Why is no registration number needed?

A zone camera is matched against the Euro standard held on the vehicle record. Fuel and first registration year show the standard a car had to meet when new, although a car registered near a changeover date still needs an official check.

Is a year enough to be sure?

Almost always, but not at a boundary. Euro 6 became compulsory for new car registrations on 1 September 2015, so a diesel registered in 2015 could be either side of it. Where a year straddles a boundary the answer says so rather than picking one.

What standard does a car have to meet?

Euro 4 for a petrol car and Euro 6 for a diesel, in every UK zone that charges cars. In practice that means a petrol car registered new after 2005 and a diesel registered new after September 2015, which is the rule of thumb Transport for London publishes.

Does an electric car pay anything?

Not in an emissions zone. A battery electric car has no tailpipe, so there is no standard for it to fail. The London Congestion Charge is different: it is a charge for entering the zone rather than for what the vehicle emits, and the discount for electric vehicles has ended.

Clean air zones

ULEZ checker

Use the fuel type and first registration year. A registration number is not required.

Summary: A car's fuel type and first registration year can indicate whether it meets Transport for London's ULEZ emissions standard. This UK checker estimates the Euro standard from the registration timetable; the operating authority's vehicle record decides whether a charge applies.

£12.50

London ULEZ, car, a day

Transport for London, Ultra Low Emission Zone (as of 8 April 2019)

Euro 4

Petrol pass mark

Transport for London, Ultra Low Emission Zone (as of 8 April 2019)

Euro 6

Diesel pass mark

Transport for London, Ultra Low Emission Zone (as of 8 April 2019)

Results for all 15 UK zones

All 15 UK charging, low emission and congestion zones, each with its own verdict rather than a single national answer.
The Euro standard a car of that fuel and year had to meet when it was registered new, taken from the legislation rather than from a rule of thumb.
The Scottish low emission zones, where there is no charge to pay: a non compliant vehicle is not allowed in and the first camera capture is a penalty.
The zones that never charge cars at all, so you can stop worrying about them.

Common questions

Why is no registration number needed?: A zone camera is matched against the Euro standard held on the vehicle record. Fuel and first registration year show the standard a car had to meet when new, although a car registered near a changeover date still needs an official check.
Is a year enough to be sure?: Almost always, but not at a boundary. Euro 6 became compulsory for new car registrations on 1 September 2015, so a diesel registered in 2015 could be either side of it. Where a year straddles a boundary the answer says so rather than picking one.
What standard does a car have to meet?: Euro 4 for a petrol car and Euro 6 for a diesel, in every UK zone that charges cars. In practice that means a petrol car registered new after 2005 and a diesel registered new after September 2015, which is the rule of thumb Transport for London publishes.
Does an electric car pay anything?: Not in an emissions zone. A battery electric car has no tailpipe, so there is no standard for it to fail. The London Congestion Charge is different: it is a charge for entering the zone rather than for what the vehicle emits, and the discount for electric vehicles has ended.

Sources

Each result page carries the sources for every zone it names.