Updated 30/09/2026

How to read the calculator results: cash flow, yield and IRR

What the investment calculator's metrics mean: cash flow, yield, IRR, the pessimistic scenario, the traffic light and what-if scenarios. Why an average mortgaged flat comes out orange and what it takes to get green.

At the top, the calculator shows a colour-coded verdict and three numbers. Each of them answers a different question.

Simple and Detailed mode

Simple mode is for a first estimate: four questions (where and for how much, how much of your own money, how you will let the flat, and for how much). The calculator fills in the rest from city and market data. The result is put in plain language: how much you top up each month or have left over, how much your money earns a year and how the flat performs against an ETF.
Detailed mode shows all parameters at once: purchase costs, the fixed-rate period and the rate after it, owners' association (SVJ) payments (building management, repair fund and utility advances), property management, the tax regime and the sale. It adds a comparison of long-term and short-term letting, sensitivity to the interest rate and rent, and a year-by-year table.

Your data is not overwritten when you switch modes, so you can move between them freely.

The three main numbers

Every month (monthly cash flow): how much the flat brings you each month, or how much you top up. After rent, all costs, the mortgage payment and taxes, in the first year after the flat is let. Below it is the average over the whole investment period, because rent rises and interest falls over time.
Annual return on your money (IRR): the average annual return on the cash you put into the flat (own funds, purchase costs and monthly top-ups), including the sale at the end. It is directly comparable with the interest on a savings account or the return on an ETF.
Versus ETF: how much more (or less) you would have at the end than if you had invested the same money at the same moments in an ETF returning, for example, 6 % a year.
Net yield (Detailed mode): the annual operating profit (rent minus operating costs) divided by the price plus purchase costs. It does not depend on financing, so it is useful for comparing flats with each other.

The traffic light: how we rate an investment

IRR alone is not enough. For a mortgaged flat it is easily pushed up by an optimistic assumption about price growth, even when the rent does not cover the interest. The traffic light therefore combines three tests.

Test	What it measures	For green	Red when
1. Return	after-tax IRR with your assumptions	at least 8 %	below 4 %
2. Resilience	Return (IRR) in the pessimistic scenario	at least 4 %	below 0 %
3. Cost coverage	whether the rent covers running costs, interest and taxes	yes (you top up at most the principal)	-

Green - Good investment: passes all three tests.
Red - Weak investment: the IRR is below 4 %, or the investment would lose money in the pessimistic scenario.
Orange - Investment with caveats: everything else. Next to the result you always see which test it failed.

The pessimistic scenario

It is not a disaster scenario, just a somewhat worse reality than the default assumptions:

price growth of at most 2 % a year (just the inflation target of the Czech National Bank (ČNB), i.e. zero real price growth),
a mortgage rate 1 percentage point higher once the fixed-rate period ends,
for a long-term let, 1 fewer let month a year; for a short-term let, occupancy 10 points lower.

Why these thresholds

4 % is the return on a safe alternative. The fixed-rate Czech retail government bond (Dluhopis Republiky) 2026 pays 4.544 % a year. If the flat earns less, it is not worth the risk or the work.
8 % is the long-term return on equities and housing (around 7 % in real terms in 1870-2015) plus a premium for the fact that a single flat is illiquid, undiversified, often bought on credit and requires work.
2 % in the pessimistic scenario: real house prices have grown by only about 1 % a year over the long run (in real terms, prices have tripled since 1900). The ČNB also warns that flat prices are well above the level households can safely finance and that rental yields are only 2-3 %. The last ten years, with growth of 10-16 % a year, were the exception.
Cost coverage separates "I top up the principal, so I am saving into my own flat" from "I top up the interest, so I am losing money on running the flat". The traffic light tolerates topping up principal, but not topping up interest and costs.

That is why an average flat bought at market price and financed with a mortgage usually comes out orange in 2026: the rent does not cover the interest and the return depends on rising prices. Green goes mainly to flats bought well below market value, with above-average rent or with a larger share of your own money.

What would make this a good investment

If the flat does not come out green, the calculator works out what would need to change: the highest purchase price, the lowest rent (for Airbnb, the nightly rate) or the lowest share of your own funds at which it would pass all three tests. Each value holds on its own, with all other inputs unchanged. The Try it button carries it over into the calculation. It is useful as a basis for negotiating the price: you know where your limit is.

What if: scenarios

The "What if…" table shows how the monthly cash flow, the return and the traffic light change when one of the assumptions does not go to plan:

Pessimistic scenario (the same one the traffic light uses to test resilience),
Higher mortgage rate, 1 percentage point higher for the whole term,
Lower rent by 10 % and More vacancy,
Slower price growth (2 % a year) and Flat prices (0 %),
Sell earlier by 5 years and a Negotiated discount of 5 % off the purchase price.

In the Custom scenario you combine changes yourself (rate, rent, price growth, holding period). If the investment only looks good in the default scenario and turns red in two or three of the variants, it is on thin ice.

Where the return comes from

The "Where the return comes from" chart breaks the total profit down into cash flow, principal repaid (the debt the rent has paid off for you) and appreciation after selling costs and tax. For most mortgaged Czech flats, cash flow is negative and the return comes mainly from principal and appreciation. That is why the price growth you enter matters so much.

Other metrics

DSCR: operating profit / loan payments. Below 1, the rent does not cover the payments. Banks want at least 1.2 from investors.
Break-even rent or occupancy: the value at which the flat pays for itself.
Additional cash invested: the sum of all monthly top-ups over the investment period.
Comparison with an alternative: how much you would have if you had invested the same money at the same moments at a return of, for example, 6 % a year.
Sensitivity and tornado chart (Detailed mode): which parameters affect the result most and what happens if the rate or occupancy changes.

Estimates and accuracy

Values you do not enter are filled in by the calculator with an estimate based on the city and the market. You can recognise them by their grey colour and the ≈ sign. The more real numbers you enter (rents from listings nearby, the actual repair fund contribution, your bank's offer), the more accurate the result.

The fastest way to real numbers is Paste a listing: open a listing on Sreality, Bezrealitky or iDNES, select the whole page (Cmd+A or Ctrl+A), copy it and paste it in. The calculator recognises the price, floor area, city and monthly fees. The text stays in your browser and is not sent anywhere. For a rental listing, it uses the amount as the monthly rent, which is handy for estimating the rent of a similar flat.

Sources

This text is indicative and does not replace tax, legal or financial advice. For binding decisions consult a Czech tax adviser. · Privacy policy · Terms of use ·