Calculate equilibrium moisture content and fingerboard width change at any humidity and temperature. Prevent cracking and fret sprout.
Enter the relative humidity (0–100%), the temperature in °C, the fingerboard width in millimeters, and the wood species. The tool estimates equilibrium moisture content with a log-linear fit to the USDA Wood Handbook: EMC ≈ −11.54 + 5.38 × ln(RH), with a small correction of −0.04% per °C above 20 °C. At 50% RH and 20 °C that lands near 9.5% EMC.
Width change is computed against an 8% reference using each species' shrinkage coefficient — rosewood 0.28, maple 0.30, ebony 0.35 percent per percent of moisture change. A 52 mm rosewood board at 50% RH (9.51% EMC) swells about 0.2 mm; the same board at a dry 30% RH falls to roughly 6.76% EMC and shrinks about 0.2 mm, which is when fret ends start to protrude.
Crack risk keys off EMC: below 5% is flagged high (humidify immediately), 5–7% is moderate, and 7% or above is low. The sweet spot for guitars is 45–55% RH at 18–22 °C, holding most tonewoods near a 7–9% equilibrium. This is a planning approximation calibrated to a 20 °C reference, not a substitute for a calibrated hygrometer in your case or shop.
45–55% relative humidity at 18–22 °C keeps most tonewoods near their equilibrium moisture content of 7–9%, which minimizes stress. In the calculator, 50% RH at 20 °C returns about 9.5% EMC and a low crack-risk rating — comfortably in the safe zone.
When the fingerboard dries out and shrinks across its width, the metal frets do not — so their ends protrude past the board edges and snag your hand. That is fret sprout, and it is a low-humidity symptom. Dropping a 52 mm rosewood board from 50% to 30% RH shrinks it by roughly 0.2 mm in this model, which is enough to feel.
It uses a log-linear fit to the USDA Wood Handbook: EMC ≈ −11.54 + 5.38 × ln(RH), with a −0.04% correction per °C above 20 °C. Equilibrium moisture content is what the wood drifts toward at a given humidity, so it predicts how a board will move once it acclimatizes to your room or shop.
Yes. Each species has its own shrinkage coefficient — rosewood 0.28, maple 0.30 and ebony 0.35 percent of width per percent of moisture change — so an ebony board moves about a quarter more than rosewood under the same humidity swing. Denser, higher-coefficient woods need the most stable environment to stay crack-free.