About ShipMargin
This started as an audit rather than a product. We went through the calculators that rank for shipping and ecommerce fee searches — around forty of them — to find out what a seller pricing a product would actually be told.
The results were worse than expected.
- The page ranking first for "dimensional weight calculator" printed the USPS divisor as 166. It changed to 139 on 12 July 2026. It also omitted DHL entirely.
- The most feature-complete marketplace fee calculator on the web — genuinely the deepest input model we found — states on the page that its fees are current as of 27 November 2022.
- A top-ranking page for a rate-calculator query returned "No shipping rates found". Another's calculator URL redirects to a 404 and has done for long enough that people still link to it.
- The largest tool farm in the ecommerce niche — 97 calculators across fifteen marketplaces — publishes no effective dates at all.
- Of nineteen ecommerce profit calculators, one modelled VAT on marketplace fees, none modelled Amazon's Digital Services Fee, none modelled a return rate, and none modelled import duty — in the year after the United States abolished its $800 de minimis exemption.
So the premise here is narrow and slightly boring: be right, show the working, and put a date on everything. There are 31 calculators, all free, none behind a login, and every published rate carries a source link and a confidence level you can see.
What we will not do
We will not invent a shipping price. Every site showing you a precise per-parcel rate is showing either its own negotiated rate, which you cannot buy, or a number with no provenance. Our cost tools take your rate as an input and get exactly right the parts that go wrong in practice: chargeable weight, surcharge stacking and cross-border duty.
We will not give you a precise duty rate from a category dropdown. Duty is set by an eight or ten digit commodity code. "Footwear" spans single digits to the high thirties. We give a typical range, say that it is a range, and link the official tariff lookup for your destination.
We will not present a secondary source as a primary one. Amazon publishes its FBA rate card only behind the Seller Central login, so our figures are a careful reproduction, and the page says so rather than implying a precision we do not have.
We will not hide an uncertain number inside a result. Where we cannot confirm something — the exact EU interim parcel duty, TikTok Shop's disputed US rate, Nigeria's additional levies — it becomes an editable input with a warning, switched off by default, rather than a silent assumption.
How this is built
It is a static site. Every calculation runs in your browser from published rules, so there is no API to fail, no rate limit to hit, and nothing to send anywhere. That is partly a performance decision and partly a reliability one: a calculator that depends on a live rate feed is a calculator that eventually returns "No shipping rates found".
The calculation engines are covered by a test suite that checks them against published worked examples — DHL's own 4.8 kg case, the USPS 11-to-13-pound divisor change, the UK VAT-on-duty sequence, India's three-levy stack, Amazon's three price bands. See the methodology page for every formula and the sources page for every citation, including a list of what we know we have not verified.
Corrections
If a figure here disagrees with your invoice, your invoice is probably right and we want to know. Send us the source you are working from via the contact page. Corrections are published with the date they were made, and the source register records when each figure was last checked.
We are not affiliated with any carrier or marketplace. Their names appear here to describe their published rules, which is the only way to write about how shipping is priced.