Royal Mail Postage Rate Calculator (UK)
Use this page to estimate inland UK postage for letters and parcels using Royal Mail size categories and your own up-to-date rate table. It is designed for quick checks while packing orders, sending documents, or comparing services.
Estimate Royal Mail postage by size category and weight band
Royal Mail inland prices depend on two checks that are easy to miss when you are packing quickly: (1) which size category your item fits (Letter, Large Letter, Small Parcel, Medium Parcel), and (2) which weight band you land in for the service you are buying (for example “up to 100 g”, then “each additional 100 g or part thereof”). This calculator helps with both.
It does not hard-code a specific year’s prices. Instead, you enter the base price and the per-step increment from the latest Royal Mail price guide (or your business rate card). That keeps the arithmetic correct even when Royal Mail updates tariffs.
The result is an estimate intended to help you choose packaging, confirm you are in the right category, and avoid underpaying postage. For buying labels, always cross-check the final price in the official Royal Mail flow or your shipping platform, especially if you add options like Signed For, Special Delivery, or compensation upgrades.
What you enter (and how to measure it)
The calculator uses the packed item, not the product alone. A few millimetres of cardboard, bubble wrap, or tape can push a Large Letter into Small Parcel. Likewise, a label and extra padding can move you into the next weight band.
- Length, width, thickness (mm): measure the packed item at its widest points. Thickness is often the deciding factor for mailers.
- Weight (grams): weigh the packed item including packaging. The calculator converts grams to kilograms internally.
- Base band weight (kg): the first weight allowance included in your chosen price line (for example 0.1 kg for “up to 100 g”).
- Base price for base band (£): the price that covers the base band.
- Additional step size (kg): the increment used after the base band (for example 0.1 kg for “each additional 100 g or part”).
- Price per additional step (£): the amount added for each additional step.
How to use the calculator (quick workflow)
- Enter your item’s dimensions in millimetres and weight in grams (use the packed item).
- Choose the Royal Mail service you plan to buy (e.g., 1st/2nd Class, Tracked 24/48, Signed For, Special Delivery) and copy the relevant base band, base price, step size, and step price into the form.
- Select Estimate Postage. The results panel shows the size category, the number of additional steps (rounded up), and the estimated total.
- If the category shows Exceeds Royal Mail Medium Parcel limits, you will likely need Parcelforce or another courier; the rate-table arithmetic here may no longer match the service you need.
Royal Mail size categories (typical retail inland limits)
The calculator checks your dimensions and weight against common inland limits. Always verify against the latest Royal Mail guidance for your service, but these are widely used reference limits:
- 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.
Tip: the script treats the larger of your two flat dimensions as “length” for category checking, so you can enter length/width in either order. Thickness is checked as entered, so measure the thickest point (including seams, clasps, or bulges).
Formula used for the estimate (rate-table arithmetic)
Let w be the actual weight in kilograms, b the base band weight (kg), s the additional step size (kg), B the base price (£), and u the price per additional step (£). Royal Mail bands are typically “or part thereof”, so any fraction over a band rounds up.
| Component | Definition |
|---|---|
| Extra weight | max(0, w − b) |
| Steps | ceil(Extra weight / s) |
| Total postage | B + Steps × u |
Practical interpretation: if your base band is 0.1 kg (100 g) and your step size is 0.1 kg, then 101 g and 199 g both count as “one extra step”, while 200 g exactly may still be within the second band depending on how the table is written. When in doubt, follow the wording in the official guide.
Worked example (realistic)
Suppose you are sending a padded envelope measuring 320 × 220 × 12 mm and weighing 180 g. That fits Large Letter limits. Your chosen service’s price guide says: base price for the first 100 g is £2.70, and each additional 100 g (or part) adds £0.60.
- Convert weight: 180 g = 0.180 kg
- Base band: b = 0.100 kg
- Extra weight: 0.180 − 0.100 = 0.080 kg
- Step size: s = 0.100 kg → steps = ceil(0.080 / 0.100) = 1
- Total: £2.70 + 1 × £0.60 = £3.30
If the same envelope weighed 205 g (0.205 kg), extra weight would be 0.105 kg and steps would round up to 2, producing £3.90. This rounding is the most common reason people under-estimate postage.
