Two weighings and a division
The coupon in the defaults goes on the scale at 152.4 g and comes out of the oven at 87.0 g. The water was 65.4 g and the wood was 87.0 g, so the moisture content is 65.4 / 87.0 = 75.2 percent. Note which number is on the bottom. Divide by the wet weight instead and you get 42.9 percent, which is a perfectly real quantity — the water fraction of what you are lifting — but it is not moisture content and using it anywhere in drying arithmetic produces answers that are wrong by tens of percent.
Because the denominator is oven-dry weight, moisture content above 100 percent is ordinary. A green softwood sample at 120 percent is not a typo; it means the board is more than half water by weight.
Letting the coupon speak for the board
The coupon is destroyed by the test, so it cannot be the thing you track. What it gives you is the moisture content at the moment of cutting, and that is enough: the board it came out of weighed 2,540 g at that same moment, so the board's oven-dry weight is 2,540 / 1.752 = 1,450 g. That figure never changes again. From then on, every weighing of that board is a moisture content — subtract 1,450, divide by 1,450.
At 1,780 g today the board is carrying 330 g of water on 1,450 g of wood, which is 22.8 percent. Pulling at 7 percent means the board reaching 1,551 g, so there are 229 g still to come off.
Why the table is in weights and not percentages
Because the scale is what you have in your hand at the kiln door. Each percent of moisture content on this board is 14.5 g — one hundredth of the oven-dry weight — so the whole schedule can be written as a column of scale readings and checked in seconds without arithmetic. The step and row count in the form are there so you can print a table that matches how often you actually weigh.
The failure modes worth knowing
Three things break this test, and all of them push the answer the same way. A coupon that sits around before its first weighing reads low. An oven that stops short of constant weight reads low. A coupon cut from a board that is not representative of the charge — an end, a sapwood edge, the outside of the pile — reads whatever that piece happens to be doing. The result in every case is a calculated oven-dry weight that is too high and a set of pull weights that let you take the charge out wetter than you meant to.
Questions people ask
How do I know the coupon is actually oven-dry?
By weighing it more than once. Take it out, weigh it, put it back, and weigh it again after another interval; when two consecutive weighings agree it has stopped losing water. There is no way to tell from one reading, and the calculator has no way to know whether you did this — it takes the oven-dry weight you give it as gospel because it has nothing else to go on.
Can I use the kitchen oven?
That is a question about your oven and your household, not one this page will answer. What the arithmetic needs is a temperature high enough to drive off water and stable enough to hold the coupon at constant weight, and whatever you use has to be something you are content to put resinous wood into. Follow whatever your own reference and your own equipment allow.
Why does the board weight have to be taken at the same moment as the coupon?
Because the coupon only tells you the moisture content at the instant it was cut. Pairing that percentage with a board weight taken an hour later gives an oven-dry weight that is too low by however much the board dried in the meantime, and every pull weight after it inherits the error. Cut, weigh the coupon, weigh the board, all in one go.
How many sample boards should a charge have?
More than one, and beyond that it is a question for whoever set your procedure. A single board tells you about a single position in the pile. The reason kilns run several — wettest-looking, driest-looking, and something from the middle — is that the spread between them is the actual information, and a charge is only as finished as its wettest sample. This page handles one board at a time; run it once per board.
My board reads below zero percent. What happened?
The scale is showing less than the calculated oven-dry weight, which is not physically possible for the same piece of wood. Either the oven did not take the coupon to constant weight, so the oven-dry figure came out too high, or the board weight and the coupon weight were not taken at the same moment, or the two are not from the same piece. The page flags it rather than printing a negative moisture content, because a negative one has no meaning.