Lumber Drying Calculator
Work out how much water has to leave a stack of green lumber, what it will weigh and shrink to, and the moisture content air drying can actually reach.
How to use this calculator
- 1Enter the rough sawn dimensions, before planing. The shrinkage figures tell you what will be left.
- 2Green hardwood is usually 60% to 100% moisture content. If you need it accurately, weigh a cut-off, dry it in an oven until the weight stops falling, and work it out from the two weights.
- 3Set the location honestly. The air drying floor it produces is the number that decides whether the plan works at all.
- 4Check progress with a meter, not a calendar. Wood is dry when it reads dry and has stopped changing, whatever the schedule said.
How the calculation works
Ovendry weight = wet weight / (1 + MC / 100)
Water to remove = ovendry weight x (start MC - target MC) / 100
Air drying floor = equilibrium moisture content at the ambient temperature and humidity
Final width = start width x (1 - S (FSP - target MC) / 100 FSP) / (1 - S (FSP - start MC) / 100 FSP)- MC
- Moisture content as a percentage of the ovendry weight - which is why green wood can be over 100%
- S
- Green-to-ovendry shrinkage for the direction concerned
- FSP
- Fibre saturation point, below which the wood starts to shrink
- EMC
- Equilibrium moisture content - where the wood settles for a given temperature and humidity, and the floor air drying cannot pass
Moisture content in wood is expressed against the ovendry weight rather than the wet weight, which is why figures above 100% are ordinary rather than impossible. A board at 100% MC is half water by weight.
The equilibrium moisture content comes from the Hailwood-Horrobin sorption model as fitted in the Wood Handbook. It is what makes the air drying floor calculable rather than a rule of thumb.
No time constant appears in these equations, because drying rate depends on air movement, temperature, humidity, species permeability and thickness in ways no single formula captures. The time figure on this page is a scaled trade rule and is presented as a range for that reason.
Worked example
40 boards of 4/4 red oak, air dried under cover
- 1.Forty boards at 1 x 8 x 96 in is 213 board feet, about 17.8 cubic feet of wood.
- 2.Green red oak at 75% moisture content weighs roughly 64 lb per cubic foot, so the stack starts near 1,130 lb.
- 3.Taking it to 7% means removing 68% of the ovendry weight in water - 440 lb, which is 53 gallons.
- 4.But outdoors under cover at 60 F and 75% humidity the equilibrium is 14.6%, so the stack will stop there. The last seven and a half points need a kiln or a heated room.
Result: About 53 gallons of water - but air drying stops at 14.6%, not 7%
The same oak finished in a heated shop
- 1.Same stack, but the target environment is now a heated shop at 68 F and 40% relative humidity.
- 2.The equilibrium moisture content there is 7.7%, so an 8% target is within reach.
- 3.This is the standard two-stage approach: air dry outdoors until the wood stops losing weight at around 15%, then bring it inside and sticker it again.
- 4.The indoor stage removes far less water than the outdoor one, but it is the stage that decides whether the furniture survives.
Result: Reachable indoors - 7.7% equilibrium against an 8% target
Why 8/4 is not twice the wait
- 1.Twenty boards of 8/4 white oak, 10 in wide and 8 ft long - 267 board feet.
- 2.The trade rule of a year per inch suggests two years. Reality is worse, because moisture has to diffuse out through the thickness rather than evaporate off the surface.
- 3.Scaling the rule by thickness to the power of 1.5 gives about 34 months as a mid-point, with a range from roughly a year and a half to four years.
- 4.White oak is also the species most prone to honeycombing if it is pushed - the surface dries, closes up, and traps water in the core, which then checks internally where you cannot see it until you saw the board.
Result: Roughly 1.5 to 4 years, not the two the folk rule implies
The floor nobody mentions
Wood does not dry to a number of your choosing. It dries towards equilibrium with the air around it and then stops. That equilibrium - the equilibrium moisture content - is set by temperature and relative humidity, and it is entirely calculable.
Outdoor air across most of the temperate world averages somewhere between 65% and 80% relative humidity over a year. At those humidities the equilibrium moisture content of wood is around 12% to 16%. An air-drying stack will fall to that figure and then simply track the seasons around it, rising in the wet months and falling in the dry ones, forever.
Indoor furniture needs 6% to 8%, because that is the equilibrium of a heated house. The gap between the two is not a matter of patience. It is a different environment, and it takes a kiln, a dehumidification chamber, or the cheapest option of all - several weeks stickered in the heated space the furniture is destined for.
Why thick stock takes so much longer
Drying is limited by diffusion. Water at the surface of a board evaporates readily; water in the middle has to travel to the surface through the wood structure first, and that journey is slow. The result is that drying time rises faster than thickness does.
The familiar rule of a year per inch is a linear approximation to a distinctly non-linear process, and it is reasonable for 4/4 stock in a temperate climate. Applied to 8/4 it under-predicts, and applied to 12/4 it under-predicts badly - thick oak can take four or five years to air dry properly.
