We are using 100 watts power output into the antenna as an example.
Go down the left column and find the gain of your antenna. (6)
This would be 6 dbd in our example.
Using the chart, read across to the right of the column from "6" and find your answer of 398 watts ERP!
These figures are assuming that you are getting 100 watts to the antenna from the end of your feedline and do not reflect any loss in it or the antenna and you know the exact gain figures for the antenna.

You should see by using the chart that with every 3db increase, your effectively doubling the ERP.

HERE'S ANOTHER GOOD...... SIMPLE EXAMPLE
 (With a bit of Ham Humor thrown in)

100 watts into a 3db gain, 2 meter 5/8 wave mag mount on the roof of your mobile.
 
It's a Ford, built in 1994, V8 engine, automatic transmission, one crack in the windshield with 198,000 miles on it and you still owe 18 payments of $287.57 after refinancing a year ago.

The oil needs changing and it has 4 new tires and you discover the battery is dead!

To Calculate the correct ERP answer you realize you wanted to replace the battery anyway so  you can check your power output into your antenna with your watt meter to make sure your rig is putting out that 100 watt signal to the antenna.

During the process of finding and replacing the battery with a new one, you have spent 4 hours finding a ride, getting past the crowds at Wally World to buy the battery, getting back home, finding the right tools but you can't find them because you loaned them to your teenage son for bike repair, disturbing your neighbor to borrow his tools, helping him take his grill outside, (his tools were burried under it in his garage), opening the hood or your car, ripping the water hose trying to take it off so you could install the battery, destroying the battery clamps, mashing your fingers, cutting your hand, asking your neighbor to take you back to Wally World to get new clamps, water hose, and antifreeze, discovering that you spent all your cash for the new battery, going to the bank, driveup out of order, standing in a long line to get cash from your account, back to Wally World and the auto parts store, getting the new parts, installing them, cranking the Ford, only to discover that the XYL ran it out of gas, borrowing a gas can from your neighbor, walking to the gas station around the corner, walking back in the summer sun (110 degrees), putting the gas in, starting the car, closing the hood and discovering that you did not tighten the hose on the bottom of the radiator that you just installed, new antifreeze spewing all over the new battery, smoke pours out of the alternator, sparks fly and the engine dies!

You throw up your hands, call a wrecker, wait and wait, get it towed to your friendly auto repair shop and after a grand total of $487.50 and 3 hours later, not counting the cost of the battery, parts and gas..... you get in the car which is now back home, it now starts, you fire up the rig, key the microphone and watch the power meter.............no output!
Is Ham Radio Fun or What!

NOW BACK TO THAT SIMPLE EXAMPLE
100 watts verified on a repaired 2 meter rig ($187.50 after waiting 3 weeks for repair!).
Your antenna gain (on the F-o-r-d) of 3 db..........

Read down the chart in left column, find 3db.........then look in the right column..... 
Your 100 watts going to that 3db gain antenna sitting on top of that............Ford!
= 199 watts ERP!