A square foot is an area — one foot by one foot. A linear foot is a length — just one foot measured along a line. They are not interchangeable, and multiplying a linear-foot price by a square-foot area is a common DIY estimating mistake.
The rule is simple: pick the units your material is priced in, measure in those units, and only convert if you have to.
The direct answer
- Floors, walls, ceilings, lawns, roofs — area, so square feet (sq ft, ft²). Use the square footage calculator on length × width.
- Baseboard, crown molding, countertop edging, rope, fencing, lumber by the stick — length, so linear feet (lin ft, ft). Measure the perimeter or the run.
- Lumber — the sticker sometimes says "per linear foot" and sometimes "per board foot." A board foot is 144 cubic inches of lumber (1 ft × 1 ft × 1 in). That is a different unit again; do not confuse it with a square foot or a linear foot.
When each one applies
Flooring (square feet). Flooring covers area. A 14 × 18 ft room is 252 square feet. Prices for the material are quoted per square foot or per box, and the flooring calculator rounds the area up to whole boxes.
Baseboard and trim (linear feet). Baseboard runs along the perimeter of the room. For the same 14 × 18 ft room, the perimeter is 2 × (14 + 18) = 64 linear feet. Subtract door openings (about 3 ft each) and add 10% for miter cuts and offcuts. If baseboard costs $1.80 per linear foot and you buy 70 ft to cover the room, that is $126.
Paint (square feet of wall). Paint covers the wall area. A 14 × 18 ft room with 9 ft walls has 576 sq ft of gross wall before openings. The paint calculator handles the subtraction and the US-gallon rounding.
Fence or railing (linear feet). A 100-ft run of fencing is 100 linear feet. The material cost scales with the length, not with any area.
Why the confusion is costly
Say a product is quoted at "$3.50 per linear foot" and the material is sold in 3-ft wide rolls. For 252 sq ft of floor, the linear length needed is 252 ÷ 3 = 84 linear feet, costing $294 before waste. Multiplying $3.50 × 252 would give $882 — but that treats a linear-foot price as a square-foot price. The dealer is pricing a length of material whose width is a product spec, not the surface area of the room.
The flip side is also real: a baseboard quote of "$2.10 per square foot" is nonsense (baseboard has no meaningful area), and should prompt a question before anything is signed. "$X per linear foot" and "$X per box" are also distinct: a box has a known coverage printed on its label, while a linear-foot price depends on the roll or plank width. Ask which unit the price is in, and what the product's usable width is when it is sold by the linear foot.
Worked example: a 10 × 12 ft bedroom
- Floor area → 10 × 12 = 120 sq ft. At $4.50/sq ft the flooring material runs $540.
- Perimeter → 2 × (10 + 12) = 44 linear feet. Subtract one 3-ft doorway: 41 ft. Add 10% for cuts: 46 lin ft. At $1.80/lin ft that is $82.80 of baseboard.
- Wall area → 44 × 9 (ceilings) = 396 sq ft; subtract one door (21 sq ft) and one window (15 sq ft) → 360 sq ft; two coats and 350 sq ft/gal → 720 ÷ 350 ≈ 2.06 → 3 gallons of paint.
Three measurements, three units: square feet for the floor, linear feet for the trim, square feet for the paint. Open the square footage calculator to run the floor example with the price already filled in.
Common mistakes
- Multiplying a linear-foot price by an area. This can triple a quote. Ask which unit the price is in.
- Measuring one wall and multiplying by four. Rooms are usually not square. Measure every wall separately for the perimeter.
- Confusing linear feet and board feet. Board feet is a volume unit used for rough lumber. "$3 per board foot" and "$3 per linear foot" give different totals on the same 2×4.
- Ignoring the waste allowance. Baseboard has a cutting waste too — 5–10% is typical — not just flooring.
NIST publishes the base definitions here (one inch is exactly 2.54 cm, one foot exactly 0.3048 m), which is why unit conversions in the length converter are deterministic to the last decimal. The hard part is matching the measurement to the material; the arithmetic is the easy part.