If you don't like memorizing charts then there are some shortcuts that can be used. For example, if you have 1 to 9 outs and the turn and river cards are yet to be seen then you can multiply your outs times 4 to come within 1 percentage point of the true chances of hitting your card. This covers the inside straight draw(4 outs), 2 overs(6 outs), the open ended straight draw(8 outs) and the flush draw(9 outs).
For example, 2 overs is 6 outs multiplied by 4 is 24% which is your approximate chance of getting at least one of your overs.