Have been meaning to pass this simple method to the masses, a lot of guys I've givin it to swear by it.....
Set your altimeter to ..................................1013
read the indicated height.........................ie...400ft
now guess the current temperature.............ie 34 degrees
now with that 34 temp figure, deduct 15
that gives 19..........now multiply that by 120..........gives you 2280,now add on the original height as prev shown 400
2280 + 400........gives the density altitude rite here, rite now as 2680ft
This is so close to accurate.............hope it helps.........cheers