Most bike lights are now rated in Lumens. A Lumen is a measure of the total visible light emitted and is commonly used for all kinds of bulbs. If you are used to buying light bulbs in the old Watts measurements a quick guide is:
40 Watt Bulb = 450 Lumens
60 Watt Bulb = 800 Lumens
There is no fixed agreement on how many Lumens you need for a bike light as it depends on the type of cycling you are doing. As a rough guide we say that:
- Daytime running lights to be seen in daylight = 100+ Lumens
- Urban commuting lights to be seen in town = 50 to 200 Lumens
- Rural riding lights to see where you are going = 400 to 600 Lumens
- Trail riding lights to see everything = 600+ Lumens
One thing to worth noting is that most lights advertise their headline rate. They may say they do 1000 lumens but if you read the small print they might only do that for an hour. The same light will probably work at 500 or 600 lumens for 2 or 3 hours so it’s worth buying a light with more lumens than you need to get decent run times.
Typically you will want a brighter front light than rear light. It’s common for a 500 lumen front light to be paired with a 100 lumen rear light. This is because you need a lot more power to see where you are going than to be seen.