If there are three 100 foot hills on the ride, that's 300 feet of elevation gain. You don't subtract the downhill parts.

On a reasonably long ride, 100 feet per mile is considered very hilly. That's a 30 mile ride with 3000 feet of climbing, for instance. But like Retro Grouch says, that doesn't include the downhills, so if the ride returns to the starting point, the climb portions are probably at least 200 feet per mile, assuming it's mostly all hills.

Elevation gains are often reported by GPS devices, such as a Garmin. Or web sites, like ridewithgps.com, will add up the elevation changes using mapping data. Both of these count up every small change in elevation, even a few feet at a time. So the elevation numbers on an easy ride can look pretty impressive. Mapmyride.com has tried to fix this by only counting 10 meter changes in elevation, that's 33 feet. But that tends to under-report a ride with many smaller hills. So there's no perfect solution.


~~~~~~~~~~~~~~
Checking out a possible route in advance can be useful. Many organized rides have had someone post their ride to ridewithgps.com. The biggest difficulties can be long, steep climbs. Long shallow climbs are pretty easy, and small "roller" hills are easy, too.

See these three rides. Click the Terrain checkbox on the Map pulldown at the top right corner. And hover your cursor over the red elevation graph to see the grades at different points in the ride, or drag to select just a section of the ride and get it's distance and elevation reported on the right side Metrics tab

Here's a ride I consider very flat. See the ridewithgps recorded route. The middle of this 40 mile ride is at 950 feet above sea level, and the lowest is just at 687 feet. There's only a couple of hills that are even 100 feet high. Yet, the small rollers, probably only 2-4% grade and 40-70 feet high at the most, add up to 1600 feet in elevation gain over the 40 miles.

This Blue Ridge Parkway ride is hilly: 3700 feet in 49 miles. Some of the 300-400 foot climbs are over 6% grade. But at least there's some downhills mixed into the big climb from mile 12 to mile 21. And the return trip is more downhill than uphill, that helps a lot.

Here's a local ride for April Fools Day. It's 2800 feet in 43 miles, but it's very difficult. The main problem is the steep climbs, many over 8-10%, and a couple over 18%.

~~~~~~~~~~~~~~~~~~~~

The other quoted number is grade percentage. A mile long hill that is 264 feet high is exactly a 5% grade. That's 264 / (1 mile x 5280 feet) = .05

(On a hill, the grade is actually the vertical distance divided by the horizontal distance. But bikes measure the sloped distance up the hill, which is a little longer than the horizontal distance. For any reasonable grade, the two are so close together that they don't really change the calculation. So we just measure the road distance and the elevation changes)