Australia Post Postage & Parcel Estimator
An Australia Post postage estimator you keep current
Australia Post uses separate, regularly updated tables for letters, domestic parcels, international services, and each speed tier. Instead of hardcoding figures that go stale, this page lets you copy the current numbers from the official price guide and applies them to your own measurements. It settles the three things senders get stuck on: is this still a letter or has its thickness pushed it into a larger category, does cubic weight beat the scale reading, and how many price bands does the billable weight land in? It does not fetch live prices, so check your inputs against Australia Post's stamp prices and parcel guidance before you pay.
Live-data limitation: this calculator does not store or fetch any live Australia Post prices. Verify every rate and service rule against the official Australia Post price guide before relying on the estimate.
Plain-text formula: cubicWeightKg = (lengthCm × widthCm × heightCm) / cubicDivisor; billableWeightKg = max(actualWeightKg, cubicWeightKg) when cubic weight applies; extraBands = ceil(max(0, billableWeightKg − baseBandKg) / bandSizeKg); estimatedPostage = basePrice + extraBands × pricePerBand.
How the estimator works
Pick the service type first. A letter / document is priced on weight plus an optional millimetre size check, because a few extra millimetres of thickness can quietly bump a small letter into the pricier large-letter range. A parcel keeps your scale weight and, when cubic pricing is on, also computes cubic weight from length, width, height, and a divisor; most parcel services bill on whichever weight is larger. The divisor is editable because domestic, courier, and air-freight services use different volumetric rules. The final block is the manual rate table — a base band weight and price, then an additional band size and price per band. Keeping the table separate from the maths is what lets the page survive price updates: you only retype the figures that changed.
Formula and pricing logic
The arithmetic runs in four steps: convert the weight to kilograms (grams ÷ 1000), compute cubic weight for parcels that use it, take the larger of actual and cubic weight as the billable weight, then feed that into the rate table. Cubic (volumetric) weight multiplies the three parcel dimensions and divides by the service divisor:
The billable weight is then max(actual weight, cubic weight). The base band covers everything up to its weight for the base price; any excess is divided by the additional band size and, by default, rounded up because most tables charge each band "or part thereof":
Total price = base price + (number of extra bands × price per additional band)
An exact pro-rata option is included for the rare table that charges weight proportionally. Alongside the total, the page prices a second box for comparison, notes how far you are into the final band and the cubic-neutral volume (actual weight × divisor) below which cubic weight stops mattering, draws a price-versus-weight staircase with a marker at your billable weight, flags editable service-limit ceilings, and can copy a shareable link that encodes the whole form.
Worked example and common situations
Suppose you are posting a parcel that weighs 2.2 kg on the scale and measures 40 cm × 30 cm × 20 cm. Assume the service uses a cubic divisor of 4000 cm³ per kg, the base band covers the first 1.0 kg for AUD 10.00, and each additional 1.0 kg band costs AUD 4.00. The cubic weight is 40 × 30 × 20 ÷ 4000 = 6.0 kg. Because 6.0 kg is higher than the 2.2 kg scale weight, the billable weight becomes 6.0 kg.
The first kilogram is covered by the base price, leaving 5.0 kg above the base band. At a 1.0 kg step that is 5 extra bands, so the estimate is AUD 10.00 + (5 × AUD 4.00) = AUD 30.00 — expensive despite the modest scale weight, purely because of bulk. Repack the same item into a 30 × 20 × 15 cm box and cubic weight falls to 2.25 kg (30 × 20 × 15 ÷ 4000). Billable weight becomes 2.25 kg, which rounds up to 2 extra bands and the estimate drops to AUD 18.00 — a saving from packaging alone, with the contents unchanged. Letters behave differently: the same document can qualify as a small letter loose but slip into large-letter or parcel treatment once padded, which is exactly what the optional letter-dimension fields flag.
| Situation | What usually drives the price | Why it matters |
|---|---|---|
| Thin envelope under common small-letter limits | Letter category plus actual weight | A few extra grams may matter, but staying within thickness limits is usually more important. |
| Large but light parcel | Cubic weight | Bulky packaging can make a parcel bill as if it were much heavier than the scale reading. |
| Dense compact parcel | Actual weight | When the box is small for its mass, cubic weight often becomes irrelevant. |
| Item just over a weight threshold | Band rounding | Going a fraction over a band can trigger the whole next step in the table. |
Limitations and final checks
This is not an official Australia Post calculator: it does not fetch live prices and does not add optional services such as tracking upgrades, extra cover, signature on delivery, pickups, or oversized surcharges — all of which come from the official pricing. It assumes you enter the correct base band, band size, price increment, and divisor, and that the table follows a base-plus-step shape; irregular tables need adjusting by hand, and the letter checks are reference guides against widely quoted limits, not a product-specific guarantee. For a binding price use Australia Post's own tools or a post-office counter. Sending from the UK instead? The Royal Mail postage rate calculator applies the same manual rate-table approach.
Australia Post postage estimates: frequently asked questions
How does this Australia Post postage estimator work?
It combines the rate-table values you enter from the current Australia Post price guide with your item's actual weight, cubic weight, and letter size checks. It does not fetch live prices, so it stays accurate as long as you supply current rates.
What is cubic weight and why can a light parcel be expensive?
Cubic (volumetric) weight converts a parcel's volume into an equivalent weight — length × width × height in centimetres divided by a service divisor such as 4000 cm³ per kg. You are billed on the greater of actual and cubic weight, so a 50 × 40 × 30 cm box bills as 15 kg even if its contents weigh 2 kg. The calculator shows which weight drives your price.
What are the common Australia Post letter size limits?
As a widely used reference, a small letter fits within 240 × 130 mm, is at most 5 mm thick, and weighs no more than 250 g. A large letter fits within 360 × 260 mm, up to 20 mm thick and 500 g. Anything larger or heavier is normally priced as a parcel, so confirm the current product rules before posting.
Does the estimate include GST, surcharges, or extras?
No. The estimate applies only the base-plus-bands structure you enter. Optional services such as signature on delivery, extra cover, or tracking upgrades, plus any surcharges or temporary price changes, must be added from the official Australia Post pricing.
Optional mini-game: Sort the Mail Counter
This mini-game does not change the calculator's result, but it makes the core idea memorable. Each shipment rushes toward the scanner with just enough detail to classify it. Your job is to route it by the rule that decides postage: Small Letter, Large Letter, Bill by Actual Weight, or Bill by Cubic Weight. The faster and more accurately you sort, the higher your score and streak. As the round goes on, the mail mix becomes trickier, with more borderline letters and bulky parcels that punish lazy guessing.
Tip: when cubic weight is higher than actual weight, reducing the box size can lower postage even if the contents stay the same.