You could always just add the four sides — that never fails. But a rectangle has matching pairs, so there's a faster way that means less to add and fewer slips.
Why the Shortcut Works
A rectangle has two long sides (both the length) and two short sides (both the width). So the four sides are really length + width + length + width. That's just two lengths and two widths — which is the same as adding one length and one width, then doubling. We write it as Perimeter = 2 × (length + width).
The Two Moves
One — add the two different sides. One length plus one width. Two — double the answer. Multiply by 2, because there are two of each.
A Worked One
A rectangle 8 long and 5 wide: add the different sides, 8 + 5 = 13; then double, 2 × 13 = 26. So the perimeter is 26 units. If those were metres, that's 26 m of edge.