This is probably the most common question we get from people coming into the shop to use our floor pumps (also known as track pumps). All the pumps have a pressure gauge with a scale marked in PSI (pounds per square inch) and/or Bar.
The answer is simple: Look carefully for numbers on the sidewall of the tyre. One set of numbers will describe the size of the tyre, but the other set will indicate the recommended inflation pressure in units of PSI and/or Bar and - more rarely - kPa (Kilopascals).
Some tyres will just show a maximum pressure, while others show a minimum-maximum range. Never go below the min pressure as you will risk more punctures, damaging the sidewalls of the tyre and, worst of all, it will make your bike much harder work to ride!
On smooth roads, the higher the pressure the faster you'll be able to ride but at the expense of it being less comfortable.
In wet or slippery conditions, consider running tyres at a lower pressure to give them more grip (but always stay above any minimum recommended pressure).