Cubic Pricing: The Best-Kept Secret in Small Parcel Shipping

Every week, I encounter e-commerce sellers who are completely unaware of Cubic Pricing. They’re often sending out compact, heavy items like candles, ceramic mugs, jars of honey, hand tools, and books. Unfortunately, they end up paying weight-based rates that are sometimes double what they could be paying. Cubic Pricing offers a solution by flipping the traditional pricing model on its head. Instead of basing costs on weight, it focuses on the space your package occupies. For sellers of dense, heavy products, this can mean a significant reduction in shipping expenses.

How Cubic Pricing Works

Traditional shipping rates depend heavily on the weight of your package. Carriers measure the package, calculate its dimensional weight, and charge you based on whichever is higher: the actual weight or the dimensional weight. This means that heavy items in small boxes often incur higher costs due to their weight, while light items in large boxes suffer from dimensional weight charges.

Cubic Pricing, however, disregards the weight of the package entirely—up to a limit of 20 lbs for Priority Mail and 70 lbs for some Ground services. Instead, it focuses solely on the cubic volume of your package.

The Cubic Volume Formula

To calculate cubic volume, use the formula:

Length (inches) x Width (inches) x Height (inches) / 1,728 = Cubic Feet

Once you've measured your box and done the math, you can determine which tier your package falls into.

USPS Priority Mail Cubic Tiers

The USPS Priority Mail Cubic system divides packages into tiers based on their cubic volume. This means that a small, densely packed box can cost significantly less to ship than a larger, lighter one. The tiers range from 0.1 cubic feet to 0.5 cubic feet, each with its own pricing and maximum weight limit of 20 lbs.

What This Looks Like in Real Dollars

Consider a practical example. Suppose you sell handmade soy candles, each weighing 2.5 lbs and shipping in a 7" x 7" x 5" box.

Weight-based Priority Mail Commercial (Zone 5):

• 2.5 lbs = approximately $9.80

Cubic Priority Mail (Zone 5):

• 7 x 7 x 5 = 245 cubic inches = 0.142 cubic feet = Tier 0.2
• Tier 0.2, Zone 5 = approximately $8.15

The savings amount to $1.65 per package. While this may not seem substantial for a 2.5 lb candle, it becomes far more significant with heavier items. For instance, if you ship cast iron trivets weighing 8 lbs in the same size box:

Weight-based Priority Mail Commercial (Zone 5):

• 8 lbs = approximately $16.40

Cubic Priority Mail (Zone 5):

• Same box = same Tier 0.2 = same $8.15

In this case, you save $8.25 per package, which is a 50% reduction. The heavier the item (up to 20 lbs), the more you save, as the cubic rate remains constant regardless of weight.

The Cubic Pricing Sweet Spot

Cubic Pricing offers the most savings when certain conditions are met:

• Weight: The savings increase as the package weight approaches the 20 lbs limit.
• Box Size: Smaller, compact boxes benefit more from cubic pricing.
• Density: Denser packages (heavy for their size) are ideal candidates.
• Zone: Higher shipping zones (5-8) yield greater savings compared to lower zones (1-3).

Products That Save Big on Cubic

Certain products are particularly well-suited for Cubic Pricing. For example, soy candles, hardback books, pottery, hand tools, gourmet food jars, small appliances, skincare products in glass jars, hardware, coffee beans, and small bags of pet food all stand to benefit significantly. Depending on the product, you can see savings ranging from 15% to 60% compared to weight-based rates.

Cubic Pricing by Zone: Full Rate Comparison

Here's a comparison between Priority Mail Commercial rates and Cubic Priority Mail rates for a 10 lb package in a Tier 0.3 box (approximately 10" x 8" x 6"):

At Zone 8, you save nearly $15 per shipment. If you ship 50 of these packages a month, that's $750 in savings from just one pricing adjustment.

How to Access Cubic Pricing

Cubic Pricing isn't available at your local post office counter. Instead, it's accessible through Commercial Pricing on approved shipping platforms.

