Basic rule of thumb
4%*number of outs to hit by the river after the flop
2%*number of outs to hit by the river after the turn

A bix more complex but a little more accurate
After the flop to hit the river
4% for the first 10 outs
3% for any out after 10
So if you have 13 outs its 49%

After the turn to hit the river
2.2% per out