Estimate format, check limits, and calculate postage costs instantly
Postage Cost & Format Estimator
Enter your item's weight and dimensions below.
Max 20,000g (20kg) for standard parcels.
Please enter a valid weight greater than 0.
Longest side
Shorter side
Thickness
Please enter all dimensions greater than 0.
United Kingdom (Mainland)
International (Estimate Only)
Currently optimized for UK Mainland prices.
Estimated Classification:
Enter details…
Estimated Cost: £0.00
Based on Royal Mail size and weight guide (2024 Estimates).
Max Weight for Band–
Dimensions Status–
Volumetric Weight–
Comparison of Service Options for Your Package
Service Type
Delivery Speed
Estimated Cost (£)
Enter values to see comparison
Figure 1: Cost comparison across different service tiers for your specific weight.
Understanding the Royal Mail Post Weight Calculator
Whether you are an eBay seller, a small business owner, or simply sending a birthday gift, understanding how postage costs are calculated is essential for saving money. A royal mail post weight calculator is a critical tool that helps you determine not just the cost, but the specific "format" classification of your item. Royal Mail uses a combination of weight and physical dimensions to categorize mail, meaning a light but bulky item can cost significantly more than a heavy but small one.
What is a Royal Mail Post Weight Calculator?
A royal mail post weight calculator is a digital utility designed to interpret the complex pricing matrix of postal services. Unlike simple weight scales, this calculator considers the "volumetric" or dimensional constraints that dictate whether your item travels as a "Large Letter" or a "Small Parcel."
This tool is primarily designed for:
eCommerce Sellers: Who need to forecast shipping margins accurately.
Office Managers: Who need to stamp mail correctly without overpaying.
General Public: To avoid the "grey card of doom" (surcharge for underpaid postage).
A common misconception is that weight is the only factor. In reality, a rigid greeting card that is 6mm thick might be pushed from the "Letter" category to "Large Letter," nearly doubling the cost, despite weighing less than 20g.
The Royal Mail Pricing Formula and Logic
The calculation logic relies on a "sieve" method. Your item is tested against the smallest category limits first. If it fits (both weight and size), it remains there. If it exceeds any single limit (length, width, depth, or weight), it falls through to the next, more expensive category.
Key Variables Table
Variables used in Postal Calculation
Variable
Meaning
Unit
Typical Range
Weight
Total mass of item + packaging
Grams (g)
0g – 20,000g
Dimensions (L, W, D)
The physical size of the package
Centimeters (cm)
Min: 10x7cm
Format
The pricing tier (Letter, Parcel, etc.)
Category
N/A
Zone
Destination (UK, Europe, World)
Region
UK / Intl
The mathematical logic follows this hierarchy:
Letter: Max 100g | 24 x 16.5 x 0.5 cm
Large Letter: Max 750g | 35.3 x 25 x 2.5 cm
Small Parcel: Max 2kg | 45 x 35 x 16 cm
Medium Parcel: Max 20kg | 61 x 46 x 46 cm
Practical Examples of Use
Example 1: The "Thick" Birthday Card
Scenario: You are sending a handmade card with a badge attached. It weighs 45g.
Inputs: Weight: 45g, Size: 20cm x 15cm x 0.8cm.
Analysis: While 45g is well under the 100g limit for a standard Letter, the depth is 0.8cm. The limit for a Letter is 0.5cm.
Result: The calculator upgrades this to a Large Letter. Instead of paying ~£1.35, you pay the Large Letter rate.
Example 2: The Shoe Box
Scenario: Returning a pair of trainers. Weight 1.2kg.
Inputs: Weight: 1200g, Size: 35cm x 25cm x 12cm.
Analysis: The weight (1.2kg) fits within Small Parcel (up to 2kg). The dimensions (35x25x12cm) also fit comfortably within the Small Parcel limits (45x35x16cm).
Result: Classified as Small Parcel. If the box were slightly larger (e.g., 50cm long), it would jump to Medium Parcel price, costing significantly more.
How to Use This Royal Mail Post Weight Calculator
Weigh Your Item: Use accurate kitchen or digital scales. Round up to the nearest gram. Enter this in the "Item Weight" field.
Measure Dimensions: Measure the Length, Width, and Depth. Order does not matter for the calculator, but ensure you measure the widest points (including packaging).
Select Destination: Currently, the tool defaults to UK Inland pricing structure.
