial') ? 100 : 50;
if (declaredValue > includedCoverage) {
var valueToInsure = declaredValue – includedCoverage;
// Simplified formula: ~$2.60 for up to $600 extra, then higher.
// We will use a simplified linear estimation for the tool: $1.85 per $100 unit.
var units = Math.ceil(valueToInsure / 100);
insuranceCost = units * 1.85;
// Minimum extra insurance charge usually applies, but simplified for this tool
if (insuranceCost < 2.60) insuranceCost = 2.60;
}
}
// 6. Total Calculation
var total = basePrice + sigCost + insuranceCost;
// 7. Update UI
document.getElementById('baseCostDisplay').innerHTML = '$' + basePrice.toFixed(2);
document.getElementById('sigCostDisplay').innerHTML = '$' + sigCost.toFixed(2);
document.getElementById('insCostDisplay').innerHTML = '$' + insuranceCost.toFixed(2);
document.getElementById('totalCostDisplay').innerHTML = '$' + total.toFixed(2);
// Show results
document.getElementById('results').style.display = 'block';
}