Since I cycle the same routes most of the time so a measurement only needs to be done once, I use the method described as "nerdy" since it's more accurate than the bike indicators anyway, and very much cheaper. Here's the original posted method:
Do you get puzzled over all this talk of 5% or 12% hills, all just mumbo jumbo, or even if you understand it, do you know how to classify your hills to compare them?
Here's how, the easy way.
You need a spirit level, you may have one already in a set square or separately, but if not, buy a cheap one from a DIY or tool shop. If you haven't got one, also get an extending rule with centimetres on it. Then get a strip of straight metal or wood and cut it to exactly 50 centimetres long. Tape an expanding rule on one end so that the tape extends down against the end of the 50 cm strip. Tape the spirit level next to it on the top surface of your straight strip and secure it firmly with tape or rubber bands.
That's your measuring apparatus complete. If the hill you want to measure isn't at home, you can tie that strip to your crossbar to ride to your destination.
Now on a representative sloping part of the hill, rest one end of your strip on the road surface with the spirit level above and the bubble kept centralised. With the other hand, hold the rule extended and vertical against the road directly above the other end of the strip and note the number of half centimetres measured where it meets the underside of the strip right at the end. Thats the percentage measure of your hill, it's as easy as that, just the distance from the road surface vertically to the 50 cm point on the underside of your strip. If it's 5.5 cm, there's 11 half centimetres so it's an 11% hill. Just make sure the bubble is central as you read the measurement.
Here's a photo of my sophisticated device:
If you want to express the hill as one in something instead of a percentage, just divide the answer into 100. So the 11% hill mentioned, with 100 divided by that 11 is a 1 in 9 hill.
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