Review Classification: The tool will instantly tell you if your item is a Letter, Large Letter, or Parcel.
Check the Chart: Look at the dynamic chart to compare 1st Class vs 2nd Class costs to decide if speed is worth the extra price.
Key Factors That Affect Royal Mail Results
When using a royal mail post weight calculator, several external factors influence the final price beyond just the raw numbers.
Packaging Weight: A heavy cardboard box can add 200-500g to your total. Always weigh the item inside its final packaging.
Dimensional Volumetric Weight: For international shipping, if a package is light but large, carriers charge based on volume ((L x W x H) / 5000) rather than actual weight.
Compensation Cover: Standard prices usually include compensation up to £20. Higher value items require "Signed For" or "Special Delivery" services, increasing the cost.
Seasonal Surcharges: Carriers often apply peak surcharges during Christmas or Black Friday periods, which might not be reflected in standard base rate tables.
Format Efficiency: Folding a document in half (A4 to A5) can reduce it from Large Letter to Letter, saving over 50% on postage.
Online vs Branch Pricing: Royal Mail often offers cheaper rates if you buy postage online compared to buying at the Post Office counter.
Frequently Asked Questions (FAQ)
Q: Does this calculator include "Signed For" pricing? A: Yes, the comparison table attempts to show Signed For estimates where applicable for the weight band.
Q: What happens if my parcel is 1g over the limit? A: Strictly speaking, you move to the next price band. 101g is a Large Letter, not a Letter.
Q: Why is my small but heavy item so expensive? A: Weight bands are strict. Even if an item is small (like a dense metal part), if it exceeds 2kg, it cannot be a Small Parcel.
Q: Are tubes calculated differently? A: Yes, tubes have specific length + diameter rules. This calculator assumes rectangular packaging.
Q: Can I use this for international mail? A: The current logic applies UK Mainland classifications. International mail uses different zones (Europe, World Zone 1, 2, 3) and weight steps.
Q: What is the maximum weight for a Medium Parcel? A: Typically 20kg. Anything heavier usually requires a dedicated courier service like Parcelforce.
Q: Does 1st Class guarantee next day delivery? A: No, 1st Class is an aim (usually 1-2 days). Only "Special Delivery Guaranteed" services guarantee next day arrival.
Q: How accurate is the price estimation? A: Prices change annually (usually in April). This tool provides estimates based on standard rate cards, but you should always verify the final price at the point of purchase.
Related Tools and Resources
Explore our other shipping and finance tools to optimize your logistics:
// — 1. CONFIGURATION & DATA —
// Royal Mail Format Limits (Approximate standard limits)
// Dims in cm, Weight in g
var formats = {
letter: { maxW: 100, maxL: 24, maxWidth: 16.5, maxD: 0.5, name: "Letter" },
largeLetter: { maxW: 750, maxL: 35.3, maxWidth: 25, maxD: 2.5, name: "Large Letter" },
smallParcel: { maxW: 2000, maxL: 45, maxWidth: 35, maxD: 16, name: "Small Parcel" },
mediumParcel: { maxW: 20000, maxL: 61, maxWidth: 46, maxD: 46, name: "Medium Parcel" }
};
// Approximate Pricing Data (UK Inland) – Simplified for demonstration
// Structure: [First Class, Second Class, Signed For First, Signed For Second]
var prices = {
letter: {
"0-100": [1.35, 0.85, 2.75, 2.25]
},
largeLetter: {
"0-100": [2.10, 1.55, 3.80, 3.25],
"101-250": [2.90, 2.10, 4.60, 3.80],
"251-500": [3.50, 2.70, 5.20, 4.40],
