Simbrief could be a useful tool. But if you look for quick math.
- Take 400knts as your average cruise speed. (Depending 9n aircrafts. For jets its 400knts. For dash-8 and other props. might be around 300knts.
- file ypur fpl. And get flight time out of it. ( Say your fpl is 800nm. So it would be 2 hr flight.)
- take 15-30min extra fuel. So in this case it would be 2.5 hr fuel.
You’ll get how many hour of fuel you have based on your aircraft fuel burn rate. In the top right corner of weight and balance.
** The flight time shown is always on lower side. So even if you just take exact 2hr fuel in the above scenario. You’ll still have enough for taxi. And hold if needed for atleast 15min.