For guitar owners
Tools
Free, and built on the same standards the archive applies to its own instruments. Nothing you enter is transmitted, stored, or seen by us. Everything runs in your browser.
Instrument Dossier Builder
Make a record of your guitar
Five steps to a printable document: identification, label, condition, provenance, photographs. The format the archive uses for its own twenty five instruments.
Open the builder →Humidity Risk
When is your guitar actually at risk
Five questions, then relative humidity and wood moisture month by month, built on the USDA equilibrium moisture content tables rather than the usual unsourced number.
Open the calculator →Why these exist
The archive spent a year learning that most owners discover they have no documentation only after a loss, and that most humidity advice online is a number with no source behind it. These are the two that seemed worth building.
Wood moves
A guitar is thin wood held under permanent tension. Spruce and cedar tops are worked down to around two and a half millimetres, and at that thickness wood responds to the moisture in the air around it. It takes water up when the air is damp and gives it back when the air is dry, and it changes dimension as it does. Across the grain that movement is significant. Along the grain it is almost nothing.
That difference is the whole problem. A soundboard is glued to braces and to sides that restrain it. When the top wants to shrink and the structure will not let it, the wood has one way out.
What dry air actually does
Below about 35% relative humidity, a guitar starts to lose moisture faster than it can equalise. The top goes flat, then slightly concave. The action drops. The strings begin to buzz against frets that are now proud of a sunken fingerboard. If it continues, the top splits, usually along a grain line and usually near the bridge or the lower bout.
A crack from dryness is not a sign of rough handling. It is the predictable result of a room, and it happens to instruments that are never taken anywhere.
What damp air does
Above about 60%, the reverse. The top swells and rises, the action climbs, and the instrument becomes harder to play. Sustained high humidity also softens hide glue, which is what most Spanish workshops used and what many still use. Joints that have held for eighty years can begin to creep.
Damp damage is slower and less dramatic than dry damage, which is exactly why it goes unnoticed for longer.
The range, and why the swing matters more
Between 45% and 55% relative humidity is where a guitar is comfortable, and most instruments tolerate 40% to 60% without complaint. But the number on any given day matters less than how fast it moves. Wood accommodates gradual change. A guitar carried from a damp car into a heated room in January is being asked to do something it cannot do quickly, and that is when tops split.
Stability beats precision. A room that sits steadily at 40% is kinder than one that swings between 35% and 60% every week.
Older instruments have less margin
An instrument built before the war has already survived decades of whatever its previous rooms did to it. Old glue joints, old repairs, and wood that has been cycling for a century have less tolerance left than a guitar built last year. The oldest instruments in this archive are kept with their strings slackened for exactly this reason: less tension across a top means less force available to open a crack when the wood moves.
What to actually do
Measure before you correct. A hygrometer in the case, or in the room where the case lives, tells you whether you have a problem. Most people who assume they have one do not, and some who assume they are fine are running at 25% all winter without knowing.
The calculator on this page is built to turn a reading into a risk assessment rather than a number. It does not replace a conservator, and it does not diagnose an instrument that already shows a symptom. If a top has sunk or a seam has opened, that is a luthier's question, not a hygrometer's.