Where the rate comes from
The open floor rate is geometry and nothing else. A head that cuts a 20 inch path, walked at 15 feet a minute, covers 1.67 feet times 15 feet every minute, which is 25 square feet a minute or 1,500 an hour if the machine never stops. It stops constantly. Emptying the vacuum, changing tooling, dragging cable past a column and turning at the end of every run are the reason a 55 percent duty figure is realistic and 80 percent is a fantasy.
The two inputs people get wrong are the working width and the speed. Working width is the path actually cut after the overlap between adjacent runs, not the diameter printed on the machine. And walking speed while cutting is far slower than walking speed. Both are worth timing once on the real floor rather than estimating, because both scale the answer directly.
The perimeter is the job
Run the defaults on this page. Six hundred square feet, two passes, and 8 percent of the area is edge work. That 8 percent takes more of the day than the other 92 percent, because a hand-held grinder with a shroud covers around 60 square feet an hour against 825 for the walk-behind on the defaults here, and the operator is bent over for all of it.
This is the estimating error that shows up on small floors. A 24 by 20 garage has 480 square feet of slab and 88 feet of wall, and a six inch band along that wall is 44 square feet that has to be done by hand. Add the door threshold, the corners and the ring around a floor drain and it climbs. On a large warehouse bay the perimeter band is a rounding error. On a two-car garage it is half the day.
Diamonds
| Input | What it is | Where the figure comes from |
|---|---|---|
| Area per set | Square feet of cutting a set of segments lasts | Your own record from a similar slab. Nothing else is reliable. |
| Area cut in total | Floor area multiplied by passes | Calculated here |
| Sets needed | Cut area divided by set life, rounded up | Calculated here |
Segment life is the least predictable number in the whole exercise. Bond hardness has to be matched against slab hardness in the opposite direction to intuition, a soft slab wears a soft bond away quickly and a hard slab glazes it, and the same set of tooling can go three times as far on one floor as on another in the same building. The calculator will do the division; it cannot supply the figure and neither can anyone who has not ground that slab.
Before and after
What the preparation is for is a surface the coating can key into, and the profile the coating needs is stated by the coating manufacturer rather than chosen here. Once the floor is prepared, the material quantity comes from the spread rate calculator or from published coverage in the garage floor coating calculator. If the slab has not been tested for moisture yet, that comes before the grinder, not after, and the arithmetic is in the slab moisture test conversion. Cracks and joints that need routing out are also better done before the floor is prepared than after, and the joint layout on a new slab is the control joint layout calculator.
Questions people ask
How many passes does a floor need?
That depends entirely on what is on it and what the coating requires underneath it, and neither is something this page can decide. Bare concrete that only needs opening up is a different job from a slab carrying an old coating, and a slab carrying an old coating is a different job again from one with adhesive residue or a cured sealer that clogs tooling. What the calculator does is let you price the difference: change the pass count from two to four and watch the hours and the tooling both double, which is the number to have in hand before agreeing a price on a floor nobody has tested a patch of.
Why is the hand grinder rate so much lower than the machine rate?
Because a hand-held grinder with a dust shroud cuts a path of a few inches, is held by a person rather than driven by machine weight, and works in the places where you cannot stand comfortably. Around 60 square feet an hour is the sort of figure people record for it against several hundred for a walk-behind. The consequence is that edge work dominates the schedule on small floors and disappears on large ones, and the calculator shows the share of hours next to the share of area precisely so that difference is visible before it becomes a surprise.
What surface profile do I need before coating?
The coating manufacturer specifies it, usually by reference to a standard profile scale, and it varies with the product and the film thickness being applied. A thin sealer and a trowelled build coat do not want the same surface. This page will not tell you which one you need, because getting it wrong in either direction is expensive: too little profile and the coating has nothing to key into, too much and the first coat disappears into the texture without building film. Get it off the technical data sheet for the specific product, and if the sheet is ambiguous, ask the manufacturer rather than guessing.
Is dust from grinding concrete actually dangerous?
Yes. Concrete contains crystalline silica, grinding it produces particles small enough to reach deep into the lung, and the resulting disease is irreversible and develops over years rather than showing up as a bad afternoon. Dry cutting inside a closed building without extraction is the worst case and it is also the most common one on small jobs, because the vacuum is the expensive part of the rental and it is the part people skip. What is required in terms of extraction and respiratory protection is set by the applicable occupational health rules and by the equipment manufacturer. This page names the hazard and stops there deliberately.
Does shot blasting replace grinding?
They are different processes producing different results and the choice is usually made by what is on the slab and what profile the coating wants. Blasting is fast on open floor and leaves a uniformly textured surface. It does not reach edges any better than a grinder does, so the perimeter arithmetic on this page applies either way, and it does not remove a soft or gummy coating the way grinding does. You can use this calculator for either by putting the blaster working width and travel speed in the machine boxes, since the production arithmetic is the same shape. The tooling line will not mean much for a blast machine, so leave the area per set blank.