Subtract your departure city's UTC offset from your destination's UTC offset. For example, New York (UTC-5) to London (UTC+0) is 5 time zones eastward.
Strategic light exposure/avoidance, adjusting your sleep schedule a few days before travel, staying hydrated, and getting sunlight at the right times on arrival are commonly recommended strategies — though they're not modeled numerically in this calculator.
Most people notice little to no jet lag under about 3 time zones. This calculator will still give you a small number in that range, but real-world effects that small are often negligible.