Where to Get Cubic Rates

Several platforms offer access to Cubic Pricing:

• atoship: Offers cubic rates without a monthly fee and automatically applies the best rate.
• Pirate Ship: Provides cubic rates and a "Simple Export Rate" for international shipments, also without a monthly fee.
• Stamps.com: Charges $19.99/month but is a well-known legacy platform.
• ShipStation: Requires a commercial account with fees starting at $25/month.
• EasyPost API: Charges per label and is developer-focused.

The most convenient platforms will automatically compare cubic against weight-based rates, ensuring you always get the best price without manual calculations.

Cubic Pricing vs. Flat Rate: When to Use Which

USPS Flat Rate boxes are a straightforward option because they have a single price, irrespective of weight or zone. However, they aren't always the most economical choice.

Small Flat Rate Box (8.6" x 5.4" x 1.6")

For many scenarios, the cubic equivalent is cheaper:

• 1 lb, Zone 3: Cubic is $6.80 vs. Flat Rate $10.40
• 5 lbs, Zone 5: Cubic is $7.90 vs. Flat Rate $10.40
• 5 lbs, Zone 8: Cubic is $9.80 vs. Flat Rate $10.40
• 10 lbs, Zone 8: Cubic is $9.80 vs. Flat Rate $10.40

Medium Flat Rate Box (11" x 8.5" x 5.5")

Again, cubic rates usually win:

• 3 lbs, Zone 3: Cubic is $8.85 vs. Flat Rate $17.10
• 5 lbs, Zone 5: Cubic is $10.45 vs. Flat Rate $17.10
• 15 lbs, Zone 5: Cubic is $10.45 vs. Flat Rate $17.10
• 20 lbs, Zone 8: Cubic is $13.50 vs. Flat Rate $17.10

Large Flat Rate Box (12" x 12" x 5.5")

The savings trend continues with larger boxes:

• 5 lbs, Zone 5: Cubic is $11.40 vs. Flat Rate $22.45
• 15 lbs, Zone 5: Cubic is $11.40 vs. Flat Rate $22.45
• 20 lbs, Zone 8: Cubic is $15.20 vs. Flat Rate $22.45

In most cases, cubic pricing is more economical than flat rate options unless the package is extremely heavy and traveling to a high zone.

Ground Advantage Cubic Rates

USPS Ground Advantage offers a similar concept to Cubic Pricing for heavier packages, without officially labeling it as such. These rates often surpass weight-based alternatives for dense packages.

For packages over 20 lbs, Ground Advantage is often the best option, especially when packaging is optimized.

Real Business Case Study

Consider a home goods seller on Shopify, shipping ceramic planters. Each package averages 7 lbs and uses a 9" x 9" x 7" box.

Before (Weight-Based Priority Mail Commercial):

• Average rate: $14.20/package
• Monthly volume: 400 packages
• Monthly shipping cost: $5,680

After (Cubic Priority Mail):

• Box dims: 9 x 9 x 7 = 567 cu in = 0.328 cu ft = Tier 0.4
• Average cubic rate: $10.10/package
• Monthly volume: 400 packages
• Monthly shipping cost: $4,040

This business saved $1,640 monthly, totaling $19,680 annually, without changing anything but the pricing structure.

Tips to Maximize Cubic Savings

Package down: Reducing your package size by just a fraction can change your tier and save $1-3 per package.

Measure inside dimensions: Carriers measure outside dimensions. Thinner packaging can help avoid higher tiers.

Use poly mailers: For non-fragile items, poly mailers have almost zero cubic volume, drastically cutting costs.

Mix your shipping methods: Use Cubic for heavy, compact items, weight-based for light, bulky items, and Flat Rate for the heaviest items to higher zones.

Know the tier boundaries: Small adjustments to box dimensions can drop your package to a lower tier, saving money effortlessly.

The Bottom Line

Cubic Pricing is a game-changer for those shipping heavy, compact products. If your packages are over 3-4 lbs and fit in a box smaller than 12" x 10" x 8", Cubic Pricing could save you 25-55% on shipping costs. This significant reduction can translate into thousands of dollars in annual savings for businesses. Stop paying by the pound and start exploring how Cubic Pricing can benefit your business.