Baufinanzierungsrechner: The Math Behind It

You found a flat you like, typed a price into a bank's Baufinanzierungsrechner, and it answered "€1,600 per month". Now you're wondering whether that number is real, or just a widget designed to get your phone number. It is real arithmetic, and you can reproduce it in a spreadsheet in five minutes.

This guide walks the math line by line, using one worked loan, so the next calculator result stops being a black box. For the wider picture of how a German mortgage works, start with our Baufinanzierung guide for internationals; this article only opens the calculator.

The formula behind every Baufinanzierungsrechner

Almost every Baufinanzierungsrechner in Germany models an Annuitätendarlehen: a loan with a constant monthly payment. You choose the Sollzins (nominal interest rate) and the anfängliche Tilgung (initial repayment rate, as a percentage of the original loan per year). The monthly payment is then fixed:

monthly rate = loan × (Sollzins + Tilgung) ÷ 12

Each month, interest is charged on whatever debt is left, and the rest of the payment reduces the debt. Because the debt shrinks, the interest part shrinks and the repayment part grows, while the total payment stays the same. Dr. Klein describes the same principle: interest is only paid on the remaining debt, so the repayment share grows with every payment.

Here is the calculation as a short script you can run anywhere with Python:

def baufi(loan, sollzins, tilgung, years):
    rate = loan * (sollzins + tilgung) / 12
    balance, total_interest = loan, 0.0
    for _ in range(years * 12):
        interest = balance * sollzins / 12
        balance -= rate - interest
        total_interest += interest
    return round(rate), round(balance), round(total_interest)

print(baufi(320_000, 0.04, 0.02, 10))  # (1600, 241467, 113467)

Different tools round differently or charge interest annually rather than monthly, so your bank's schedule can differ by a few hundred euros over ten years. The structure is identical.

Worked example: €320,000 at 4% Sollzins

I ran the script above for one loan and three Tilgung settings. The 4% is an illustrative round input, not a quoted offer. For context, Interhyp's published effective rates for the week of 7–13 September 2026 were 3.99% to 4.38% for 10-year terms depending on loan-to-value.

Initial Tilgung	Monthly rate	Debt left after 10 years	Interest paid in 10 years	Years until fully repaid
1%	€1,333	€280,733	€120,733	about 40.3
2%	€1,600	€241,467	€113,467	about 27.6
3%	€1,867	€202,200	€106,200	about 21.3

Own calculation with the formula above: 4% Sollzins, constant monthly payment, interest charged monthly. Illustrative only.

The 1% row is the trap the widget hides. The monthly rate looks friendly, but after ten years you still owe about 88% of the loan, and the calculator only quotes it as a small line labelled "Restschuld".

Why the rate stays flat while the split moves

At 2% Tilgung, the first year of payments contains about €12,681 of interest and only €6,519 of repayment. In year ten, the same €19,200 of payments splits into about €9,862 interest and €9,338 repayment. That shift is why a higher Tilgung has an outsized effect: extra repayment early lowers the balance that interest is charged on for every later month.

What each input really does

Input	What it changes	Where it hides
Purchase price	Basis for closing costs and loan size	Often entered without Kaufnebenkosten
Equity (Eigenkapital)	Loan amount and loan-to-value tier	Rate tiers use the bank's valuation, not your price
Sollzins / Effektivzins	Interest on the debt vs the all-in yearly cost	Calculators show an example rate
Tilgung	Speed of repayment and total interest	Default is often the lowest one
Sollzinsbindung	How long the rate is fixed	Shorter fixes are usually cheaper today, riskier later
Sondertilgung	Extra payments to cut the balance	Only if your contract allows it

Loan-to-value matters because banks price in tiers. On the Interhyp figures above, the 10-year effective rate moves from 3.99% below 70% financing to 4.38% above 90%, so the same house costs more when you bring less equity. The rate itself is explained in detail in our Baufinanzierung explainer.

Bank, broker or neutral: comparing the calculators

The near-identical search variants (Sparkasse, Commerzbank, Check24, "kostenlos", "online") are not separate tools of separate quality. They fall into three types. This table summarises what each publishes about its own calculator, based on the providers' own pages.

Calculator	Type	What you enter	What you get back
Interhyp	Broker	Monthly payment or price, equity, fixed period, purpose	Sollzins, loan incl. estimated closing costs, monthly rate, debt after the fixed period, itemised repayment schedule
Sparkasse	Bank	Purpose, financing need, equity, fixed period, initial Tilgung	Expected monthly rate on an example rate, debt left after the fixed period
Commerzbank	Bank	Price, equity, fixed period, Tilgung, monthly rate, interest rate	Repayment plan and variants
Check24 interest calculator	Comparison portal	Property and personal details	Possible interest rates based on offers from more than 650 providers

The difference to keep in mind is the business model. A bank's calculator exists to book you into its own product. A broker's and a portal's calculator exists to pass your request to a lender panel. All of them are legitimate; none is a neutral referee.

What no calculator shows you

Closing costs are extra money. Interhyp's own example calculations assume closing costs of 10 to 15% of the purchase price. Enter €400,000 with €80,000 of equity and forget them, and the calculator finances €320,000. Add 10% of costs (€40,000) and the real loan is €360,000: at the same 4% and 2%, the monthly rate rises from €1,600 to €1,800 and the debt after ten years from €241,467 to €271,650. The full fee list is in our closing costs breakdown.

The rate is an example, not an offer. Interhyp's page says the result supports first planning and that an individual offer comes from a personal consultation. Your real rate depends on income, equity, the property and your residency situation, which we cover for non-EU buyers in buying property as a foreigner.

Refinancing risk stays outside the maths. The Restschuld is a number, not a plan. If rates are higher when your fixed period ends, the same balance costs more each month.

A 15-minute check before you trust any result

1. Recompute the monthly rate with the formula. If it does not match, find the extra input (fees, special payments).
2. Add closing costs to the loan, or make sure the calculator does.
3. Look at the Restschuld after the fixed period and ask whether you can refinance it at a rate 1–2 points higher.
4. Compare at least two calculator types, including one that shows real offers.
5. Only then give out your phone number.

If you want real offers next to your worked numbers, an intermediary can show them side by side.

For the full purchase path from viewing to notary, see our complete guide to buying a home in Germany.

FAQ

How is the monthly rate on a Baufinanzierung calculated?

Monthly rate = loan × (Sollzins + initial Tilgung) ÷ 12. The rate stays constant, but the share that goes to repayment grows as the balance falls.

What is the difference between Sollzins and Effektivzins?

Sollzins is the nominal interest rate on the loan. Effektivzins is the annual cost including certain fees and the effect of payment timing, and is the figure meant for comparing offers.

How much Tilgung is typical?

Calculators often default to 1–2%. As the worked example shows, 1% can leave about 88% of the loan outstanding after ten years, so the choice matters more than the default suggests.

Are online Baufinanzierung calculators accurate?

The arithmetic is accurate for the inputs you give. The result is only as realistic as the example rate and the costs you include. It is an orientation, not an offer.

This article is for informational purposes only and does not constitute financial or insurance advice. Compare options via licensed intermediaries such as Check24 or Verivox.