"501-750": [3.50, 2.70, 5.20, 4.40] // Usually jumps to small parcel logic if heavy, but keeping 750g band
},
smallParcel: {
"0-2000": [4.19, 3.49, 5.59, 4.89]
},
mediumParcel: {
"0-2000": [6.29, 5.49, 7.69, 6.89],
"2001-10000": [7.99, 6.99, 9.39, 8.39],
"10001-20000": [11.99, 10.49, 13.39, 11.89]
}
};
// — 2. CORE LOGIC —
function validateAndCalculate() {
var weight = parseFloat(document.getElementById('weight').value);
var l = parseFloat(document.getElementById('length').value);
var w = parseFloat(document.getElementById('width').value);
var d = parseFloat(document.getElementById('depth').value);
var weightErr = document.getElementById('weightError');
var dimErr = document.getElementById('dimError');
// Validation
var isValid = true;
if (isNaN(weight) || weight <= 0) {
// Only show error if field is not empty (user is typing)
if(document.getElementById('weight').value !== "") weightErr.style.display = 'block';
isValid = false;
} else {
weightErr.style.display = 'none';
}
if (isNaN(l) || isNaN(w) || isNaN(d) || l <= 0 || w <= 0 || d <= 0) {
if(document.getElementById('length').value !== "" && document.getElementById('width').value !== "" && document.getElementById('depth').value !== "") {
dimErr.style.display = 'block';
}
isValid = false;
} else {
dimErr.style.display = 'none';
}
if (!isValid) return;
// Sort dimensions to match logic (L is longest, D is shortest)
var dims = [l, w, d].sort(function(a, b) { return b – a; });
var length = dims[0];
var width = dims[1];
var depth = dims[2];
// Determine Format
var format = "Over Size Limit";
var costObj = null; // [1st, 2nd, S1, S2]
var formatKey = "";
// Check Letter
if (weight <= formats.letter.maxW && length <= formats.letter.maxL && width <= formats.letter.maxWidth && depth <= formats.letter.maxD) {
format = formats.letter.name;
formatKey = "letter";
costObj = prices.letter["0-100"];
}
// Check Large Letter
else if (weight <= formats.largeLetter.maxW && length <= formats.largeLetter.maxL && width <= formats.largeLetter.maxWidth && depth <= formats.largeLetter.maxD) {
format = formats.largeLetter.name;
formatKey = "largeLetter";
if (weight <= 100) costObj = prices.largeLetter["0-100"];
else if (weight <= 250) costObj = prices.largeLetter["101-250"];
else if (weight <= 500) costObj = prices.largeLetter["251-500"];
else costObj = prices.largeLetter["501-750"];
}
// Check Small Parcel
else if (weight <= formats.smallParcel.maxW && length <= formats.smallParcel.maxL && width <= formats.smallParcel.maxWidth && depth <= formats.smallParcel.maxD) {
format = formats.smallParcel.name;
formatKey = "smallParcel";
costObj = prices.smallParcel["0-2000"];
}
// Check Medium Parcel
else if (weight <= formats.mediumParcel.maxW && length <= formats.mediumParcel.maxL && width <= formats.mediumParcel.maxWidth && depth <= formats.mediumParcel.maxD) {
format = formats.mediumParcel.name;
formatKey = "mediumParcel";
if (weight <= 2000) costObj = prices.mediumParcel["0-2000"];
else if (weight <= 10000) costObj = prices.mediumParcel["2001-10000"];
else costObj = prices.mediumParcel["10001-20000"];
}
else {
format = "Parcelforce / Courier Required";
formatKey = "courier";
}
// Update UI
updateResults(format, costObj, weight, formatKey);
}
function updateResults(format, costs, weight, formatKey) {
var classDiv = document.getElementById('classificationResult');
var costDiv = document.getElementById('costResult');
var tableBody = document.getElementById('comparisonTableBody');
classDiv.innerText = format;
if (formatKey === "courier" || !costs) {
costDiv.innerText = "Check Courier Rates";
tableBody.innerHTML = "
Item exceeds standard Royal Mail sizes.
