Measuring Your Garage

Ordering too little is the expensive mistake. Ordering slightly too much costs the difference between two kit sizes; running out with half a floor coated costs the floor, because the base coat already down is curing and there is no pause while more material ships.

Measuring takes ten minutes. Do it properly once.

Measure the floor, not the building

A two-car garage described as 24 by 24 is almost never 576 square feet of concrete. That number usually comes from a plan, a listing or a builder’s brochure, and it describes the outside of the framing. The slab inside is smaller by the thickness of the walls on every side — and the wall to the house is not the same thickness as the exterior walls.

So do not use the number you were given. Stand in the garage and measure the concrete you are going to coat, from the face of one wall to the face of the opposite wall.

Measure at floor level specifically. Drywall usually stops short of the slab, framing and baseboard sit proud of it, and a tape held at waist height can be an inch or two out on every wall. Across a two-bay garage that is several square feet you did not buy material for.

Break the floor into rectangles

1. Measure the long wall, corner to corner, at floor level. Write it down.
2. Measure the short wall the same way. Write it down.
3. Multiply the two. That is your main rectangle.
4. Do the same for every piece of floor that falls outside that rectangle, and add them on.

Measure each wall rather than assuming the opposite one matches it. Garages are rarely square, and a slab poured against an existing foundation usually is not. If two opposite walls disagree, use the longer one.

Alcoves, bay extensions and the parts that are not a rectangle

Most garages are not one clean box, and the pieces that hang off the main floor are the ones people forget.

• A bay extension — the deeper third bay, or the strip added on when the garage was extended — is its own rectangle. Measure it where it opens off the main floor and where it ends, and add it.
• An alcove is a small rectangle: the water heater recess, the bump-out beside the stair, the notch where the furnace sits. Small rectangles feel too small to bother with, and three of them are a meaningful area.
• An angled corner or a curved apron is measured as the rectangle that contains it. You will have slightly more material than that shape needs, which is the right direction to be wrong in.

The garage floor calculator takes a section for each rectangle and totals them for you, so you do not have to do the arithmetic or the rounding.

What to subtract, and what to leave in

• Leave in anything you can move. The workbench, the shelving, the freezer, the car. You will move it, coat under it and move it back. A floor with a bare rectangle under the bench is a floor with a bare rectangle under the bench for good, and it is exactly where you will later want to move the bench to.
• Subtract only what is fixed to the slab and staying there — a poured stair, a masonry pier, a built-in cabinet run bolted down, a lift base you are not removing.
• Do not subtract drains, cracks or a patch. You coat around a drain and you fill a crack. Neither saves material worth counting, and taking them out of the arithmetic is how a measurement drifts short.

Decide where the floor ends

Two edges need a decision before you measure, because both change the number.

The garage door. Most people coat out to the threshold and stop on a clean line under the closed door, where the edge sits sheltered. Some carry the coating through to the outside edge of the slab. The second option adds a strip to your measurement and puts the coating edge where the slab takes the most weather, which is the hardest place for any floor coating to hold.

The step into the house. Decide whether the tread and the riser get coated. They take material, they take time, and they take more wear per square foot than anything else in the building.

Why you buy more than the arithmetic says

Your measurement tells you how big the floor is. It does not tell you how much material the floor will take. The data sheet is explicit about why:

Material consumption is greater when applied to concrete substrates due to penetration of the primer into the surface as well as variable surface uniformity.

Manufacturer product data sheet

Two things in that sentence cost material. The first is your concrete: a rough, porous, older or freshly ground slab drinks more than a smooth one, and no data sheet can know which of those you have. The second is you. Nobody pulls a perfectly even film, and the places where you go heavy get paid for out of the places where you would otherwise have had just enough.

That is the whole substantiation for the overage, and it is worth saying plainly that it is a real constraint rather than an upsell. The calculator adds the buffer to your measured area and sizes the kit against the larger figure. Buy the kit that covers the buffered number, not the one that just covers your arithmetic.

Where the coverage numbers come from

The base coat’s data sheet gives a formula rather than a fixed rate, because coverage depends entirely on how thick you lay it: divide the film thickness you want, in mils, into 1604.

Read that as theoretical coverage on a perfect substrate, which is what the caveat above it is warning you about. It is also per coat: the system is a primer, a base coat that receives the flake, and a clear topcoat, and each layer has its own thickness. The kit sizes already carry that arithmetic, so the only number you need to produce is the area of your floor.

Then let the calculator do the rest

Take your rectangles to the garage floor calculator. It totals the sections, adds the overage and names the kit that covers it. It works without JavaScript and it does not ask for your email address.

Once you have the number, the next question is whether the slab can take a coating at all. That is concrete moisture.