Canada Post Flat Rate Boxes: Sizes, Prices & 5kg Limit

Canada Post maxes out at 5 kg per flat rate box, while USPS allows up to 70 lbs domestically — a 14× gap that shapes every shipping decision for anyone moving parcels north of the border or south into the US. This guide lays out the official 2026 pricing, the four size options, and where flat rate actually makes sense versus paying by weight.

Weight Limit: 5 kg ·
Available Sizes: 4 (Extra-Small to Large) ·
Price Range: $18.99 – $32.99 CAD ·
Shipping Coverage: Anywhere in Canada ·
Liability Coverage: Up to $100

How much do Canada Post flat rate boxes cost?

Canada Post prices its flat rate boxes in a tiered structure that climbs with size. The Extra Small runs $18.99 CAD, the Small hits $21.99 CAD, the Medium lands at $24.99 CAD, and the Large tops out at $32.99 CAD (Canada Post Official). All four include prepaid postage, so what you pay at the counter is the final rate — no zones, no dimensional weight math.

Small box pricing

The Small flat rate box at $21.99 CAD covers items weighing up to 5 kg, with drop-off available at any street letter box or local post office (Canada Post Official). For sellers shipping multiple small items, this eliminates the need to weigh each parcel individually.

Large box pricing

The Large flat rate box at $32.99 CAD offers the highest capacity for a flat rate, though it requires post office drop-off rather than street box access (Canada Post Official). At roughly $33 CAD, it undercuts comparable USPS Large Flat Rate Box pricing when converted from USD, though USPS domestic limits run far higher.

Prepaid options

Prepaid bundles simplify the process: buy the box, pack it, and ship it without any metering step. This matters for e-commerce sellers who batch shipments — the box itself becomes the complete transaction at the counter.

What sizes do Canada Post flat rate boxes come in?

Canada Post offers exactly four sizes: Extra Small, Small, Medium, and Large (PluginHive). This narrower lineup contrasts with USPS, which also includes flat rate envelopes and multiple Medium configurations. The four Canada Post sizes share one rule: whatever fits inside, as long as it stays under 5 kg, ships at the fixed rate.

Extra-small dimensions

The Extra Small box targets lightweight, compact items — jewelry, small electronics, or documents. Its precise outer dimensions do not appear on official Canada Post pages, which creates a planning gap for shippers who need exact fit data. The $18.99 CAD price point makes it the cheapest entry into flat rate shipping.

Medium and large options

Medium and Large boxes handle most standard e-commerce shipments. The Medium fits items that would otherwise require a medium-sized corrugated box, while the Large accommodates larger orders without splitting into two parcels. Some regional post offices report inconsistent Large box inventory, so calling ahead is advisable before making a large shipment plan (PluginHive).

What is the weight limit for flat rate boxes?

Every Canada Post flat rate box — regardless of size — carries a hard 5 kg (11 lbs) maximum weight limit (Canada Post Official). Canada Post describes the model as “if it fits, it ships,” but only up to that ceiling. Exceed 5 kg and the flat rate no longer applies, forcing you into weight-based pricing or a larger service tier.

Domestic limits

For domestic Canadian shipments, the 5 kg cap governs all four box sizes. This is dramatically lower than USPS domestic flat rate boxes, which allow up to 70 lbs — roughly 14 times the capacity (Easyship). Heavy-item shippers, particularly those moving dense goods like automotive parts or bulk pantry items, will hit the Canada Post ceiling quickly.

International considerations

Canada Post flat rate boxes are domestic-only — they cannot be used for cross-border or international shipments (PluginHive). For US-to-Canada shipments, USPS Priority Mail International takes over, with Medium and Large flat rate boxes capped at 20 lbs for Canadian destinations (USPS Official). Envelopes and Small boxes for international service drop to a 4 lb ceiling (USPS Official).

How long do flat rate boxes take to ship Canada Post?

Canada Post does not publish guaranteed delivery windows specific to flat rate boxes — delivery speed depends on destination, service level, and time of year. The flat rate pricing covers the postage cost, but the transit time follows standard parcel service timelines rather than a separate flat rate guarantee. Tracking comes included with every flat rate shipment, giving senders package visibility throughout the journey.

Domestic timelines

For context, Canada Post Xpresspost targets 2-3 business day delivery within Canada and often undercuts UPS on cost for comparable speeds (EcomCrew). Flat rate boxes travel through the same network, so urban-to-urban routes typically move faster than rural or northern deliveries. Peak seasons like December can stretch standard timelines by several days.

Tracking details

Every Canada Post flat rate box includes tracking at no extra charge, aligned with standard domestic parcel service. This contrasts with USPS Priority Mail Flat Rate domestic service, which also includes tracking and targets 2-3 business day delivery (Shippo). For cross-border tracking, USPS and Canada Post scanning systems share data through joint tracking protocols.

Is it cheaper to use a flat rate box or your own box?

The flat rate advantage narrows or disappears depending on your item’s weight and destination zone. For lightweight parcels under 1-2 kg moving moderate distances, the fixed price simplifies budgeting and often matches or undercuts weight-based alternatives. For heavier items approaching the 5 kg ceiling, or for short-zone lightweight shipments, bringing your own box with weight-based postage typically costs less.

Cost comparison factors

USPS weight-based rates frequently beat flat rate pricing for packages under 20 lbs traveling short zones — zones 1-4 benefit most from standard weight-based postage (ShippingEasy). The flat rate premium buys simplicity: no scale check at the counter, no zone lookup, no dimensional weight calculation. For high-volume shippers, that time savings compounds across hundreds of parcels.

When flat rate saves money

Flat rate boxes make the most sense when you regularly ship items near the 5 kg ceiling. A sender packing dense goods — books, automotive parts, weighted gift sets — benefits from a fixed price that would otherwise climb with each additional pound. USPS flat rate shines for heavy items close to 70 lbs over long distances, where the fixed rate undercuts weight-based zone pricing by a wide margin (Easyship). Canada Post flat rate serves the same role for domestic Canadian shipments within the 5 kg constraint.

Canada Post vs USPS flat rate: Specifications compared

Five distinct differences shape which system serves you better depending on whether you’re shipping domestically within Canada, domestically within the US, or crossing the border.

The 14× weight limit gap between Canada Post and USPS shapes the entire comparison. A Canadian shipper moving heavy items to the US must switch carriers entirely — Canada Post flat rate does not cross the border, and USPS domestic limits don’t apply to Canadian destinations.

As long as it doesn’t weigh over 5 kg, if it fits, it ships.

— Canada Post (Official Postal Service)

The maximum weight for all USPS flat rate boxes and envelopes is 70 pounds.

— Shippo (Shipping Platform)

Related reading: USD to CAD rate · Canada Revenue Agency My Account

Canada Post flat rate boxes carry fixed prices from $18.99 to $32.99 for loads up to 5kg, with sizes and options expanded in this sizes prices shipping guide for practical shipping.

Frequently asked questions

For Canadian shippers moving heavy items domestically, the 5 kg ceiling is the constraint. Anyone regularly shipping parcels approaching that limit should calculate whether flat rate’s fixed price beats weight-based alternatives before committing to the box. Cross-border shippers, meanwhile, face a complete carrier switch — Canada Post flat rate does not extend south, and USPS domestic limits don’t apply north of the border.