Why the two percentages do not add
Take the defaults: 12,000 board feet on the sticks, 4 percent cull, 12 percent downgrade of what survives. The cull takes 480 board feet, leaving 11,520. Twelve percent of that — 1,382 board feet — lands in a lower bin. What comes out at the grade you sawed for is 10,138 board feet, which is 84.5 percent of the load, not the 84 percent you get by subtracting 4 and 12 from 100.
Half a percent sounds like nothing until you run it backward. To net 10,000 board feet at grade you have to put 11,838 on the sticks. Load 10,000 and you finish 1,552 board feet short, which is not a rounding error, it is most of a small charge.
Cull and downgrade are different kinds of loss
A culled board is footage that left the building. At $3.40 and $0.10 salvage it costs $3.30 a board foot. A downgraded board is still on the truck and still gets loaded, stacked, planed and shipped — it just fetches $1.85 instead of $3.40, so it costs $1.55 a board foot. On a per-board-foot basis cull is the more expensive event by a wide margin.
That does not settle which rate to chase, because the two rates are not usually the same size and because they compound in a particular order. The calculator drops each one by a point in turn, on the same load and at your own three prices, and reports what each is worth. On some price sets the answer flips, which is why the page works it out rather than stating a rule.
The kiln does not know which boards will make grade
Drying cost is charged against everything that goes in the chamber. At $160 per thousand, the default charge costs $1,920 to dry. Spread over the 10,138 board feet that came out at grade, that is $0.19 a board foot rather than the $0.16 the loaded rate suggests — an 18 percent premium that exists purely because the chamber heated wood that ended up in the wrong bin. The worse the yield, the wider the gap, and it widens faster than the yield falls.
Where the numbers have to come from
Both rates are yours. There is no published cull rate for your species at your thickness on your schedule in your chamber, and anyone quoting one is quoting an average of somebody else's mill. Counting one charge honestly — tally in, tally out by bin, culls weighed or counted — gives a figure worth more than every rule of thumb in the trade, and it is the single input this page is most sensitive to.
Questions people ask
Should the downgrade rate be a share of the load or a share of the survivors?
The field asks for a share of the survivors, because that is the order the two things happen in — you pull the culls first and grade what is left. If your tally sheets record downgrade as a share of the whole load instead, convert it before entering: divide your figure by (100 minus the cull rate) and multiply by 100. Getting this the wrong way round shifts the overload factor by roughly the product of the two rates, which is small at low rates and not small at high ones.
What cull rate should I use?
Yours. This page will not supply one and there is no honest default to give: the rate depends on species, thickness, how the lumber was handled before it went on the sticks, the schedule your kiln supplier provides, the chamber, and the grade rules of whoever is buying. The number in the field is a placeholder. Count a charge in and out by bin and use what you find.
Does the salvage value field change the overload factor?
No. The overload factor comes only from the cull and downgrade rates, because it is a question about footage rather than money. Salvage value only affects the money blocks and the comparison of which rate is worth chasing — with a high salvage value a culled board costs less to lose, which can tip the comparison toward chasing downgrade instead.
Can I use this for air-dried stock as well as kiln charges?
The arithmetic does not care where the degrade happened. If you are counting cull and downgrade across a season in the yard rather than across a kiln charge, put the season tally in and set the drying cost per thousand to whatever the yard costs you to hold the stock. The block headed as the kiln charging the whole load then reads as the yard charging the whole pack, which is equally true.