USPS flat rate box picker

What it computes

This picker finds the smallest listed USPS Flat Rate Box that fits one cushioned bundle. It also shows verified domestic retail postage for your mailing date.

Start with the contents and their cushioning together. Measure the bundle's three outside dimensions, then weigh everything you will mail, including the box. The result compares four box shapes: small, two medium versions and large.

If you are still choosing the contents, care package ideas covers items and destinations before dimensions. Once the bundle is ready, this worksheet helps you check which box to collect.

How it works

The fit check compares your bundle with each box's interior dimensions. It tries each orientation with the bundle's faces parallel to the box walls. Nothing tilts, bends or compresses. Among the boxes that fit, it selects the smallest interior volume.

The dimensions come from USPS Priority Mail, checked September 14, 2026:

• Small: 8.625 × 5.375 × 1.625 inches.
• Medium, top loading: 11 × 8.5 × 5.5 inches.
• Medium, side loading: 13.625 × 11.875 × 3.375 inches.
• Large: 12 × 11.75 × 5.5 inches.

Each listed box allows up to 70 lb of total packed weight for domestic mailing. Use an actual USPS Flat Rate Box. Its flaps must close along their normal folds without bulging or enlargement.

Retail prices come from Notice 123, effective July 12, 2026 and checked September 14, 2026. Small costs $13.65, either medium costs $24.80, and large costs $34.00. These figures cover postage; they do not include packing materials or optional services.

The price window ends October 3, 2026. USPS Postal Explorer lists a proposed temporary price change starting October 4, 2026. For mailing dates outside the verified window, the tool keeps the box match and withholds the price. A blank mailing date uses today.

Examples

These are hypothetical packed bundles, all mailed September 14, 2026. Their dimensions already include cushioning, and their weights include the box.

Small bundle. An 8 × 5 × 1 inch bundle weighing 2 lb fits the Small Flat Rate Box. The verified retail postage is $13.65.

Medium top-loading box. A 10 × 8 × 4 inch bundle weighing 5 lb fits the top-loading Medium Flat Rate Box. It exceeds the small box's interior dimensions. The verified retail postage is $24.80.

Large box. An 11 × 10 × 5 inch bundle weighing 8 lb fits the Large Flat Rate Box. The verified retail postage is $34.00.

Limits

This is a size comparison, not a search for the cheapest shipping service. It excludes your own boxes, envelopes, commercial labels, military discounts and international rates. Several loose objects may fit differently after rearrangement; measure their final cushioned bundle and check closure before buying postage.

Box fit does not establish whether the contents can be mailed. Our care package shipping guide covers that separate check. Rate references need quarterly review and another check whenever USPS announces a change; verify current postage before mailing.