Short answer: if two ISV simulators give different numbers for the same car, the gap almost always comes from two things — the €500 diesel surcharge (particulates) and WLTP vs NEDC CO2. Many online simulators assume by default that the diesel emits under 0.001 g/km of particulates and skip the €500; others use the wrong CO2 table. Each of these moves the ISV (import tax) by hundreds to thousands of euros. Check both in our ISV calculator.
Why the numbers vary
ISV isn't a round figure — it comes from a formula with several inputs, and one wrong input shifts the total a lot. The most common reasons two estimates disagree:
- €500 diesel surcharge (particulates) — added or forgotten.
- WLTP vs NEDC CO2 — different tables for the same emissions number.
- Which CO2 value is used — the certificate (CoC) figure or a "reduced" one.
- Age reduction (Tabela D) — the right bracket from the first-registration date.
- Table year — 2026 keeps the 2025 values, but not everyone updates.
The €500 surcharge — the most-forgotten one (diesel)
Diesel vehicles emitting 0.001 g/km or more of particulates pay a €500 surcharge on top of the ISV. In practice almost every real diesel is over that threshold, so the surcharge applies to the vast majority. Many simulators default to "under 0.001 g/km" and don't add the €500, making the number look lower than what you'll actually pay. The surcharge is also reduced by the car's age, like the rest of the ISV.
Rule of thumb: when you budget a diesel, make sure the estimate includes the €500. Our ISV calculator adds it by default (and lets you switch it off in the rare case the CoC shows <0.001).
WLTP vs NEDC — the number-one error
CO2 can be homologated under two standards: NEDC (older cars, typically pre-2020) and WLTP (from ~2020). Each has its own environmental-component table, and the difference is huge — the same CO2 figure can produce a very different ISV depending on the standard. Using the wrong table is the most expensive mistake you can make.
How to know yours: check the Certificate of Conformity (CoC). If it shows both values (NEDC and WLTP), the WLTP one is always used. Cars before 2018 are usually NEDC; from 2020 on, WLTP.
CO2 is the certificate figure — beware "reduced" values
The tax is charged on the CO2 value from the certificate (CoC), as-is. Be suspicious of simulators that display a "reduced" CO2 on screen (a leftover from 2019 transition tables) — it can make it look like you pay less than you do. Enter the real value from the document.
Age reduction (Tabela D)
Used cars imported from the EU get an age reduction, counted from the first registration — from 10% (under 1 year) up to 80% (over 10 years). The bracket changes on the registration anniversary, so a few days' difference can move the car into another bracket and change the amount. A 3–5-year-old car from Germany often makes sense because the ISV has already "come down".
It's an estimate — the official figure is the AT's
Any simulator gives an estimate; the official amount is set by the tax authority (AT). Ours was built to match the AT simulator to the cent in our tests, and it also covers what many don't: motorcycles, light commercials, motorhomes and plug-in hybrids. Calculate your ISV at /isv and the annual IUC at /iuc — separate tools. If you're importing, see also how to import a car from Germany and whether it's worth importing.
Frequently asked questions
Why do two ISV simulators give different numbers?
Almost always because of the €500 diesel surcharge (added or not) and the WLTP vs NEDC CO2 table. The CO2 value used, the age reduction and the table year also matter.
Does the €500 surcharge apply to my diesel?
It applies if the car emits 0.001 g/km or more of particulates — which is nearly every diesel. It's only exempt if the certificate shows under 0.001 g/km, which is rare.
How do I know if my car is WLTP or NEDC?
Check the Certificate of Conformity (CoC). If it has both values, the WLTP one is used. Before 2018 it's usually NEDC; from 2020 on, WLTP.
Does the simulator give the official figure?
No — it gives an estimate. The official amount is set by the AT. Our calculator is tuned to match the AT simulator to the cent in our tests.
Can I calculate the ISV before buying the car?
Yes, and you should. Enter engine size, CO2 (from the CoC), fuel and first-registration date before paying a deposit — that avoids surprises after tax.