The source dimensions are 1080 wide by 1920 high. The target height is fixed at 1000, so width is calculated. The proportional width is fractional, making the rounding choice visible rather than implicit.
Reduce the portrait ratio
The greatest common divisor of 1080 and 1920 is 120. Dividing both terms by 120 gives 9:16. Width remains the first term even though it is smaller than height.
Calculate the unrounded width
Target width = 1000 × 1080 ÷ 1920 = 562.5. The reduced ratio gives the same result: 1000 × 9 ÷ 16 = 562.5. This is the proportional width before any whole-number decision.
Choose 563 or 562 explicitly
Nearest with half up and ceiling both return 563; floor returns 562. AspectFit keeps 562.5 visible beside the rounded width, so copying 563 × 1000 does not hide how the integer was chosen.