This is also why rushing thick stock destroys it. Push the surface to dry much faster than the core and the outer shell shrinks around a wet interior that has not shrunk yet, which puts the surface in tension and the core in compression. The surface checks, and worse, the core can honeycomb - internal splits that are invisible until the board is resawn and which make it worthless.
- End-seal immediately — End grain loses moisture ten to fifteen times faster than the faces. Unsealed ends check within days of sawing.
- Sticker in line — Stickers must sit vertically above one another and above the bearers, or the boards bridge between them and set into a curve.
- Shade and shelter — Sun and wind dry the surface far faster than the core, which is the mechanism behind most checking. A roof and shade beat a breezy open yard.
- Weight the top — The top few boards in a stack have nothing pressing them flat and are the ones that cup. Concrete blocks solve it.
How to know when it is done
Not by the calendar. The reliable method costs nothing: weigh a sample board periodically, and when its weight stops falling over a couple of weeks it has reached equilibrium with wherever it is. That tells you it is done drying in that environment, which is a different question from whether it is dry enough to use.
A pin moisture meter answers the second question, with caveats. Pin meters measure electrical resistance between two probes, which is reliable between about 6% and 25% and useless above roughly 30%, where the reading saturates. They also read whatever depth the pins reach, so a case-hardened board can read acceptably at the surface and be considerably wetter in the middle.
For anything critical, the oven-dry method settles it. Cut a small sample from well inside the board, weigh it, dry it at around 215 F until the weight stops changing, and weigh it again. The moisture content is the weight lost divided by the final weight. It destroys the sample and it is the only method that is unambiguously right.
What this assumes, and where it stops
Assumptions
- Density is derived from the published figure at 12% moisture content, adjusted for the volume change between that and the moisture content in question.
- The dimensions entered are the dimensions now, at the starting moisture content, and shrinkage is applied from there.
- The air drying floor uses the annual average conditions for the location chosen. Real stacks track the seasons around that average.
- Time estimates scale the trade rule of a year per inch by thickness to the power of 1.5, which reflects the diffusion-limited nature of drying. They are indicative only.
Limitations
- Drying time depends on species permeability, air movement, season, stack geometry and local climate in ways no formula captures. Treat the range here as an order of magnitude.
- It does not model drying defects - checking, honeycomb, case hardening, collapse - which are governed by the drying *rate* rather than the endpoint, and which are what actually ruin stacks.
- Kiln schedules are species and thickness specific and are a subject in their own right. This page tells you what has to be removed, not how to run a kiln.
- Moisture content above about 30% cannot be measured with a pin meter, so the starting figure is nearly always an estimate unless you oven-dry a sample.
- Sticker counts assume a stack roughly 48 in wide. Adjust for a wider or narrower one.
Common questions
Can I air dry lumber to furniture moisture content?
Almost nowhere. Air drying reaches equilibrium with outdoor air, which across most of the temperate world means 12% to 16% moisture content. Indoor furniture needs 6% to 8%. The gap has to be closed in a kiln, a dehumidification chamber, or by stickering the wood for several weeks in the heated space it will eventually live in - which is free, and which is what most small shops do.
Is a year per inch right?
Roughly, for 4/4 stock in a temperate climate, and increasingly wrong as the wood gets thicker. Drying is limited by how fast moisture diffuses out through the thickness, so time rises faster than thickness. 8/4 takes well over twice as long as 4/4, and thick oak can be a multi-year project. The rule is a useful order of magnitude and a poor schedule.
Why did my boards check even though I dried them slowly?
Usually the ends rather than the faces. End grain loses moisture ten to fifteen times faster than the face of a board, so the ends dry, shrink and split while the rest is still wet. End-sealing with wax emulsion or even old paint, on the day the wood is sawn, prevents almost all of it. If the checks are in the faces, the stack was probably in sun or wind.
How much lighter does lumber get?
A great deal. Green red oak at 75% moisture content is around 65 lb per cubic foot; the same wood at 7% is about 44. Roughly a third of the green weight is water that leaves. This matters for handling, for what the stack stands on, and for anyone planning to move a green log with equipment sized for dry lumber.
Does stacking it in a heated shop dry it faster?
Faster and lower, which is the point - but green wood taken straight into a dry heated space is the classic way to ruin it. The surface dries far ahead of the core, checks, and case-hardens. The right sequence is to air dry outdoors under cover until it stops losing weight, then bring it in. The indoor stage removes a small fraction of the total water and does most of the work that matters.
Sources
- Drying and Control of Moisture Content and Dimensional Changes (Wood Handbook chapter 13) — USDA Forest Service, Forest Products Laboratory
- Air Drying of Lumber (FPL-GTR-117) — USDA Forest Service, Forest Products Laboratory
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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