Flooring is sold by the box, installed by the square foot, and the difference between those two numbers is waste. Running short mid-floor is the expensive mistake here, because a second batch of the same product can be a visibly different dye lot.
For an L-shaped room, split it into rectangles, run this twice and add the results — do not use the outer envelope.
Printed on the carton and on the product page. Common values: LVP 23–30, laminate 20–24, engineered hardwood 20–26, solid hardwood 18–24, tile varies widely.
Waste is not padding
Every row you lay ends in a cut, and that offcut either starts the next row or goes in the bin. On a straight layout most of them start the next row, which is why 7% covers it. Turn the floor 45 degrees and both ends of nearly every plank get cut, the offcuts come off at angles, and very few of them begin a row cleanly — hence 15%. Herringbone cuts every piece twice and the pattern decides which offcuts you can use, so 20% is normal and some installers will ask for 25%.
The room shape adds to that separately. An island, a hearth, a closet return and a bay window each generate cuts that produce nothing reusable, and they add up faster than people expect.
Dye lots are real
Flooring is manufactured in batches and the colour drifts between them, subtly enough that you will not notice in the store and obviously enough that you will notice on your floor. Buy the whole job in one order, from one retailer, and check the lot numbers on the cartons match before you open any of them.
This is also why the leftover box matters. Five years from now, when a chair leg gouges a plank or a dishwasher leaks, having matching material on hand is the entire difference between a twenty-minute repair and re-doing the room.
Before you order
Acclimatise the material in the room it is going into — most manufacturers require 48 to 72 hours and will void the warranty if you skip it. Check the subfloor is flat before you check it is clean; most floating floor failures are flatness problems, not product problems. And measure the doorways, because transitions and the trim around them are their own small budget.
What we learned doing this ourselves: what your subfloor is actually doing, a floating floor that went wrong, skipping the underlayment, choosing LVP, and what AC ratings really mean.
Method
Area is length × width. Waste allowance is layout plus room complexity: 7% straight, 15% diagonal, 20% herringbone or chevron, plus 0%, 3% or 6% for room complexity. Box count rounds up, because retailers do not sell half cartons. These are standard industry allowances and match what most flooring manufacturers publish in their installation instructions — check yours, since a few require a specific minimum overage to keep the warranty valid.