Demystifying the math — a friendly guide to OMSP, CO₂ bands (WLTP/NEDC), the NOx levy, and a full worked example.
Why this matters (quick hook)
VRT can add thousands to the cost of a car — knowing how it’s calculated helps you make better buying decisions and avoid surprises.
The core formula (plain and simple)
Final VRT (Category A) = CO₂ component (percentage of OMSP) + NOx levy
Everything starts with the OMSP — that’s the value we apply percentages to and add NOx on top of. Read on for each piece in plain language.
Step 1 — What is OMSP (Open Market Selling Price)?
OMSP is Revenue’s assessed market value for the vehicle — not necessarily what you paid or what’s printed on the invoice. It’s the figure Revenue uses to calculate percentage-based parts of VRT (the CO₂ component).
Revenue looks at comparable Irish market sales, the exact model, trim, options, age, mileage, and condition when setting the OMSP. Small differences in OMSP directly change the VRT because CO₂ charges are percentages of this value.
Tip: when importing, have the vehicle’s spec and mileage ready — Revenue will use those to match the correct OMSP.
Step 2 — CO₂ emissions and VRT bands (Category A passenger cars)
Why CO₂ matters
The CO₂ component is a percentage of OMSP. Lower CO₂ usually means lower percentage — so lower VRT. Many VRT decisions are now based on WLTP CO₂ figures (the modern test standard).
WLTP vs NEDC — what to know
WLTP is the more accurate, modern test for CO₂. If a vehicle only has an older NEDC figure, Revenue uses conversion rules to produce a WLTP-equivalent figure before applying VRT bands. This conversion is described in Revenue guidance and related manuals.
Current CO₂ band summary (Category A)
Revenue applies CO₂-based rates to the OMSP. As of the latest Revenue guidance, the simplified banding for many passenger cars is:
- 0–120 g/km (WLTP): 8% of OMSP, or minimum €160 (whichever is greater).
- >120 g/km (WLTP): 13.3% of OMSP, or minimum €266 (whichever is greater).
Example — CO₂ charge calculations
If OMSP = €15,000 and CO₂ = 110 g/km → CO₂ charge = 8% × €15,000 = €1,200 (above the €160 minimum).
If OMSP = €2,000 and CO₂ = 130 g/km → 13.3% × €2,000 = €266 (this equals the stated minimum so Revenue would use €266 as it’s the higher rule threshold).
Step 3 — The NOx levy (nitrogen oxides)
What is NOx? NOx stands for nitrogen oxides — pollutants measured in mg/km (or mg/kWh for certain heavy duty engines).
How the NOx levy is applied
The NOx levy is cumulative and banded. Revenue charges it in bands depending on mg/km recorded (usually on the Certificate of Conformity):
| NOx (mg/km) | Charge (per mg/km) |
|---|---|
| 0 – 40 mg/km | €5 per mg |
| 41 – 80 mg/km (next 40 mg) | €15 per mg |
| Above 80 mg/km | €25 per mg |
There are caps to avoid runaway charges: currently the NOx levy cap is €4,850 for diesel vehicles and €600 for other vehicles.
NOx example
If a car records NOx = 60 mg/km:
- First 40 mg @ €5 = 40 × €5 = €200
- Next 20 mg @ €15 = 20 × €15 = €300
Total NOx levy = €500.
Step 4 — VRT Categories (B, C, D, M) — quick primer
Not every vehicle uses the Category A calculator. Revenue separates vehicles into VRT categories with different rules. See the summary below and consult the manual for edge cases.
- Category A: Passenger cars (M1) — CO₂ % of OMSP + NOx levy (described above).
- Category B: Light commercial vehicles (N1) — CO₂-based but with specific rates for vans; Budget adjustments have introduced lower bands for low-CO₂ commercial vehicles (e.g., 8% for 0–120 g/km in some recent rules).
- Category C: Larger commercials, agricultural vehicles, buses, special vehicles — often subject to flat-rate or bespoke rules.
- Category D: Exempt vehicles (for example some diplomatic or historic vehicle cases) — no VRT.
- Category M: Motorcycles — typically taxed based on engine capacity (cc) and age-related reductions may apply.
Always check Revenue’s VRT manual for the exact treatment of the category your vehicle sits in.
Putting it all together — full worked example (Category A)
Vehicle details (hypothetical)
- OMSP (Revenue-assessed): €20,000
- WLTP CO₂: 142 g/km → falls into >120 g/km band
- NOx (Certificate): 60 mg/km
Step A — CO₂ component
Rate for >120 g/km = 13.3% of OMSP (or minimum €266, whichever is greater).
CO₂ charge = 13.3% × €20,000 = €2,660
Step B — NOx levy
NOx = 60 mg/km → first 40 mg @ €5, next 20 mg @ €15.
NOx charge = (40 × €5) + (20 × €15) = €200 + €300 = €500
Final VRT due
Total VRT = CO₂ component + NOx levy = €2,660 + €500 = €3,160.
Note: If the CO₂ percentage calculation produced an amount below the stated minimum, Revenue uses the higher (minimum) value.
Practical tips & other notes
- Use Revenue’s VRT enquiry / calculator for an official estimate — the final VRT depends on Revenue’s OMSP and the exact CO₂/NOx figures at registration.
- If your car only has NEDC CO₂ figures, Revenue converts to a WLTP equivalent using published formulas — so don’t be surprised if your CO₂ number changes when Revenue assesses it.
- Electric cars: typically exempt from NOx and may qualify for VRT reliefs. Check current Revenue relief thresholds and expiry dates (some reliefs are time-limited).
- Small differences in OMSP make a difference — always provide accurate vehicle spec and mileage to Revenue to avoid surprises.
Conclusion — quick recap
- Find the OMSP — Revenue’s market value for your vehicle.
- Determine CO₂ (WLTP or converted NEDC) and apply the CO₂ % to OMSP.
- Calculate the banded NOx levy from mg/km data.
- Add CO₂ component + NOx levy = total VRT for a Category A car.
Use a trusted VRT calculator for a quick estimate, but for the final amount you’ll need the OMSP and the exact CO₂ and NOx figures Revenue will use on registration.
Want a precise instant estimate? Use our VRT calculator and enter OMSP, WLTP CO₂ and NOx to see the result immediately.
Frequently Asked Questions
1. Where does Revenue get OMSP from?
Revenue builds OMSP from real Irish market comparables, considering model, trim, options, age, mileage and condition.
2. What if my car only has an NEDC CO₂ figure?
Revenue converts older NEDC figures to WLTP-equivalents using published formulas or tools (e.g., CO2MPAS style conversion) before applying CO₂ bands.
3. Are there caps on the NOx levy?
Yes. The NOx levy is capped (for example, €4,850 for diesel vehicles and €600 for other vehicles) so very high NOx figures don’t create unlimited charges.
4. Do electric vehicles pay NOx or CO₂ charges?
Battery electric vehicles (BEVs) are typically exempt from NOx charges and may benefit from VRT reliefs; check Revenue guidance for the current relief amount and eligibility windows.
5. Where can I get an official VRT quote?
Use the Revenue VRT enquiry / ROS system and the official VRT calculator for an estimate — but remember the final figure depends on Revenue’s OMSP and the official CO₂/NOx data.
Sources: Revenue.ie(VRT calculating & applying tax pages and VRT manuals), VRT industry guides and official NOx guidance.