How Much Flooring Do I Need?

The waste allowance decides what you shop for. The box size decides what you take home — and those are usually several percent apart.

Your 10% allowance isn't what you end up with

A 20 × 15 room is 300 sq ft. Add 10% waste and you're shopping for 330.

At 22.5 sq ft per box that's 14.67 boxes — so you buy 15, and go home with 337.5 sq ft.

Your real margin over the floor isn't 10%. It's 12.5%.

Your details

Room length(ft)

Room width(ft)

Waste allowance(%)

10% for a simple rectangle laid straight. 15-20% for diagonal, herringbone or an awkward shape.

Coverage per box(sq ft)

Read it off the actual box — there is no standard, and this is what the answer turns on.

Result

Boxes to buy

Rounded up. Consider one more for dye-lot matching — see the notes.

Floor areaSquare feet.: 300
Area including wasteWhat you are actually shopping for.: 330
Those boxes coverWhole boxes, so usually more than you asked for.: 337.5
Left over after the waste allowanceThe rounding surplus — sometimes plenty, sometimes almost none.: 7.5
Your REAL margin over the floorNot the allowance you typed — what the whole boxes actually give you.: 12.5%

Open the Flooring Calculator on its own page to bookmark or share it.

The box size decides what you buy

This is the part that separates flooring from tile or paint.

The allowance decides what you shop for. The rounding to whole boxes decides what you actually get. Those land several percent apart in either direction, depending purely on how the numbers fall.

Which is why the leftover figure is worth reading before deciding whether to add a spare box. Sometimes the rounding has already handed you most of one.

Convert to boxes once, at the end

If you're flooring more than one room: add all the areas first, then convert once.

Round each room up to whole boxes separately and you pay the rounding penalty every single time. Across four rooms that easily buys two or three boxes more than the job needs.

How much waste to allow

Situation	Allowance
Simple rectangle, planks laid straight	10%
Alcoves, angles, several doorways	15%
Diagonal or herringbone	20%+

Diagonal layouts are the big jump. Every plank meeting a wall gets an angled cut, and angled offcuts are far harder to use anywhere else — the waste doesn't come back to you the way a square cut's does.

Long narrow rooms also waste more than square rooms of the same area, because more rows end at a wall.

Take the box coverage off the actual box

There's no standard. Coverage per box genuinely varies between products and manufacturers, which is why the calculator asks rather than assuming.

It's also the number the entire answer turns on — so read it off the box you intend to buy, and check it again if you switch product while choosing.

Buy one more box, but not for the reason you think

The standard advice is one extra box. The reason isn't waste — it's dye lots.

Boards from a different production run can differ subtly in shade and pattern, and matching one a few months later is often impossible. A spare box means a damaged plank is a twenty-minute repair instead of a visible patch or a whole new floor.

Write the batch number down. It's on the box, it takes ten seconds, and it's the only thing that gives you a chance of matching later.

The problem this can't solve

It knows area, not layout. So it can't tell you that your last row will land as an awkward 20 mm sliver.

The fix for that isn't more material anyway. If the final row works out uncomfortably narrow, trim the first row so the first and last are a similar sensible width. Work it out on paper before opening a box — it's much harder to fix halfway across the room.

L-shaped rooms

Split into rectangles, work out each, add the areas, then apply waste.

Don't measure the overall bounding box of an L-shaped room. That includes floor that doesn't exist, and you'll buy material for a corner of the house that isn't there.

Don't forget the rest of the job

Underlay, beading, threshold strips and expansion gaps around the edges are all separate purchases and separate decisions.

And leave the boards in the room to acclimatise for as long as the manufacturer specifies. Laying flooring that hasn't settled to the room's humidity is how a tight, well-cut floor develops gaps in month two.

Print the flooring take-off sheet — it has a row per room so you can add the areas before converting once.