A linear foot is one foot of length, measured along a line. It sounds too simple to need a guide, and then the first real job arrives: a floor to cover, a room to trim, a fence to build, and the question is no longer what a linear foot is, but how many of them the job needs. The answer depends on the job, because boards, trim, and fencing are measured three different ways. This guide works each method step by step, with worked examples, unit conversions, waste allowances, and the mistakes that leave a job short.

What a linear foot is, and what it is not

A linear foot, often written LF or lin. ft., is a foot measured in a straight line. A board 10 feet long is 10 linear feet, whatever its width. A wall 12 feet long needs 12 linear feet of baseboard. The unit says nothing about width, thickness, or area: it is length, and only length. That is the whole definition, and it is also the source of every confusion, because the jobs that need linear feet — flooring, trim, fencing — each relate length to something else, and the relation is different every time.

The confusion to clear first is with square feet. Square feet measure area, length times width. Linear feet measure length only. A 10-foot board that is 6 inches wide covers 5 square feet, and a 10-foot board that is 12 inches wide covers 10 square feet, but both are 10 linear feet. The two units convert into each other only when the width is known, which is exactly what the boards method below does, and the companion guide on linear feet versus square feet works the conversion out in full, with tables.

The second confusion is with board feet, the lumber trade unit, which is a volume: one board foot is a piece 12 inches long, 12 inches wide, and 1 inch thick, which is 144 cubic inches. A 10-foot board that is 6 inches wide and 1 inch thick is 10 linear feet and 5 board feet. The two units are used in different trades for different purposes, and this guide uses linear feet throughout, because that is what flooring, trim, and fencing are sold by.

Boards: area divided by width

Flooring and decking boards are sold by the linear foot, but the job is measured in square feet, so the calculation is a division: linear feet equals area divided by board width, with both in the same unit. The logic is plain: area is length times width, so dividing area by width leaves length. For 100 square feet covered with boards 0.5 feet wide, the calculation is 100 ÷ 0.5 = 200 linear feet. The boards do not have to be one long piece; the total is what matters, and the board count comes from dividing the total by the length of one board.

Work a full room. A room 12 feet by 15 feet has an area of 180 square feet. The boards are 6 inches wide, which is 0.5 feet. Linear feet are 180 ÷ 0.5 = 360. The boards come in 8-foot lengths, so the board count is 360 ÷ 8 = 45 boards, exactly, with no rounding needed. Add a 10 percent waste allowance for cuts and mistakes: 360 × 1.10 = 396 linear feet, and 396 ÷ 8 = 49.5, which rounds up to 50 boards. The waste allowance is added to the linear feet before the board count is computed, not after, because waste is measured in length, and rounding up at the end is what guarantees the job is not short by half a board.

Board width is the number that must be measured, not assumed. Nominal lumber sizes are not actual sizes: a board sold as 6 inches wide is 5.5 inches wide when measured, which is 0.4583 feet, and using 6 inches in the division undercounts the linear feet by about 8 percent. Flooring is usually sold by its actual face width, which is the width that shows, not the width including the tongue, so the face width is the figure to use. Measure the board, convert to feet, and then divide.

Narrower boards need more linear feet for the same area. Waste is added to linear feet, then the board count is rounded up.

Trim: add every piece

Trim — baseboards, casing, crown moulding — follows edges, not areas, so the calculation is a sum, not a division. Measure every run separately, add the runs, and the total is the linear feet to buy. A room 12 feet by 15 feet has four walls: 12, 15, 12, and 15 feet, which add to 54 linear feet of baseboard. That is the perimeter, and for a plain rectangular room the perimeter is the answer, before deductions and waste.

Deduct the openings. A doorway 3 feet wide needs no baseboard across it, so subtract 3 feet for each doorway, and subtract the width of any opening the trim does not cross. Two doorways in the 12 by 15 room bring the total from 54 to 48 linear feet. Then add waste: trim waste comes from cuts and mistakes, especially around corners and mitres, so add 10 percent, which brings 48 to 52.8 linear feet. Trim is sold in fixed lengths, usually 8, 10, 12, or 16 feet, so convert the total into lengths: 52.8 linear feet is six 10-foot lengths, which is 60 linear feet bought, with 7.2 feet left over for cuts and spares.

