What the calcium chloride arithmetic is doing
The test is a dish of anhydrous calcium chloride, weighed, sealed under a dome on the bare slab, left, and weighed again. The salt takes up whatever water vapour comes off the concrete underneath the dome, so the weight it gained is the water that emerged from a known area over a known time.
The reporting unit is pounds per 1,000 square feet per 24 hours, so the conversion is three scalings applied to one weight. Grams to pounds is a division by 453.59237. The dome footprint to 1,000 square feet is a multiplication. The actual elapsed hours to 24 is another. Run the defaults on this page and 3 grams gained over 72 hours under a dome covering 72 square inches comes out at 4.41 pounds per 1,000 square feet per 24 hours.
The elapsed time is worth being fussy about. A test placed on a Friday afternoon and lifted on a Monday morning is not 72 hours, and using the nominal figure rather than the real one moves the answer by the same percentage as the error. The same goes for the dome footprint, which is a property of the kit rather than a universal number, and which is why it is a box on this page instead of a constant in the code.
The two tests do not convert
The surface emission test and the in-situ relative humidity probe are both called moisture tests and they measure genuinely different quantities. The dish measures what is leaving the top of the slab over three days, which depends heavily on what the surface has been doing lately. The probe measures the humidity of the air in equilibrium with the concrete at the depth the hole was drilled, which is much less sensitive to the last few weeks of room conditions.
A slab that has been covered with stored material for a year has a damp surface and a dry room above it, and the two tests will disagree about it in a way that no arithmetic reconciles. There is no conversion factor between them and this page does not offer one. If a data sheet asks for a probe result, a dish result is not an answer to that question.
Why this page will not tell you the answer
Every acceptance limit in this area belongs to a product rather than to concrete. One manufacturer will accept a reading that another will not, the same manufacturer will set different limits for different products in its own range, and the limits move when formulations change. A page that printed a threshold would be inventing a specification for a product it has never seen, and the cost of getting that wrong lands on someone with a lifted floor.
So the limit is a box you fill in from the data sheet, the page prints your reading next to it, and the decision stays with the manufacturer. That is not caution for its own sake. It is the same reason the test result decides the job before the coating does.
What to do with the number
If the result is comfortable and the manufacturer agrees, preparation is next, and the hours and tooling for that are in the grinding production calculator. Material quantity follows from the spread rate calculator or from published coverage in the garage floor coating calculator. If the result is uncomfortable, the questions are about drainage and what is under the slab rather than about coatings, and the ground water side of that is the territory of the interior drain tile calculator and the crawl space ground moisture calculator. A basement that is fighting humidity in the air as well as through the slab is the basement dehumidifier calculator.
Questions people ask
How do I convert a calcium chloride result to pounds per 1,000 square feet?
Three scalings on the weight the dish gained. Divide the grams by 453.59237 to get pounds. Multiply by 1,000 divided by the dome footprint in square feet, which scales the area up to a thousand square feet. Multiply by 24 divided by the actual hours of exposure, which scales the time to a day. The dome footprint has to come from the kit instructions, since it is not the same for every kit, and the hours have to be the real elapsed time rather than the nominal three days. This page does all three and shows each step, so you can see which input is moving the answer.
Can I convert an RH probe reading into an emission rate?
No, and treating them as interchangeable is a real source of trouble. They measure different physical quantities under different conditions: the dish measures vapour leaving the surface over three days, the probe measures the humidity of the concrete at depth. The surface test responds strongly to what the room and the floor covering have been doing recently, the probe test far less so. Two slabs can give matching probe readings and very different dish readings. If the material going down specifies one test, run that test.
Does this page tell me whether my slab is ready to coat?
No, deliberately, and no calculator should. Acceptance limits are properties of the specific coating or adhesive, they differ between products from the same manufacturer, and they change when formulations do. What this page does is convert your measurement into the unit the data sheet asks for and print it beside the limit you entered, so the comparison is in front of you in the right units. The decision belongs to the manufacturer of the material going down, taken with the whole test record rather than a single number, and for a floor of any consequence it is worth having that conversation in writing.
How many places should I test?
More than one, and the number usually comes from the test standard the product references, which is why it is an input here rather than a built-in figure. Standards differ between materials and get revised, so the count on the sheet in front of you beats anything a calculator remembers. Placement matters at least as much as count. A slab is not uniform: test the coldest corner, anywhere the floor sits below the outside ground level, near an outside wall, near any known plumbing run, and somewhere in the open middle so you have a comparison. A single reading from the easiest spot in the room is close to no information.
Why did my slab pass in August and fail in February?
Because a slab on ground is in contact with the ground, and the ground changes with the season. Water table height, rainfall, outside grade drainage and how much the heating is running all move the result, and none of them is a property of the concrete. A test is a snapshot of conditions during the days it ran, which is why a result is worth recording with its date, the weather and the state of the building rather than as a bare number. Where a floor matters, testing in the season the slab is wettest tells you more than testing in the season it is driest.