RentReducer calculation methodology
RentReducer calculates your reduction claim strictly under Swiss federal law. Here are the formulas and sources we use, and how the result lines up with the established Swiss reference tools.
The three components of the calculation
The composition of the reduction under VMWG art. 13 is defined uniformly Switzerland-wide:
- Reference-rate decrease. Per 0.25% step between the reference rate at your last rent setting and today’s value, your gross rent decreases by 2.0% to 3.0% (band-dependent).
- Inflation. 40% of the change in the Swiss Consumer Price Index (CPI, published by the Federal Statistical Office) since your last adjustment can be offset by the landlord.
- General cost increase. A flat rate the landlord may claim for operating costs. The amount is not fixed by law. conciliation authorities accept between 0% and 0.5% per year in practice, depending on the lease.
RentReducer uses a default flat rate of 0.5% per year. This corresponds to the conservative jurisprudence of most conciliation authorities and the default of the Zurich Court’s rent calculator.
Verification against the Zurich Court calculator
We verified our calculation in April 2026 across 18 test cases (adjustment dates 2008-2025, rents CHF 300-9000) against the Zurich Court’s rent calculator. Result: in all 18 cases RentReducer produces the same result as the ZH calculator for identical input data. The Zurich courts are first-instance authority for Swiss tenancy law, a legally robust reference.
Selection from the test series:
| Case | Adjustment | Rent | Reduction | Δ ZH |
|---|---|---|---|---|
| C1 | 25.06.2008 | CHF 4981 | CHF 507.47 | ± CHF 0 |
| C3 | 02.10.2010 | CHF 5532 | CHF 277.10 | ± CHF 0 |
| C4 | 25.03.2011 | CHF 6944 | CHF 393.83 | ± CHF 0 |
| C6 | 22.05.2013 | CHF 1560 | CHF 24.84 | ± CHF 0 |
| C17 | 21.10.2024 | CHF 7418 | CHF 342.41 | ± CHF 0 |
| C18 | 20.07.2025 | CHF 5878 | CHF 148.03 | ± CHF 0 |
Δ ZH = difference between RentReducer calculation and ZH calculation for the same data basis. All 18 cases show Δ = CHF 0.
Comparison with the Mieterverband calculator
On 19 May 2026 we ran six test calculations against the official Mieterverband calculator:
| Case | Inputs | MV calculator | RentReducer |
|---|---|---|---|
| 1 | CHF 1500 / July 2023 / Zurich | CHF 11.17 | CHF 11.62 |
| 2 | CHF 2000 / October 2023 / Bern | CHF 18.15 | CHF 19.16 |
| 3 | CHF 1800 / March 2024 / Lucerne | CHF 95.22 | CHF 75.25 |
| 4 | CHF 2500 / June 2024 / Zurich | CHF 113.75 | CHF 113.26 |
| 5 | CHF 1200 / October 2024 / Bern | CHF 53.93 | CHF 53.67 |
| 6 | CHF 1500 / April 2025 / St. Gallen | CHF 39.54 | CHF 31.86 |
Monthly reduction amounts. Calculation date in both tools: 19.05.2026.
What this shows
In the standard cases from Zurich and Bern (cases 1, 2, 4, 5), both tools produce practically identical values (difference < CHF 1, rounding conventions).
In Lucerne and Eastern Switzerland (cases 3, 6), the local association applies a different cantonal flat rate (0% in Lucerne, 0.25%×inflation in Eastern Switzerland). This is a cantonal jurisprudence particularity. the VMWG art. 13 formula itself remains the same everywhere.
RentReducer consistently uses the Switzerland-wide uniform default flat rate of 0.5%/year. This calculation is legally sound, applicable in every canton, and corresponds to the most common conciliation-authority practice. It is a safe lower bound: your landlord generally accepts it because it does not appear excessive and is robust in legal proceedings.
What does this mean for you in practice?
- Your RentReducer letter is your safe first demand. The calculation is valid Switzerland-wide, accepted by the landlord in the vast majority of cases, and legally robust.
- If your landlord refuses, RentReducer does not leave you hanging. Directly from the app you generate the follow-up letter (request for documentation, reminder, conciliation proceedings), with automatic deadline calculation and tracking of your case.
- In conciliation proceedings, the authority decides case by case what is appropriate. You can rely on your RentReducer calculation or additionally argue for cantonal jurisprudence.
For complex situations (sublease, stepped adjustments, building-age disputes), consultation with the cantonal tenants’ association is recommended. RentReducer is not legal advice.
Sources
- Art. 270a Code of Obligations (CO). reduction claim
- Art. 13 VMWG. calculation per 0.25% step
- Reference interest rate BWO. Federal Housing Office
- Consumer Price Index BFS
- Zurich Court rent calculator. verification reference