Crown moulding and casing follow the same addition, with their own deductions. Casing runs around each door and window, so each opening adds its own perimeter: a door 3 feet wide and 7 feet high needs 3 + 7 + 7 = 17 linear feet of casing, minus the threshold if the casing does not run across the bottom. Measure each opening, add them, and do not reuse a baseboard measurement for casing, because the runs are different lengths on different planes. The calculator on this site keeps the two methods in separate tabs so they never get mixed up.

Fencing, decking, and pipe: measuring a run

A fence is a run, and the calculation is the length of the run plus the posts. A straight run 120 feet long needs 120 linear feet of fencing material, whether it is panels, pickets, or rails. The posts are counted separately: with posts every 8 feet, a 120-foot run has 120 ÷ 8 = 15 intervals, which need 16 posts, one at each end and one at each interval. A run with a gate needs the gate width deducted from the fencing and the gate itself bought separately, and a run that turns a corner needs a post at the corner.

Decking follows the boards method, with the decking calculator on this site handling the board spacing. The boards run across the joists, and the count is the deck width divided by the width of one board plus its gap, rounded up, which is a division along one direction, like the boards method, with the gap added to the width. Pipe, cable, and rope are the simplest case of all: the length of the run is the linear feet, with a small allowance for bends, joints, and terminations, usually 5 to 10 percent, because a pipe that is cut short cannot be stretched.

For a deck, work the same room numbers as a check. A deck 12 feet wide by 16 feet long, with boards 5.5 inches wide and a 1/8-inch gap, has a pitch of 5.625 inches, and 144 inches ÷ 5.625 = 25.6, rounded up to 26 rows, each 16 feet long, which is 416 linear feet before waste. The deck board guide on this site works this example in full, with the waste and the fixed lengths, and it is worth reading before ordering, because the gap is the detail most often left out.

Units: inches, yards, and metres

Every linear-foot calculation fails the same way: mixed units. Inches must be converted to feet before dividing, because 100 square feet divided by 6 inches is not 16.67 linear feet; it is 100 ÷ 0.5 = 200 linear feet, after converting 6 inches to 0.5 feet. Divide inches by 12 to get feet, and feet by 3 to get yards. A measurement of 96 inches is 8 feet. A measurement of 144 inches is 12 feet. A roll of 40 yards is 120 feet.

Metric conversions follow the same rule. One metre is 3.28084 feet, so a room 4 metres by 5 metres is 13.12 by 16.40 feet, with an area of 215.28 square feet, and boards 15 centimetres wide are 0.4921 feet wide, giving 215.28 ÷ 0.4921 = 437.4 linear feet. Work in one unit system throughout, convert at the start, and never convert in the middle of a division, because the rounding compounds. The calculator on this site takes feet and inches together and converts internally, which is the safest way to enter mixed measurements.

Convert first, calculate second. Mixing inches and feet in one division is the most common unit error.

Waste: how much to add, and when

Waste is the extra material for cuts, mistakes, and defects, and 10 percent is the usual starting point for boards and trim. It is a starting point, not a guarantee: a room with many corners, angles, or short runs needs more, because each cut wastes an offcut, while a long straight run needs less. Diagonal layouts need 15 percent or more, because every board is cut at an angle and the offcuts are shorter. Patterned layouts, such as herringbone, need the most of all, and the supplier or installer is the authority on the exact allowance for the pattern.

The order of operations matters: add waste to the linear feet, then divide by the board length, then round up. Adding waste after rounding up understates it, and rounding down after adding waste defeats it. For the 12 by 15 room above, the order is 360 linear feet, plus 10 percent to 396, divided by 8 feet to 49.5, rounded up to 50 boards. Reversing the last two steps gives 45 boards plus 10 percent, which is 49.5 boards, the same number here by luck, but the luck does not hold in general, and the correct order is the one that guarantees the count.

