Perimeter, area and metric units
Add the sides for perimeter, multiply length by width for area, and step by tens between metric units.
A worked example
A rectangle 11 cm by 6 cm has perimeter 2 x (11 + 6) = 34 cm and area 11 x 6 = 66 cm². That 11 cm length is 11 x 10 = 110 mm.
The mistake people actually make
Going from a large unit to a small one multiplies and the number grows; going the other way divides and it shrinks. For a shape with a piece cut out, find the whole area first, then subtract the missing piece.