Available in English
Four calculators, fully translated. The German site has 48 — the link at the bottom of each page takes you there.
Service charges
Check a Nebenkostenabrechnung: which costs may be passed on, how they are apportioned, and the twelve-month deadline that ends the claim.
02 — RentingRental deposit
The statutory cap of three months' net rent, payment in three instalments and the interest that belongs to the tenant.
03 — BuyingPurchase costs
Transfer tax by federal state, notary and land registry fees from the statutory schedule, and the agent's commission.
04 — InvestingRental yield
Gross and net yield including purchase costs, management, maintenance reserve and a vacancy allowance.
Why this exists
German tenancy law is unusually protective and unusually detailed. Much of it is genuinely in your favour — deposits are capped, rent increases are limited, service charges may only cover seventeen listed categories. But none of that helps if the letter arrives in German and the deadline has already passed.
These pages do two things. They calculate, and they tell you which provision applies, so you can look it up yourself. Statutes are published free of charge at gesetze-im-internet.de; many are also available there in English translation.
What this is not
This is not legal advice and cannot be. The calculators work from what you enter and from publicly available data; they cannot see your contract, your building or your municipality's rent index. For anything contested, speak to a lawyer or a tenants' association (Mieterverein) — membership costs less than an hour of legal fees and includes advice.