Wood Selection & Material Science

What these tools are actually for

Every one of these calculators exists because timber is hygroscopic: it exchanges moisture with the air around it until it reaches equilibrium, and it changes dimension while doing so. That single fact causes most of the failures in finished work.

The usual sequence is: find the equilibrium moisture content for the service environment — a Panipat workshop in June and an air-conditioned Delhi apartment are not the same environment — then use the shrinkage predictor to convert the gap between current and target MC into millimetres of movement, then allow for it in the design.

Fibre saturation point is the threshold that makes the rest meaningful: above it the wood is losing free water and barely moves, below it every percentage point of moisture change shows up as dimensional change. Acclimation tells you how long the material needs on site before you can cut to final size, which is the step most often skipped.

Green weight and durability classification are the two that matter before the timber arrives — one for handling and freight, one for whether the species belongs in that application at all.

The moisture lifecycle of timber

Every board on the planet passes through the same sequence: green (freshly felled, full of free and bound water), air-dried to roughly the outdoor equilibrium, kiln-dried to the target service moisture, delivered, acclimatised on site, and then in-service — where it continues to breathe with the seasons for the rest of its life. Most defects in finished work trace to one specific point in that sequence being skipped or misunderstood, and every tool on this page maps to one of those points.

Green to air-dry is where the green weight calculator matters. Green Teak weighs roughly 60% more than dry Teak of the same volume, and that difference is what a freight quote is really pricing. Getting green weight wrong on an inbound shipment means the container is either overweight for the axle limits or the freight forwarder has been paid for water.

Air-dry to kiln-dry is where the kiln schedule finder and vapour pressure deficit calculator apply. The kiln schedule is set by species, thickness and current moisture content, because drying too fast checks the wood and drying too slowly costs energy for no benefit. VPD is the physical driver — it is the gap between how much moisture the air could hold and how much it is holding, and it is what actually pulls water out of the boards. A kiln operator who does not think in VPD is guessing.

Kiln-dry to service is where the EMC calculator becomes central. Every location has an equilibrium moisture content that wood will settle to given time. Kiln-drying to 8% and installing in a 12% EMC environment means the wood will absorb until it reaches 12% — and it will move as it does. Kiln-drying to 12% and installing in a 6% EMC environment means the opposite: shrinkage and possibly cracking. The EMC calculator tells you the target you should have been drying to.

Delivery to installation is where the acclimation calculator and moisture tracking tool apply. Every batch of timber, no matter how well kiln-dried, needs a period in the actual installation environment before it is cut to final size. The acclimation calculator gives you the estimated days for the batch to reach EMC; the tracking tool lets you record actual readings over time so you can see when it is really stable, not just theoretically stable.

In-service is where the shrinkage predictor and fibre saturation calculator live. Above the fibre saturation point (typically 25—30% MC), the wood is losing free water from cell cavities and does not change dimension. Below FSP, every percentage point of moisture change shows up as dimensional change — and the movement is roughly twice as large tangentially as radially. The shrinkage predictor converts a moisture change into a millimetre change for a given board width and species.

Anisotropy: why grain direction changes everything

Wood shrinks and swells at three different rates in three different directions, and understanding this is the single biggest lever in wood work. In round numbers, movement along the grain is negligible (0.1—0.3% from green to oven-dry); movement radially, across the growth rings, is 3—6% for most species; and movement tangentially, around the growth rings, is 6—12%. That is why a flat-sawn board cups when it dries and a quarter-sawn board stays flat. The direction the log was cut through determines how the board will behave for the rest of its life.

This is also why the shrinkage predictor asks for grain orientation, not just species. A 300 mm wide flat-sawn White Ash board losing 4% MC will move roughly 8 mm. The same board quarter-sawn will move roughly 4 mm. If a design allows for the smaller number and the timber turns up flat-sawn, the design is wrong.

Why India needs its own numbers

Most published wood-science data was measured in North American or European conditions and referenced to standardised interior climates — 20 °C, 65% relative humidity, giving an EMC of around 12%. That is not what an interior in Chennai, Delhi, or Kochi looks like. Delhi in July can sit at 30 °C and 80% humidity, giving a wood EMC closer to 16%. An air-conditioned Bengaluru office in winter might be at 22 °C and 40% humidity — EMC around 7—8%. That is a 9-point swing across a single country and a single season, which will move a solid wood panel roughly 6—8 mm across a normal cabinet width.

The EMC calculator here uses the Hailwood—Horrobin sorption model from the USDA Wood Handbook, but every calculation on the site is set up so you enter your own temperature and humidity for the actual place the wood will live. A default answer for Wisconsin is the wrong answer for Panipat. The EMC calculator lets you enter the real conditions and get a real number.

Common misconceptions

• "Kiln-dried" means the wood is dry and stable. Kiln-drying reduces moisture to a target, but the wood begins moving toward its local EMC the moment it leaves the kiln. Kiln-dried timber that has been stored outdoors for two months in a monsoon is not kiln-dry anymore. Always check current MC with a meter before installation.
• Sealing wood stops movement. A good film finish slows moisture exchange — it does not stop it. The wood still reaches EMC eventually; it just takes longer, which makes it slower to respond to short-term humidity swings. Over a full season it moves the same amount as unsealed wood.
• Old wood does not move. Antique furniture that appears stable is stable because it has spent decades reaching equilibrium with its environment. Move it to a different climate — from a London apartment to a Chennai villa — and it will move as much as any new board.
• All species need the same drying schedule. Oak, dried at the same rate as Pine, will honeycomb inside where you cannot see the damage. Every species has its own schedule for a reason; the kiln schedule finder returns the right one for the species and thickness combination.
• MC is uniform across a board. A freshly kiln-dried thick board can be 8% at the surface and 15% at the core. Meters read only the surface; a moisture core test or an oven-dry sample is the only reliable way to know the true interior value on thick stock.

How these tools connect to the rest of the site

Wood science sits underneath everything else. The joinery calculators use the shrinkage figures from here to size expansion gaps and choose fastener locations. The ThermoWood tools use the same moisture-and-movement principles applied to timber that has been thermally modified, which halves the shrinkage but changes several other properties too. The species database is where the actual radial, tangential and volumetric shrinkage figures for each timber are published, sourced from the USDA Wood Handbook, CIRAD and IWST. Every number on those pages plugs directly into these calculators.

If a project involves choosing between two species based on how they will behave in a given environment, work in this order: EMC calculator for the environment, species pages for candidate timbers' shrinkage figures, shrinkage predictor to compare how each behaves for that MC change, then the joinery expansion gap calculator to see whether the design can accommodate the movement of the timber you prefer.