Category examples and common packing mistakes
Many “wrong postage” problems come from assuming an item is a Letter when it is actually a Large Letter, or assuming a Large Letter when the thickness pushes it into Small Parcel. Use the examples below as a quick mental checklist before you buy postage.
| Item you are sending | Likely category | What usually goes wrong | What to measure twice |
|---|---|---|---|
| A few A4 sheets in a thin envelope | Large Letter | Assuming Letter pricing because it “feels like a letter” | Thickness at the thickest point (staples, folds) |
| A paperback book in a cardboard mailer | Large Letter or Small Parcel | Mailer corners or stiffeners push thickness over 25 mm | Thickness including the mailer seam |
| A small cosmetics box | Small Parcel | Measuring only length/width and ignoring depth | Depth (thickness) and total packed weight |
| A shoe box | Medium Parcel | Choosing Small Parcel based on weight alone | Longest side and overall girth |
Service notes (Tracked, Signed For, Special Delivery)
Different services use different price tables, but the same workflow applies: confirm the size category, then apply the correct base band and increments for that service. If you switch from 2nd Class to Tracked 48, for example, keep your dimensions and weight the same but update the rate inputs to match the Tracked 48 line in the guide.
If you are comparing services, run the calculator multiple times with the same dimensions and weight and only change the rate inputs. That gives you a like-for-like comparison and makes it obvious when a service uses different weight steps.
Troubleshooting and sanity checks
If the estimate looks wrong, it is usually one of the issues below. These checks are also useful if you are building a packing process for a small shop and want consistent results.
- Units mismatch: dimensions must be in millimetres and weight in grams. If you measured in centimetres, multiply by 10 before entering.
- Base band vs step size: make sure your base band matches the first line of the table you are using. For “up to 100 g”, base band is 0.1 kg.
- “Or part thereof” rounding: if your table says “each additional 100 g or part”, then 101 g counts as one full step.
- Packaging creep: tape, labels, and stiffeners can add both thickness and weight. Weigh and measure the final packed item.
- Category boundaries: if you are close to a limit (e.g., 25 mm thickness for Large Letter), consider choosing packaging that gives you a buffer.
- Exceeds Medium Parcel: if the calculator flags this, the correct service may be Parcelforce or a courier with different pricing rules.
A quick check: if you increase weight slightly (for example from 150 g to 151 g) and your step size is 0.1 kg, the price should only change when you cross a band boundary. If it changes unexpectedly, re-check the base band and step size you entered.
Frequently asked questions (practical)
Is this an official Royal Mail calculator?
No. This is an independent estimator that performs the same kind of band arithmetic you see in the price guide. You supply the rates. For official pricing and purchasing, use Royal Mail’s website or your postage provider.
Why does the calculator ask for prices instead of selecting a service?
Royal Mail prices change over time and can differ between retail and business accounts. By letting you enter the base price and step increment, the calculator stays useful even when tariffs update. It also works for comparing multiple services: you can paste in the numbers for each service line.
Does it include compensation, add-ons, or discounts?
No. The estimate is the core postage arithmetic only. If you add Signed For, Special Delivery options, Saturday delivery, or account discounts, those adjustments are not included unless you incorporate them into the rate inputs yourself.
Can I use it for international shipping?
The size-category checks are based on common UK inland retail limits. International services can have different maximum weights and steps by destination. You can still use the weight-band arithmetic by entering the correct international base band and step size, but always verify destination limits and restrictions.
Limitations and assumptions
- Rates are user-supplied: the estimate is only as accurate as the base/step values you enter from the current Royal Mail guide (or your contract rates).
- Retail inland size rules: category limits here reflect common inland guidance; international services and Parcelforce can differ.
- Add-ons not included: optional extras (compensation upgrades, Saturday delivery, business discounts, surcharges) are not modelled.
- Packaging matters: thickness and weight should include all packaging; a few millimetres can change category.
- Edge cases: unusually shaped items, tubes, or items with protrusions may be assessed differently in practice.
Use the output as an arithmetic check and a way to test “what-if” changes before you buy a label. If you are shipping high-value items, confirm service terms and compensation limits directly with Royal Mail.