How do I measure a room that is not a rectangle?

Split it into rectangles, work out each one, and add them together. An L-shaped room is two rectangles; a bay window is a rectangle plus a shallow one. Measuring the longest points and multiplying overstates the area every time.

Should I measure to the wall or to the baseboard?

To the wall, then take off nothing. Flooring runs under the baseboard in most installations, and the waste allowance you add later covers the difference either way.

How much waste should I add?

That depends on the material and the layout, not on the area — so it is not applied here. The tile and flooring calculators add it, because there the offcuts are what force you into another box.

Home & construction

Square Footage Calculator

Enter the length and width to get the area in square feet, square yards and square metres.

The link carries your figures, so it reopens on exactly these numbers.

The formula

square feet = length × width × areas

square yards = square feet ÷ 9

perimeter = (length + width) × 2 × areas

Worked example

A 20 ft × 14 ft living room

20 × 14 = 280 ft²
280 ÷ 9 = 31.11 yd²
(20 + 14) × 2 = 68 ft of perimeter

Where this goes wrong

Dividing square feet by three to get square yards

A square yard is three feet by three feet, so it holds nine square feet, not three. Divide 280 ft² by 3 and you order 93 yd² instead of 31 — three times too much carpet. Areas scale with the square of the length factor, every time.

Questions

How do I measure a room that is not a rectangle?: Split it into rectangles, work out each one, and add them together. An L-shaped room is two rectangles; a bay window is a rectangle plus a shallow one. Measuring the longest points and multiplying overstates the area every time.
Should I measure to the wall or to the baseboard?: To the wall, then take off nothing. Flooring runs under the baseboard in most installations, and the waste allowance you add later covers the difference either way.
How much waste should I add?: That depends on the material and the layout, not on the area — so it is not applied here. The tile and flooring calculators add it, because there the offcuts are what force you into another box.