Search "[airport A] to [airport B] flight distance km" — many flight-tracking and mapping sites list great-circle distances between airport pairs.
Short flights burn proportionally more fuel per kilometer during takeoff and climb, and fewer passengers on smaller regional jets means each person's share is larger. Long-haul flights are more fuel-efficient per kilometer once at cruising altitude, but no single number is exactly right for every flight.
Some carbon calculators multiply CO2-only figures by roughly 1.9–3x to account for aviation's other atmospheric effects at altitude. This is scientifically debated and not universally standardized, so it's left out here — you can apply your own multiplier to the result if you want to include it.