Royal Mail Postage & Parcel Estimator (UK)

Royal Mail prices change periodically. This calculator stays accurate by applying stable UK sizing rules and letting you enter the current rate for your chosen service.

What This UK Calculator Solves

In the UK, Royal Mail is still the everyday carrier for letters and small parcels. Whether you are mailing paperwork, shipping a small online order, or sending a gift across the country, postage depends on two things that are easy to get wrong: the size category and the weight band. Prices also vary by service level (1st Class, 2nd Class, Tracked 24/48, Signed For, Special Delivery) and are updated over time. That means a calculator that hard‑codes last year’s prices will drift out of date quickly.

This estimator takes a different approach. It:

• Validates your item against Royal Mail size limits (Letter, Large Letter, Small Parcel, Medium Parcel) that rarely change.
• Computes the billable weight band for whichever weight step your rate table uses.
• Applies the arithmetic using your current base price and per‑step increment from Royal Mail’s latest price guide.

By separating rules from rates, the calculator remains accurate even when Royal Mail revises prices.

Royal Mail Size Categories (Retail Inland)

Royal Mail’s retail categories are based on maximum dimensions and maximum weights. As of recent guidance, the common inland bands are:

• Letter. Up to 240 mm × 165 mm × 5 mm, up to 100 g.
• Large Letter. Up to 353 mm × 250 mm × 25 mm, up to 750 g.
• Small Parcel. Up to 450 mm × 350 mm × 160 mm, up to 2 kg.
• Medium Parcel. Up to 610 mm × 460 mm × 460 mm, up to 20 kg.

If your item exceeds Medium Parcel limits, Royal Mail usually routes it to Parcelforce or another courier, where pricing is different and dimensional weight may apply. This calculator flags that case.

Weight Bands and How Tables Work

Royal Mail price guides list rates by size category, service, and weight band. For example, a Large Letter 1st Class might have a price for 0–100 g, another for 101–250 g, and so on. To avoid hard‑coding a specific year’s bands, this calculator lets you choose your base band and step size. If your table uses “first 100 g included, then +£X per additional 100 g (or part),” you set base band = 0.1 kg and step size = 0.1 kg. If your table uses larger steps, set them accordingly.

The Underlying Formulas

Let w be the actual weight of your item in kilograms, b the base band weight (also in kg), s the additional step size (kg), and your current rates from the Royal Mail guide be base price B and step price u.

Royal Mail bands are “per part thereof,” meaning even a small fraction over a band rounds up. That is exactly what the ceiling function does.

Worked Example

You are sending a padded envelope that measures 320 mm × 220 mm × 12 mm and weighs 180 g. That fits within the Large Letter limits (max 353×250×25 mm, max 750 g). You are using a Large Letter Tracked 48 table that says: base price for first 100 g is £2.70, and each additional 100 g (or part) adds £0.60.

Convert weight to kg: 180 g = 0.180 kg. Base band b = 0.100 kg. Extra weight = 0.080 kg. Step size s = 0.100 kg, so steps = ceil(0.080 / 0.100) = 1. Total postage = £2.70 + 1×£0.60 = £3.30.

If the envelope weighed 205 g, extra weight would be 0.105 kg, steps = ceil(1.05) = 2, giving £3.90. This rounding behaviour is where many people mis‑estimate costs.

Comparison Table: Choosing the Right Category

Special Delivery, Tracked, and Signed For

Royal Mail offers several premium services that share the same size rules but use different price tables. “1st Class” and “2nd Class” are the traditional untracked inland services. “Tracked 24” and “Tracked 48” add end‑to‑end tracking and typically include some compensation if a parcel is lost. “Signed For” adds a signature at delivery on top of 1st or 2nd Class. “Special Delivery Guaranteed” is the fastest and most secure inland service, aimed at high‑value items with time guarantees and higher compensation limits.

The important thing for calculator use is that the math is the same: you still choose the correct size category and weight band, then read the base price and increments from the guide for that service. If you switch services, update your base and step prices accordingly.

International Shipping Differences

International Royal Mail services (International Standard, International Tracked, International Signed, and Global Express) reuse similar categories but have different weight steps and maximum weights by destination. Some destinations also have restricted items. This calculator can still help with band arithmetic if you plug in the correct international base band and step size, but always verify maximum weights for your destination on the Royal Mail country guide.

Why Volumetric Weight Usually Doesn’t Apply Here

Unlike some couriers, Royal Mail retail inland pricing is generally driven by size category limits, not by a volumetric divisor. If your parcel fits a category, you pay based on actual weight. Volumetric charging becomes relevant mainly when parcels exceed Medium Parcel limits and are routed to Parcelforce or other couriers, which is why this calculator flags that case rather than trying to compute courier volumetrics. If you regularly ship large lightweight boxes, a Parcelforce‑specific calculator would be more appropriate.

Limitations and Assumptions

This calculator intentionally does not hard‑code prices, because Royal Mail updates them. It assumes:

• You enter the correct base and step prices from the latest Royal Mail price guide for your chosen category and service.
• Size limits reflect current retail inland guidance; verify if you are shipping internationally or via Parcelforce.
• We do not include add‑ons like Saturday delivery, compensation upgrades, or business account discounts.

Use the output as an accurate arithmetic check and a way to test “what‑if” weight or size changes before you buy a label.