Keep the offcuts. A board cut for one row leaves an offcut that starts the next row, and a trim offcut fills a short run, so the waste allowance is not all thrown away. The allowance is insurance, not consumption: buy it, use what the job needs, and keep the rest for repairs, because matching a floor or a trim profile years later is harder than storing a few boards now.

From linear feet to a shopping list

Turn the linear feet into an order in three steps. First, the total linear feet, with waste, from the method above. Second, the board or trim length available, from the supplier: 8, 10, 12, or 16 feet are the common lengths, and longer lengths cost more per foot but waste less on long runs. Third, the count: total linear feet divided by the length, rounded up. For the 12 by 15 room, 396 linear feet divided by 10-foot boards is 39.6, rounded up to 40 boards, which is 400 linear feet bought. The extra 4 linear feet are the rounding, and they join the waste allowance as spares.

Check the order against the area as a sanity check. The room is 180 square feet. One board covers 0.5 feet by 10 feet, which is 5 square feet of face, so 180 ÷ 5 = 36 boards would cover the area with no waste and no rounding, and the order of 40 boards is 4 boards above that, which is the waste and the rounding together. If the area check and the linear-feet method disagree by more than a board or two, one of them has the wrong width, the wrong length, or the wrong unit, and the linear-feet method is the one to trust, because it models how boards are actually laid.

Mistakes: width, rounding, and double counting

First, using the nominal width instead of the actual width: a 6-inch board is 5.5 inches, and the division must use 5.5. Second, forgetting the unit conversion: dividing square feet by inches instead of feet gives an answer twelve times too small. Third, rounding down: a board count of 49.5 must be 50 boards, because 49 boards leave half a board of floor uncovered. Fourth, adding waste before the division in the wrong place, or not at all: a job with no waste allowance is a job that runs short on the last row.

Fifth, double counting: measuring the same board twice when it covers two rooms, or counting a shared wall twice when trimming two rooms. A board that runs through a doorway belongs to one room or the other, not both, and a wall shared by two rooms needs one run of baseboard, not two. Sixth, using area where length is needed: a fence is a run, not an area, and a trim job is a sum of edges, not a division of an area. And seventh, measuring the room and forgetting the openings: a doorway is 3 feet of baseboard not needed, and a room with three doors is 9 feet of trim saved, which is a board and a half on a 10-foot length.

Frequently asked questions

How do I calculate linear feet for flooring?

Divide the floor area in square feet by the board width in feet. For 100 square feet and 0.5-foot-wide boards, that is 100 divided by 0.5, which is 200 linear feet. Then divide by the length of one board and round up to get the board count, and add a 10 percent waste allowance to the linear feet before dividing.

How many linear feet are in a 12×12 room?

It depends on what you are measuring. For baseboard, the perimeter is 48 linear feet, minus doorways. For flooring boards 6 inches wide, the area is 144 square feet, so the linear feet are 144 divided by 0.5, which is 288 linear feet, plus waste. The room size alone does not fix the answer; the width or the edge being measured does.

What is the formula for linear feet?

For boards, linear feet equals area in square feet divided by board width in feet. For trim, linear feet equals the sum of all the runs, minus openings, plus waste. For a run such as a fence, linear feet equals the length of the run. All three give a length in feet; the formula differs because the job differs.

How do I calculate linear feet for a fence?

Measure the length of the run in feet; that is the linear feet of fencing material. Count the posts separately: divide the run by the post spacing to get the intervals, and add one post. A 120-foot run with posts every 8 feet needs 120 linear feet of fencing and 16 posts. Deduct the gate width from the fencing and buy the gate separately.

How many linear feet is 100 square feet?

It depends on the width. At 6 inches wide, 100 square feet is 200 linear feet. At 12 inches wide it is 100 linear feet, and at 3 inches wide it is 400 linear feet. Square feet and linear feet convert only through a width, which is why the question needs the board width before it has an answer.

Should I add a waste allowance?

Yes, 10 percent is the usual starting point for boards and trim, added to the linear feet before the board count is computed. Diagonal layouts need 15 percent or more, and long straight runs may need less. The allowance covers cuts, mistakes, and defects, and the offcuts are kept for repairs.