Cyclist often refer the the gradient of a climb in terms of a percentage. The gradient when measured in percentage is just the (horizontal distance / vertical climb) * 100. So you get the idea if I travel 100 m and rise 8 metres that’s 8% gradient. While that may not sound difficult over a longer distance that will really start to bit.
Here’s my spin on the gradients.
At 1-2% you’ll barely notice it, like riding into a bit of a head wind
3-4 % most people will do this quite easily. It rates as hill for a non-rider but to a rider it is a mere bump
5-7% will start to bother non-riders, riders will start to find it interesting
8-9% non-riders will start to look for other ways around
10-15% is going to hurt a non-rider and they may give, Riders will be challenged by this
15% or greater and you’re in the “praying to your deity of choice” territory. This is going to be tough. Non riders simply will not have enough strength to push themselves up this sort of a climb, Good riders will be challenged by this
As a guide at 10% gradients roads are usually labelled with a warning of steep descent.