Choosing the correct box size isn’t just about fitting a product—it’s a strategic decision that can reduce costs, improve delivery efficiency, and reflect positively on your brand. At Unipak, we specialise in 100% recycled, plastic-free corrugated boxes for trade and e-commerce. This guide walks you through the key considerations when selecting the right box size for your application.

1. Understand Your Product Dimensions

Before you consider any box, measure your product's:

- Length (L) – the longest side
- Width (W) – the shorter side on the same face as length
- Height (H) – the dimension from base to top

Use millimetres (mm) for accuracy. If you’re unsure, use our Box Size Calculator for instant matching.

2. Factor in Internal Protection & Void Fill

You’ll likely need internal packaging such as:

- Paper void fill
- Tissue wrap
- Corner buffers
- Product literature or packing slips

Tip: Add 10–20mm to each dimension to allow for internal protection.

3. Consider Dimensional Weight vs Actual Weight

Many UK carriers now charge by volumetric weight.
Formula: (L × W × H) ÷ 5000 = Dimensional weight (kg)

Choosing a box that minimises void space can save you money, especially for lightweight but bulky items.

4. Choose the Right Box Grade: Single or Double Wall?

Single Wall: Best for small to medium items
Double Wall: Best for heavy or fragile products

5. Align with Courier Requirements

Some couriers enforce box size thresholds. Example:

- Royal Mail Small Parcel: Max 450×350×160mm
- Royal Mail Medium Parcel: Max 610×460×460mm

Shipping within these dimensions can qualify your dispatch for lower postage bands.

6. Sustainability & Branding Considerations

Ensure your packaging is:

- Right-sized to reduce environmental impact
- Fully recyclable
- Neutrally branded if dispatching under white label

7. Use the Right Tools: Box Calculator & Trade
Services

Unipak’s Box Size Calculator takes the guesswork out. Input your product dimensions to find the closest-fit box.

Trade buyers can request access to our full spec sheets, volume pricing, and account-based ordering.

Summary Checklist

Before placing your next packaging order, ensure:

✅ Product dimensions are measured accurately
✅ Internal fill has been accounted for
✅ Courier rules are considered
✅ Box board grade matches weight and fragility
✅ Volumetric weight won’t cost more than necessary
✅ Sustainability credentials align with your brand

Need Help? Speak to a Box Specialist

Our team can recommend the optimal size, strength, and style for your needs. Call, chat live, or contact us today.

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