Moisture Content Calculator

What moisture content is counting

Moisture content answers one question: how much of a sample's weight is water. Put 100 pounds of freshly harvested corn in a drying oven until the weight stops falling, and 80 pounds come back out. The 20 pounds that left were water, so water made up one fifth of the starting weight.

That same 20 pounds becomes two different percentages depending on which weight sits under the line. Divide by the 100 pounds you started with and you get 20%. Divide by the 80 pounds of dry material and you get 25%. Both describe the identical sample, and neither one is wrong.

Trades settled on different habits. University of Wisconsin's corn storage guide figures grain moisture on a wet basis, because the elevator weighs the load as it arrives. Woodworkers and soil labs tend to report dry basis, since the dry matter is the part they are following through a process. South Dakota State's grain moisture bulletin lays out the wet basis arithmetic and the shrink that follows from it.

Wet basis and dry basis, side by side

The two formulas differ by one word. Wet basis puts the total sample weight in the denominator: (wet weight - dry weight) / wet weight x 100. Dry basis puts the oven-dry weight there instead: (wet weight - dry weight) / dry weight x 100.

Because the dry basis denominator is smaller, that number is always the larger of the two. The gap is small at low moisture and widens fast as water climbs. A sample at 10% wet basis is 11.1% dry basis, a difference most people would ignore. At 40% wet basis the same sample reads 66.7% dry basis, and mixing up the two would badly misstate how much water is present.

Going the other way is just as quick. Dry basis equals wet basis divided by (100 minus wet basis), then multiplied by 100. Wet basis equals dry basis divided by (100 plus dry basis), then multiplied by 100. A 25% dry basis reading is 25 / 125 x 100, or 20% wet basis, which lands back on the corn example.

Worked example: 100 pounds of corn

Start with a load that weighs 100 pounds at harvest and 80 pounds after drying. Water weight is 100 minus 80, or 20 pounds. Wet basis is 20 / 100 x 100 = 20%. Dry basis is 20 / 80 x 100 = 25%. The dry matter, 80 pounds, does not change during drying, which is the anchor for every other calculation on this page.

Now dry that same load down to the 15.5% wet basis that No. 2 corn is graded at. At 15.5% moisture the dry matter is 84.5% of the final weight, so the final weight is 80 / 0.845 = 94.67 pounds. The load drops from 100 pounds to about 94.7 pounds, a shrink of 5.3 pounds, or 5.3% of the original weight. Only 5.3 pounds of water left, not the 20 pounds the sample started with, because the target is a share of a smaller final weight.

That distinction is where most shrink arguments start. A seller who reads 20% moisture and assumes the load must lose 20% of its weight to reach 15.5% will expect 80 pounds back and feel shorted by 14.7 pounds that were never water to begin with.

Worked example: a board that loses 500 grams

Wood is usually reported on a dry basis, and the numbers look strange until you see why. A green white oak board weighs 1,200 grams. Kiln drying brings it to 700 grams. Water lost is 500 grams, so wet basis is 500 / 1,200 x 100 = 41.7%. Dry basis is 500 / 700 x 100 = 71.4%, which is the figure a woodworker would quote.

The dry basis number is the one that behaves predictably. Wood shrinks and swells in proportion to how much water it holds relative to the wood substance, not relative to its current total weight. A board at 71% dry basis holds 71 grams of water for every 100 grams of wood fiber, and that ratio is what drives movement across the grain.

Lumber settles toward the moisture of the air around it. Indoors in a heated house, kiln-dried stock usually sits somewhere between 6% and 9% dry basis depending on the season and the region. A board that reads 15% dry basis in a shop is still moving, and a tabletop built from it will show gaps by February.

Typical numbers people quote

Half the confusion around moisture is that the same phrase means different things in different trades. The table below lists numbers you are likely to meet and, more importantly, the basis each one is stated on.

Grade limits are worth reading closely. The corn and soybean figures above are maximums for those grades, not targets, and a load one point wetter is discounted rather than rejected. Firewood sellers who advertise "seasoned" are claiming a dry basis number under 20%, which is the range where a split piece will usually light without a starter.

Anything measured in a lab, from soil to compost to a food sample, is likely on a dry basis because the lab dries a subsample to constant weight and divides by what remains. Ask which basis before comparing your number to a published range. The calculator on this page gives both at once so the comparison is never ambiguous.

Where a moisture number turns into money

Grain is sold by weight, and water is part of that weight, so every point of moisture has a price. The table below shows what 1,000 pounds of grain weighs after drying to 15.5% wet basis from three starting points. Dry matter stays fixed in each row, and only water leaves.

The pattern is not linear. Five extra points of moisture at the wet end cost far more weight than five points at the dry end, because each point is a share of a weight that is itself falling. Buyers apply shrink factors and drying charges on top of the weight loss, and those terms live in the elevator's tariff sheet rather than in the moisture itself.

Drying also costs fuel and time. Purdue Extension's guide to bin drying and cooling treats airflow and temperature as the two levers, and both get more expensive as the starting moisture rises. A load at 25% needs roughly twice the water removed as one at 20%, which is why harvest timing has a real dollar value.

How to weigh a sample so the number holds up

The formula is simple, so the error almost always comes from the weights. Three habits keep the result honest.

First, use a sample big enough for your scale. A 100 gram sample on a scale that reads to 0.1 gram gives a moisture figure good to about 0.1 point. The same scale on a 5 gram sample is off by 2 points before you start. Kitchen scales that resolve to 1 gram are fine for a 2 pound sample of firewood and useless for a soil test.

Second, dry until the weight stops changing. An oven at 105 degrees Celsius for 24 hours is the standard for most materials, and many grain labs use 130 degrees on a shorter schedule. A microwave works for a quick check if you weigh every few minutes and stop when two readings match. Wisconsin's guide notes that kernel damage and sample handling can bias a reading before the drying even begins.

Third, treat a hand-held moisture meter as a screening tool. Those meters read electrical resistance between pins, and the conversion to moisture depends on the species, the temperature and the density of the material. They are handy in a woodpile and unreliable as the final word on a load of grain. The oven method is the reference the meters are calibrated against.

What the calculator returns

Enter the wet weight, the dry weight and the unit you weighed in. The tool returns moisture content on both bases, the water weight, the dry matter weight, and what the sample would weigh after drying to a target moisture you supply.

The dry matter line is the one to watch. If it changes between two tests of the same material, the sample changed or a weighing error crept in, because drying removes water and leaves the fiber, starch or solids behind. That single check catches most bad readings.

The target line answers the question this page exists for: how much will this weigh when it is dry enough to sell, burn or install? Set the target to 15.5% for corn, 20% for firewood or 8% for interior lumber and the tool converts the dry matter into a finished weight. If the target is higher than the sample's current moisture, the tool says so instead of returning a negative loss.