Bike lights are rated in Lumens. A Lumen is the measurment 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
One thing worth noting is that most lights advertise their headline rate. Packaging may say 1000 lumens. Reading the small print they might only do that for an hour. That light may run 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 running times.
More power is needed to see where you are going rather than to be seen. Therefore 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.
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:
Our 2021 Cycling Light Guide has a selection of our favourite lights in different lumen ranges.
Or you can just jump right in and shop all cycling lights.