Determine your parcel format and estimated Royal Mail postage cost
Before sending a parcel with Royal Mail, you need to know which format category it falls into. The format determines the price you pay, and getting it wrong can mean surcharges, delays, or your item being returned. Royal Mail classifies items into five categories based on dimensions and weight: Letter, Large Letter, Small Parcel, Medium Parcel, and Large Parcel. This calculator takes your parcel's length, width, depth, and weight, then determines the correct Royal Mail format. It also calculates the volumetric weight, which couriers use to price bulky but lightweight items, and provides estimated postage costs for both 1st and 2nd class service. Knowing your parcel's format before you visit the Post Office or buy postage online helps you budget accurately and choose the most cost-effective option. For items that exceed Medium Parcel limits, you will need Parcelforce or a third-party courier.
To use the parcel size calculator: 1. Measure the length of your parcel in millimetres. This should be the longest dimension. 2. Measure the width in millimetres. This is the second longest dimension. 3. Measure the depth in millimetres. This is the shortest dimension. 4. Weigh your parcel in grams. Include all packaging materials in the weight. 5. View the results. The calculator shows: - The Royal Mail format category your parcel fits into - The volumetric weight (used by couriers for pricing) - The chargeable weight (whichever is greater: actual or volumetric) - Estimated 1st and 2nd class postage costs - Whether your parcel exceeds standard Royal Mail limits 6. If your parcel is classified as "Large Parcel", it exceeds Royal Mail's standard dimensions. You will need to use Parcelforce or a courier such as Hermes, DPD, or UPS.
The calculator uses two main processes: Format determination: Your parcel's dimensions and weight are compared against Royal Mail's format limits in order from smallest to largest. The first format where all four measurements (length, width, depth, weight) fall within the limits is your category. The limits are: Letter (240x165x5mm, 100g), Large Letter (353x250x25mm, 750g), Small Parcel (450x350x160mm, 2kg), Medium Parcel (610x460x460mm, 20kg). Volumetric weight calculation: Couriers calculate volumetric weight as (length in cm x width in cm x depth in cm) / 5000. This represents the space your parcel takes up relative to a standard density. The chargeable weight is whichever is greater: the actual weight or the volumetric weight. For example, a parcel measuring 40x30x20cm weighing 1kg has a volumetric weight of 4.8kg, so the courier would charge based on 4.8kg. The estimated postage costs shown are the base rates for the determined format. Actual costs may vary depending on the specific weight band within that format.
When packaging items for post, keep dimensions as small as practical to stay in a cheaper format category. A few millimetres can make the difference between a Large Letter (from GBP 2.10) and a Small Parcel (from GBP 4.45). Packaging tips: Use appropriately sized boxes or padded envelopes rather than oversized packaging. Bubble wrap adds minimal dimensions. Jiffy bags are ideal for items that just exceed letter depth limits. For fragile items, ensure adequate protection but avoid excessive void fill that increases dimensions unnecessarily. For regular senders, Royal Mail offers postage calculators on their website and the Click and Drop platform for printing labels at home. Third-party comparison services like ParcelHero aggregate prices from multiple couriers, which can be especially cost-effective for heavier items where courier rates undercut Royal Mail parcel pricing.