Kiln Capacity, Stickers, and the Wood Fraction of a Charge
How much wood actually fits in a kiln, why stickers are not just spacers, and how to back out the fill factor from a real charge.
Wood fraction is t / (t + s)
The wood volume in a kiln charge is the stack volume times t / (t + s) where t is board thickness and s is sticker thickness. For 25 mm boards on 19 mm stickers the wood fraction is 25 / (25 + 19) = 56.8%. A 2.4 × 1.2 × 3.0 m stack then holds 8.64 × 0.568 = 4.91 m³ of theoretical wood, which the Kiln capacity tool multiplies by the fill factor (default 0.9) to give 4.42 m³ actual — about 1 873 board feet per charge.
Thicker stickers waste kiln volume
1″ (25 mm) stickers on 1″ (25 mm) boards give 50% wood. 1.25″ (32 mm) stickers drop the fraction to 43.9% — a 6% loss of wood per charge. The trade-off: thicker stickers give more airflow and reduce sticker imprint (the wet mark that stays under the sticker). For 25 mm hardwoods, 19 mm stickers are the standard. For thicker stock (50 mm+) or softwoods that need aggressive drying, 25 mm stickers are appropriate.
Stickers must be dry, uniform thickness, and aligned
Stickers should be dried to the same MC as the charge (or drier), all the same thickness to within 0.5 mm, and stacked vertically — every sticker directly above the one below it. Misaligned stickers create hollows in the stack and uneven drying. The standard sticker is a piece of the same species, 19–25 mm thick, 25–40 mm wide, and as long as the stack width. Use only one width of sticker per charge; mixing widths creates gaps.
Fill factor accounts for what geometry misses
A perfectly geometric stack of equal-width boards would have a fill factor of 1.0. In practice: end gaps, slightly off-square stacks, varying board widths within a charge, and the fact that stickers aren't always full length all eat into the geometric volume. 0.90 is a reasonable default for well-stacked hardwood charges. To back out a fill factor for your kiln, weigh a finished charge (kg dry mass) and divide by the geometric volume × species density — the ratio is your fill factor.
Match the kiln volume to your production
A small dehumidification kiln (2 m³ charge) running 6 charges/month produces about 100 m³ of dried stock per year — enough for 2–3 small furniture workshops. A 30 m³ conventional kiln produces closer to 1 500 m³/year, suited to a 30-person factory. The Kiln capacity tool turns the stack size into cubic metres and board feet per charge, which you multiply by your cycle time and number of charges to plan throughput.