Heights and flatness are different measurements
A laser gives you heights against an arbitrary plane. A flooring instruction sheet asks for a gap under a straightedge. The two are related, but one does not convert into the other by subtraction, and the mistake gets made in both directions.
A floor can fall steadily by half an inch across a room and show almost nothing under a ten foot straightedge, because the straightedge tips onto the slope and rides it. That floor is out of level and dead flat, and most flooring products care about the second. Equally, a floor that measures within an eighth of an inch overall can hide a sharp dip between two stations that swallows a quarter inch under the straightedge. The reduction on this page walks the straightedge along the readings and reports what it would actually show.
How the gap is worked out
At each position the straightedge rests on the two highest points it can reach within its length — not the ends, and not the highest point in the room. That resting line is the upper hull of the readings inside the window. The gap is the largest vertical drop from that line down to the floor, and the page reports the worst one along with where it happens.
Which is why the reading spacing matters. A straightedge finds a dip; a tape at two foot centres walks over it. If the spacing is more than about a third of the straightedge length the page says so and stops, because a coarse grid cannot reproduce a straightedge rocking on two high points and a comfortable number would be a false one.
Flat, not level
The volume estimate comes two ways for a reason. Flooding the whole area up to the highest point makes a floor that is flat and level and takes a great deal of material. Filling only what sits more than the allowed gap below that plane makes a floor that is flat and still slopes, and it usually takes a fraction of the compound. Most flooring instructions ask for the second. Doors, thresholds, appliance openings and the top step of a stair all lose whatever height gets added, which is the other reason not to flood a floor that never needed it.
What one line cannot tell you
Floors are directional. A wood floor is usually worst across the joists and best along them, a slab is often at its worst at a cold joint or a saw cut, and dips cluster under partition walls, around plumbing penetrations and where a panel has swollen along its edges. A single line of readings is one slice through all of that. Run at least one more at right angles, and put the tightest spacing where the first pass found the worst gap.
Questions people ask
How do I turn laser readings into a flatness figure?
Take readings at even spacing along a line, then slide a window the length of your straightedge along them. In each window the straightedge sits on the two highest points inside it, and the gap is the biggest drop from that line down to the floor. That is what this page does. Subtracting the lowest reading from the highest gives you the range, which is a different measurement and usually a larger one.
What flatness does my floor need?
Whatever the installation instructions for the product you bought state, over the straightedge length they state it over. That figure is a field on this form rather than a constant in the code, because it varies by product and by whether the floor is floated, glued or nailed, and because it is a warranty condition rather than a general rule. A page that printed a number would be guessing at your product.
Why does the range say half an inch but the straightedge gap say an eighth?
Because the floor is sloping rather than dipping. A straightedge laid on a steady slope tips onto it and reads almost nothing, which is the right answer: flatness is about local dips and bumps, and a floating floor on a gentle slope has no idea it is on one. Level is a separate question and it matters for appliances, thresholds and doors rather than for the covering.
How far apart should I take readings?
Close enough that the straightedge spans at least three of them, and closer than that where the floor is bad. At twelve inches a ten foot straightedge covers eleven readings, which finds most things. At three feet it covers four, and a sharp dip between two stations disappears. The page refuses to answer when the spacing is too coarse for the straightedge length rather than returning a comfortable number.
Should I fill the low spots or grind the high ones?
The page prints both volumes so the comparison is visible, and neither is automatically right. Filling adds height and weight and needs primer, cure time and often more than one lift. Grinding removes nothing structural, but it makes dust, and grinding concrete releases respirable silica that has to be controlled under the rules that apply where the work is done. Which one wins usually turns on whether the floor is a few bumps or one long dish.