";
drawChart([0,0,0,0]);
updateMetrics(weight, 0, true);
return;
}
costDiv.innerText = "Estimated 1st Class: £" + costs[0].toFixed(2);
// Update Metrics
var limit = 0;
if (formatKey === "letter") limit = 100;
else if (formatKey === "largeLetter") limit = 750;
else if (formatKey === "smallParcel") limit = 2000;
else if (formatKey === "mediumParcel") limit = 20000;
updateMetrics(weight, limit, false);
// Update Table
var html = "";
html += "
Standard 1st Class
1-2 Days
£" + costs[0].toFixed(2) + "
";
html += "
Standard 2nd Class
2-3 Days
£" + costs[1].toFixed(2) + "
";
html += "
Signed For 1st
1-2 Days
£" + costs[2].toFixed(2) + "
";
html += "
Signed For 2nd
2-3 Days
£" + costs[3].toFixed(2) + "
";
tableBody.innerHTML = html;
// Update Chart
drawChart(costs);
}
function updateMetrics(currentWeight, limit, isOver) {
var l = parseFloat(document.getElementById('length').value);
var w = parseFloat(document.getElementById('width').value);
var d = parseFloat(document.getElementById('depth').value);
if (isOver) {
document.getElementById('maxWeightMetric').innerText = "Exceeded";
document.getElementById('dimStatusMetric').innerText = "Too Large";
document.getElementById('volWeightMetric').innerText = "N/A";
} else {
document.getElementById('maxWeightMetric').innerText = limit + "g";
document.getElementById('dimStatusMetric').innerText = "Fits " + document.getElementById('classificationResult').innerText;
// Volumetric calculation (L*W*D)/5000 is standard courier formula
var volWeight = (l * w * d) / 5000;
document.getElementById('volWeightMetric').innerText = volWeight.toFixed(2) + "kg";
}
}
// — 3. CHART LOGIC (Canvas API) —
function drawChart(costs) {
var canvas = document.getElementById('priceChart');
var ctx = canvas.getContext('2d');
// Handle high pixel density
var dpr = window.devicePixelRatio || 1;
var rect = canvas.getBoundingClientRect();
canvas.width = rect.width * dpr;
canvas.height = rect.height * dpr;
ctx.scale(dpr, dpr);
var width = rect.width;
var height = rect.height;
var padding = 40;
var barWidth = (width – (padding * 2)) / 9; // 4 bars + spacing
ctx.clearRect(0, 0, width, height);
if (costs[0] === 0) return; // No data
var maxVal = Math.max(costs[0], costs[1], costs[2], costs[3]) * 1.2;
var labels = ["1st", "2nd", "Sign 1st", "Sign 2nd"];
var colors = ["#004a99", "#6c757d", "#28a745", "#17a2b8"];
// Draw Axis
ctx.beginPath();
ctx.moveTo(padding, padding);
ctx.lineTo(padding, height – padding);
ctx.lineTo(width – padding, height – padding);
ctx.strokeStyle = "#ccc";
ctx.stroke();
// Draw Bars
for (var i = 0; i < 4; i++) {
var val = costs[i];
var barHeight = (val / maxVal) * (height – (padding * 2));
var x = padding + (i * 2 * barWidth) + barWidth;
var y = height – padding – barHeight;
ctx.fillStyle = colors[i];
ctx.fillRect(x, y, barWidth, barHeight);
// Text Label
ctx.fillStyle = "#333";
ctx.font = "12px sans-serif";
ctx.textAlign = "center";
ctx.fillText("£" + val.toFixed(2), x + (barWidth/2), y – 5);
// X Axis Label
ctx.fillText(labels[i], x + (barWidth/2), height – padding + 15);
}
}
// — 4. UTILITIES —
function resetCalculator() {
document.getElementById('weight').value = '';
document.getElementById('length').value = '';
document.getElementById('width').value = '';
document.getElementById('depth').value = '';
document.getElementById('classificationResult').innerText = "Enter details…";
document.getElementById('costResult').innerText = "Estimated Cost: £0.00";
document.getElementById('comparisonTableBody').innerHTML = "
Enter values to see comparison
";
document.getElementById('weightError').style.display = 'none';
document.getElementById('dimError').style.display = 'none';
var ctx = document.getElementById('priceChart').getContext('2d');
ctx.clearRect(0,0, 1000, 1000); // clear canvas
}
function copyResults() {
var format = document.getElementById('classificationResult').innerText;
var cost = document.getElementById('costResult').innerText;
var weight = document.getElementById('weight').value;
var l = document.getElementById('length').value;
var w = document.getElementById('width').value;
var d = document.getElementById('depth').value;
var text = "Royal Mail Weight Calculation:\n";
text += "Inputs: " + weight + "g (" + l + "x" + w + "x" + d + "cm)\n";
text += "Format: " + format + "\n";
text += cost + "\n";
text += "Generated by Royal Mail Post Weight Calculator";
var tempInput = document.createElement("textarea");
tempInput.value = text;
document.body.appendChild(tempInput);
tempInput.select();
document.execCommand("copy");
document.body.removeChild(tempInput);
var btn = document.querySelector('.btn-copy');
var originalText = btn.innerText;
btn.innerText = "Copied!";
setTimeout(function(){ btn.innerText = originalText; }, 2000);
}