Concrete Moisture Calculator
Enter your RH probe or calcium chloride readings plus site conditions to check whether the slab is dry enough to coat, with a clear go or no-go for each test.
Last updated August 16, 2026
Check Your Concrete Moisture Before Coating
Enter your on-site moisture test readings to get an instant pass, caution, or fail result for each one, plus an overall go / no-go recommendation.
Test Method
Select the test you carried out on site.
Test Readings
Enter the result from your on-site test.
Site Conditions (optional)
Add these for a dew point / condensation check. You'll still get an RH result without them.
Optional Details
Add more context for a more complete assessment.
Enter your test readings above to see the site assessment.
Thresholds shown are industry guidelines — not manufacturer approvals. Always check your specific product's Technical Data Sheet (TDS) for its rated moisture limits before applying.
How the Concrete Moisture Calculator Works
- Select the test method you used: RH probe (ASTM F2170), calcium chloride (ASTM F1869), or both
- Enter your moisture reading: RH% for a probe test, or MVER in lbs/1,000 ft²/24 hr for a calcium chloride test
- Enter substrate temperature, air temperature, and air humidity for a dew point condensation check
- Optionally enter surface pH, slab age, slab position, and vapor barrier status for a more complete picture
- Review the color-coded assessment for each factor and the overall site recommendation
- Always cross-check results against your product's Technical Data Sheet before applying
Examples
Frequently Asked Questions
What is the difference between an RH probe test and a calcium chloride test?
Both measure concrete moisture but in different ways. The RH probe (ASTM F2170) is inserted into a hole drilled at 40% slab depth and measures the relative humidity inside the slab, so it reflects the true moisture condition at the depth where it matters for coatings. The calcium chloride test (ASTM F1869) sits on the slab surface for 60 to 72 hours and measures how much moisture vapour is being emitted from the top.
The RH probe test is now the industry-preferred method and is required by most flooring manufacturers. The calcium chloride test is still widely used but measures surface emission only, which can underestimate moisture in thick or dense slabs.
What RH level is acceptable for epoxy flooring?
Most standard epoxy floor coating systems specify a maximum of 75% to 80% RH. Some moisture-tolerant systems are rated to 85%. If readings exceed 85%, a Moisture Vapor Barrier (MVB) is typically required before the main coating system. Above 95% RH, you should investigate the moisture source rather than just applying an MVB, because there may be an active water intrusion issue that a surface treatment alone won't fix.
What is the dew point check and why does it matter?
If the substrate surface temperature is at or below the dew point of the surrounding air, moisture will condense on the slab, the same way a cold drink sweats on a warm day. Applying epoxy over condensation causes immediate adhesion failure. The general rule is the substrate must be at least 3°C (5°F) above the dew point before applying. This calculator uses the Magnus formula to estimate dew point from your air temperature and relative humidity readings.
Do I still need to test if the slab looks and feels dry?
Yes. Always test. Concrete can hold significant moisture below the surface that is completely invisible and undetectable by touch. This internal moisture migrates to the surface after coating is applied and causes blistering, bubbling, and delamination. The only way to know the true moisture condition of a slab is to measure it with a calibrated instrument. A slab that looks and feels dry can still fail a moisture test.
What is a Moisture Vapor Barrier and when do I need one?
A Moisture Vapor Barrier (MVB) is a specialised low-viscosity epoxy primer designed to block moisture vapor transmission through the slab. It is applied before the main coating system when RH levels are too high for a standard primer. Most MVB products are rated to 95% to 100% RH and 10 to 15 lbs MVER.
MVB products manage vapor transmission only. They do not fix an active water leak. If you have standing water, water intrusion through cracks, or hydrostatic pressure, that must be resolved before any surface coating is applied.
Important Notes
- This calculator interprets readings. It does not perform moisture testing, and on-site testing with calibrated equipment is required before every coating application
- RH probe tests must equilibrate for the time specified by the test kit manufacturer (typically 24 to 72 hours) before reading
- Test at multiple locations across the slab, because moisture levels vary significantly from one area to another
- New concrete requires a minimum of 28 days cure time before coating, even if moisture readings appear acceptable
- Always confirm your specific product's moisture limits in the Technical Data Sheet before proceeding