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Why Moisture Meter Readings Vary by Material

A reading of 18 on drywall, 18 percent on oak and 18 on concrete are three different amounts of water. Meters are calibrated for one material and offer scales or corrections for the rest. Knowing which scale you are on is half the job.

By the Akermin Editorial Team · 3 min read

Moisture meters measure an electrical property, resistance for pin meters and dielectric change for pinless ones, and infer water from it. The relationship between water content and that electrical property is different for every material: wood species vary in density and chemistry, gypsum and plaster behave differently again, concrete and brick differently still, and salts or preservatives change everything. Meters are calibrated for one reference (usually Douglas fir for pins, a set wood density for pinless) and provide scales, groups or corrections for the rest. A number is only meaningful on the scale it was read from.

Wood: species and density

Pin meters are typically calibrated to Douglas fir. Other species conduct differently at the same moisture content, so meters provide species groups or correction tables; oak, for example, may read a couple of points different from the calibration species. Pinless meters respond to density as well as water, so a dense hardwood and a light softwood at the same true moisture read differently unless the meter’s specific-gravity setting is adjusted. Manuals include the tables; using them is the difference between a 12 percent and a 15 percent reading.

Drywall, plaster and masonry: reference scales

Gypsum, lime plaster, brick and concrete have no single water-to-resistance relationship, so meters offer a relative or reference scale (often 0 to 100 or a WME “wood moisture equivalent” figure) rather than a true percentage. The number is useful only in comparison: a suspect patch reading 40 beside a known-dry area reading 12 is clearly wet; 40 on its own is not a percentage of anything. Some meters have a drywall mode that maps the scale to an approximate percentage; treat it as indicative.

Things that distort readings

Factor Effect Meter type affected
Salts (efflorescence, sea air, de-icing) Reads high; salts conduct Pin mainly
Preservative treatment Reads high Pin
Surface condensation or rain Reads high at the surface Both
Metal, wiring, pipes, foil-backed board False high Pinless
Density change (knots, different board) Shifts reading with no water change Pinless
Temperature Warm material reads higher; some meters compensate Pin
Glue lines, laminates, tile Averaged into the reading Pinless
Frozen material Reads very low Both

Working comparatively

  1. Identify the material and select the matching scale or correction.
  2. Take a baseline on a known-dry area of the same material, same orientation, same depth.
  3. Read the suspect area the same way.
  4. Report the difference and the scale, not a bare number.
  5. Where an absolute figure matters (flooring installation, timber grading), use the method the relevant standard specifies: oven-dry test for wood, in-slab RH probes for concrete.

Typical wood moisture content

Condition Moisture content
Interior furniture and flooring, heated home 6 to 9%
Interior joinery, general 8 to 12%
Framing in a dry house 10 to 16%
Exterior timber, sheltered 12 to 18%
Fungal decay possible Above 20%
Fiber saturation About 28 to 30%

For how the two meter types differ, see pin vs pinless moisture meters. Moisture meters are covered in the thermal and inspection category.

Extech MO265 Combination Pin and Pinless Moisture Meter with RJ45

Extech MO265 Combination Pin and Pinless Moisture Meter with RJ45

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Extech MO257 Pinless Moisture Meter, Depth 0.78 to 1.6 in

Extech MO257 Pinless Moisture Meter, Depth 0.78 to 1.6 in

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

Can I use a wood moisture meter on drywall?

Yes, on its reference or relative scale, comparing suspect areas with a known-dry area. The wood percentage scale does not apply to gypsum.

Why does pressure-treated wood read high?

Preservative salts make it conductive, so a pin meter reads more moisture than is present. Some meters have a correction; otherwise treat readings as comparative.

What is a normal moisture reading for concrete?

Handheld meters give only a relative scale on concrete. Flooring standards use in-slab RH probes or calcium chloride tests for an absolute figure.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.