Letter (Max 24cm x 16.5cm x 5mm)
Large Letter (Max 35.3cm x 25cm x 2.5cm)
Small Parcel (Max 45cm x 35cm x 16cm)
Medium Parcel (Max 61cm x 46cm x 46cm)
1st Class (Next day aim)
2nd Class (2-3 working days)
Estimated Postage Cost:
£0.00
function calculatePostage() {
// Get input elements
var typeSelect = document.getElementById('packageType');
var weightInput = document.getElementById('weightGrams');
var classSelect = document.getElementById('serviceClass');
var resultBox = document.getElementById('resultBox');
var priceDisplay = document.getElementById('priceDisplay');
var detailDisplay = document.getElementById('detailDisplay');
var errorDisplay = document.getElementById('errorDisplay');
// Get values
var type = typeSelect.value;
var weight = parseFloat(weightInput.value);
var service = classSelect.value;
var price = 0;
var valid = true;
var errorMsg = "";
// Reset displays
errorDisplay.style.display = "none";
resultBox.style.display = "none";
// Basic Validation
if (isNaN(weight) || weight 100) {
valid = false;
errorMsg = "Standard Letters cannot exceed 100g. Please select 'Large Letter'.";
} else {
if (service === '1st') price = 1.35;
else price = 0.85;
}
}
// LARGE LETTER
else if (type === 'largeLetter') {
if (weight > 750) {
valid = false;
errorMsg = "Large Letters cannot exceed 750g. Please select 'Small Parcel'.";
} else {
// Weight tiers: 100g, 250g, 500g, 750g
if (weight <= 100) {
price = (service === '1st') ? 2.10 : 1.55;
} else if (weight <= 250) {
price = (service === '1st') ? 2.90 : 2.40;
} else if (weight 2000) {
valid = false;
errorMsg = "Small Parcels cannot exceed 2kg (2000g). Please select 'Medium Parcel'.";
} else {
price = (service === '1st') ? 4.19 : 3.49;
}
}
// MEDIUM PARCEL
else if (type === 'mediumParcel') {
if (weight > 20000) {
valid = false;
errorMsg = "This calculator only supports Royal Mail parcels up to 20kg.";
} else {
if (weight <= 2000) {
price = (service === '1st') ? 6.29 : 5.49;
} else if (weight <= 10000) {
price = (service === '1st') ? 7.99 : 6.99;
} else {
// 10kg – 20kg
price = (service === '1st') ? 11.99 : 10.49;
}
}
}
// Output
if (valid) {
priceDisplay.innerHTML = "£" + price.toFixed(2);
var className = (service === '1st') ? "1st Class" : "2nd Class";
var typeName = typeSelect.options[typeSelect.selectedIndex].text.split(' (')[0];
detailDisplay.innerHTML = className + " " + typeName + " (" + weight + "g)";
resultBox.style.display = "block";
} else {
errorDisplay.innerText = errorMsg;
errorDisplay.style.display = "block";
}
}
Understanding UK Postage Costs and Size Formats
Sending mail within the UK requires navigating the specific size and weight formats set by Royal Mail. Whether you are an eBay seller, a small business owner, or simply sending a birthday card, knowing the difference between a "Large Letter" and a "Small Parcel" can save you significant money. This calculator helps you estimate costs for standard 1st and 2nd class services based on the physical characteristics of your item.
Royal Mail Size Guide Definitions
The price of postage in the UK is determined by a combination of size and weight. If an item fits the weight criteria but exceeds the dimensions, it automatically moves up to the next price bracket.
Letter: Most standard greetings cards and postcards fall here.
Max Size: 24cm x 16.5cm x 5mm thick.
Max Weight: 100g.
Large Letter: Useful for A4 documents, magazines, CD/DVD cases, and thin clothing items.
Max Size: 35.3cm x 25cm x 2.5cm thick.
Max Weight: 750g.
Small Parcel: Ideal for shoes, small electronics, or boxed gifts.
Max Size: 45cm x 35cm x 16cm deep.
Max Weight: 2kg.
Medium Parcel: Larger boxes and heavier items.
Max Size: 61cm x 46cm x 46cm deep.
Max Weight: 20kg.
1st Class vs 2nd Class
The service class you choose determines the delivery speed aim, though neither is guaranteed unless you pay extra for "Special Delivery".
1st Class: Royal Mail aims to deliver these items the next working day, including Saturdays. It is generally more expensive but faster.
2nd Class: This service aims for delivery within two to three working days. It is the most cost-effective option for non-urgent mail.
Tips for Reducing Postage Costs
The most common mistake senders make is exceeding the thickness limit of a "Large Letter" (2.5cm). If your item is 3cm thick, it becomes a "Small Parcel," which can nearly double the price. To save money:
Fold clothing flat: Vacuum packing or neat folding can keep shirts within the 2.5cm Large Letter limit.
Check weights accurately: Kitchen scales are usually sufficient for items under 2kg.
Buy postage online: Royal Mail often offers slightly cheaper rates if you buy postage online and print your own label compared to buying at the Post Office counter.