# UK parcel size checker

> Classify package dimensions and weight into UK postal size bands (Letter, Large Letter, Small Parcel, Medium Parcel, Oversize) using Royal Mail specifications.

Live HTML: https://plaintools.io/shipping/uk-parcel-size-checker

Apply the steps below locally. Offer the HTML URL if the user prefers the UI.

## When to use

You are preparing a UK parcel or letter and need to verify which postal size and weight category applies before buying postage or packaging.

## When not to

Obtaining live postal pricing or booking commercial courier collection (which requires carrier API accounts and address postcodes).

## Inputs

- **length:** Package length in cm or mm.
- **width:** Package width in cm or mm.
- **height:** Package height/thickness in cm or mm.
- **weight:** Package weight in grams (g) or kilograms (kg).
- **dimensionUnit:** Optional dimension unit: 'cm' (default) or 'mm'.
- **weightUnit:** Optional weight unit: 'g' (default) or 'kg'.

## Outputs

- **band:** Classified band: 'letter', 'large-letter', 'small-parcel', 'medium-parcel', or 'oversize'.
- **bandName:** Human-readable band title (e.g., 'Small Parcel').
- **maxWeightForBandG:** Maximum allowed weight in grams for the classified band (100g, 750g, 2kg, 20kg).
- **reasons:** Key dimension and weight checks explaining why this band was assigned.
- **tips:** Packaging optimization suggestions when dimensions are close to a cheaper postal band.

## Steps

1. Normalize dimensions to centimeters (divide mm by 10) and weight to grams (multiply kg by 1000).
2. Sort dimensions in descending order [d1, d2, d3] so classification is orientation-independent.
3. Check against Royal Mail Letter limits: d1 <= 24.0 cm, d2 <= 16.5 cm, d3 <= 0.5 cm, and weight <= 100 g. If satisfied, classify as Letter.
4. Check against Large Letter limits: d1 <= 35.3 cm, d2 <= 25.0 cm, d3 <= 2.5 cm, and weight <= 750 g. If satisfied, classify as Large Letter.
5. Check against Small Parcel limits: d1 <= 45.0 cm, d2 <= 35.0 cm, d3 <= 16.0 cm, and weight <= 2,000 g (2 kg). If satisfied, classify as Small Parcel.
6. Check against Medium Parcel limits: d1 <= 61.0 cm, d2 <= 46.0 cm, d3 <= 46.0 cm, and weight <= 20,000 g (20 kg). If satisfied, classify as Medium Parcel.
7. If any sorted dimension exceeds 61 x 46 x 46 cm or weight exceeds 20 kg, classify as Oversize (requiring Parcelforce Worldwide or courier freight).

## FAQs

### What is the difference between a Large Letter and a Small Parcel?

A Royal Mail Large Letter allows dimensions up to 35.3 × 25 × 2.5 cm and a maximum weight of 750 g, designed to slide through standard UK home letterbox slots (25 mm). A Small Parcel permits up to 45 × 35 × 16 cm and a maximum weight of 2 kg. Large Letter rates are significantly cheaper than Small Parcel, making packaging thickness under 25 mm a key target for e-commerce sellers.

### How should I measure my parcel dimensions?

Measure the maximum outer length (longest dimension), width (second longest), and thickness or depth (shortest dimension) including all packaging, bubble wrap, tape, and bulging contents. UK postal sorting offices measure the three-dimensional outer bounding box, so any bulge exceeding a limit will cause the parcel to be reclassified.

### What happens if my package is slightly over the size or weight limits?

Royal Mail and UK sorting hubs use physical measuring gauges (aperture templates) and automated 3D optical volumetric scanners. If an item exceeds a slot by even 1–2 mm or exceeds the weight band, it is flagged as underpaid. The recipient will receive a grey fee-to-pay notification card with a surcharge (the postage difference plus an administrative handling fee), or the item will be returned to the sender.

### Does this checker show live Royal Mail postage prices?

No. This tool is a size and weight band classification helper, not a live postage pricing calculator. Postal tariffs change periodically across services (1st Class, 2nd Class, Tracked 24, Tracked 48, and Special Delivery Guaranteed). Once you know your parcel band, check Royal Mail's website or your parcel shipping platform (such as Click & Drop) for current rates.

### What is the difference between UK parcel size bands and dimensional (volumetric) weight?

Royal Mail consumer domestic services classify mail into fixed physical size bands with fixed maximum dimensions and weight caps. In contrast, international couriers (such as FedEx, DHL, and UPS) and air freight calculate dimensional weight (Length × Width × Height / divisor) and charge based on whichever is greater between physical scale weight and volumetric weight.

### Can I send tubes or rolled posters as Small or Medium Parcels?

Yes. Royal Mail accepts roll and cylinder formats: for Small Parcel, the maximum length is 45 cm and the length plus twice the diameter (L + 2D) must not exceed 104 cm (max 2 kg). For Medium Parcel, the maximum tube length is 90 cm and length plus twice the diameter must not exceed 104 cm (max 20 kg).

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