What it does
Paste the list of hostnames you plan to put on Let's Encrypt certificates, and this tool groups them by registered domain (eTLD+1) and shows how they map onto Let's Encrypt's issuance limits. It tells you the fewest certificates the set needs, flags where a wildcard would collapse several subdomains, and warns when a naive one-certificate-per-name approach would exceed the weekly per-domain limit. Everything runs locally in your browser; nothing is sent anywhere.
Why the registered domain matters
Let's Encrypt's headline limit counts new certificates against the registered domain, not each hostname. So www.example.com, api.example.com, and blog.example.com all draw from a single weekly budget under example.com. The tool computes that grouping with the Public Suffix List, so it is correct even for multi-label suffixes: shop.example.co.uk and www.example.co.uk group under example.co.uk, three labels deep, not co.uk. Getting this boundary wrong is the usual reason people are surprised by a rate-limit error.
What it computes
For each registered domain it shows the count of intended names (against the per-domain weekly limit), the fewest certificates needed if you pack up to 100 names per certificate, and any wildcard candidates: a parent with two or more direct subdomains, which a single *.parent name could cover instead. The summary totals the names, the distinct registered domains, and the minimum certificates across everything, and raises a warning if any one registered domain would exceed the weekly limit when issued one certificate per name. IP-address inputs are listed separately, since they use their own unit, and anything that is not a valid hostname (including a bare public suffix like co.uk) is called out.
The limits it uses
The numbers shown are a dated, sourced snapshot of Let's Encrypt's published limits: 50 new certificates per registered domain per 7 days, 300 new orders per account per 3 hours, 5 certificates for the same exact set of names per 7 days, and up to 100 names per certificate. The single most useful fact for planning is that renewals coordinated by ARI are exempt from all rate limits, so once you are issued, keeping certificates renewed is not a rate-limit concern. Because Let's Encrypt can change these numbers, the tool shows the snapshot date and links the source; check that page for the current values before relying on a tight margin.