Baseboards follow the edge of a room, not the floor area. A garden bed border counts the walk around the rectangle. You have length and width and need the outer distance—twice each dimension added. The form does that without you typing (l+w)×2 from memory wrong at midnight.
Area is for paint or sod inside; perimeter is for trim and rail length. Same labels, different question.
Perimeter of a 12 by 9 rectangle is 42 with the defaults—people still add 12+9 twice mentally and get 21 once in a while.
Border math on real jobs
Openings and gates are not modeled here—this is a full rectangle. Deductions live in your cut list. For the surface inside the same room, area for 12 by 9 is the paired number.
A square lot uses the square perimeter page. A curved edge uses circumference with radius 6. If the plan is in centimeters and lumber is in feet, 274 centimeters in feet is a length conversion, not perimeter by itself.
- Perimeter is linear units, not square.
- Confirm which sides are length versus width—it does not change the sum.
- Buy extra stock for cuts; the math is idealized.
Ordinary DIY and classroom geometry, not structural engineering.