Moisture content is a fraction, and the argument is always about the denominator. Divide the mass of water by the total mass of the wet sample and you get the wet basis figure. Divide the same mass of water by the oven-dry mass instead and you get the dry basis figure. Both are correct, both are in daily industrial use, and they give different numbers for the same handful of material — 24% on one basis is 31.6% on the other.
This calculator reports both from a single pair of weighings, converts a figure from either basis to the other, and works out how much water has to leave to reach a target. Arb Digital built it that way because the single most expensive mistake in this area is not an arithmetic error but a basis mismatch between a specification and a test result.
What This Moisture Content Calculator Does
Weigh the sample as received, dry it to constant mass, weigh it again, and enter both figures. The headline result is the wet-basis moisture content; the grid gives the dry-basis figure, the actual mass of water lost, the dry-matter percentage, and the mass of water that would still have to be removed to reach the target you set.
If you were given a moisture content rather than masses, switch the first selector to conversion mode, enter the figure and say which basis it is on, and the calculator returns its equivalent on the other. That conversion is not a scaling factor — it is non-linear, and the gap between the two bases widens sharply as material gets wetter.
The target calculation assumes the dry matter is conserved, which is the correct assumption for drying and for humidification. It reports a positive number when water must be removed and states plainly when water would have to be added instead.
How to Use It
- Enter the wet mass as received. Weigh promptly — a sample left open on a bench is already changing.
- Enter the oven-dry mass. Dry to constant mass at the temperature your method specifies, then weigh again after cooling in a desiccator.
- Enter the container tare if you weighed in a dish, and it will be subtracted from both readings automatically.
- Read both bases. The headline is wet basis; the first grid item is dry basis. Quote whichever your industry uses, and always say which one it is.
- Set a target and its basis to see how much water still has to be driven off.
The Formula: How It's Calculated
With mw as the wet mass and md as the oven-dry mass, the mass of water is simply mw − md. The two definitions differ only in what that is divided by:
MCwet basis = (mw − md) ÷ mw × 100
MCdry basis = (mw − md) ÷ md × 100
Converting between them: MCdb = MCwb ÷ (100 − MCwb) × 100 and MCwb = MCdb ÷ (100 + MCdb) × 100. The oven-dry basis definition used across the timber industry, and the reason a green log can be more than half water by weight, is set out in Penn State Extension's guide to calculating the green weight of wood species. The wet-basis convention that dominates grain handling, along with the storage moisture figures that follow from it, is covered by University of Minnesota Extension in drying wheat and barley.
A worked example, matching the defaults on this page: a 500 g sample dries to 380 g, so 120 g of water left. Wet basis is 120 ÷ 500 = 24.0%. Dry basis is 120 ÷ 380 = 31.58%. Dry matter is 76.0%. To reach 12% wet basis, the dry matter must end up as 88% of the total, so the final mass is 380 ÷ 0.88 = 431.8 g, meaning 68.2 g of water still has to go.
Which Industries Use Which Basis
Wet basis dominates wherever material is bought and sold by weight, because the buyer is paying for the whole delivered mass and wants to know what fraction of it is water. Grain trading, food processing, feed, paper and pulp, and pharmaceutical loss-on-drying testing all work on the wet basis. When a grain contract specifies 14% moisture, that is wet basis, and the shrink calculation applied at the elevator follows from it.
Dry basis dominates wherever the dry matter is the thing that stays constant and the water is a variable passenger. Timber and wood products are always quoted on the oven-dry basis, which is why green softwood can be described as 120% moisture content without anyone blinking. Geotechnical soil testing uses the dry basis. Textiles use a closely related idea called regain, which is dry basis by another name. Biomass fuels are reported both ways, and the two figures appear in the same documents often enough that heating-value calculations go wrong regularly.
The gap between the two bases is small when material is dry and enormous when it is wet. At 5% wet basis the dry basis figure is 5.26% — close enough that a careless swap might not be noticed. At 50% wet basis the dry basis figure is 100%. At 60% it is 150%. Any process working with genuinely wet material has to be explicit about which basis it means.
Why the Oven-Dry Mass Is the Hard Part
Both formulas look trivial, and the arithmetic is. All of the difficulty sits in defining "dry", because almost nothing is a clean separation of water from solid. Standard methods therefore specify a temperature, a duration and an end condition — typically drying to constant mass, meaning successive weighings differ by less than a stated amount.
Temperature is a compromise. Too low and bound water never leaves, so the result reads dry. Too high and volatile organics, oils or structural water go with the moisture and the result reads wet. Materials that decompose or oxidise near the drying temperature are worse still: a sample can gain mass in the oven when a fat oxidises, giving a negative apparent moisture content.
Cooling matters too. A hot sample must be cooled in a desiccator before weighing, because hot air convection off the pan disturbs the balance and because a hygroscopic material starts reabsorbing moisture from room air the moment it leaves the oven. Weighing it warm, or leaving it on the bench for ten minutes first, both put a bias into the number in opposite directions.
Equilibrium Moisture Content and Why Material Will Not Stay Where You Put It
Hygroscopic materials exchange moisture with the air around them until they reach an equilibrium set by the air's relative humidity and temperature. That equilibrium moisture content is a material property, not a target you choose, and it is the reason timber dried to 8% in a kiln drifts to a different figure once it is installed, and why grain in a bin re-wets under the wrong aeration conditions.
The consequence for measurement is that a sample begins changing the moment it is exposed. Sealed sampling, prompt weighing and consistent handling are what make repeated tests agree. The consequence for process control is that removing water is only half the job; keeping it out means controlling the air the material sits in, which is where the relative humidity calculator, the absolute humidity calculator and the dew point calculator come in. How fast the water actually leaves a free surface is a separate question handled by the evaporation rate calculator.
Drying Loss, Shrink and the Money Side
Drying removes water, so the mass that comes out is smaller than the mass that went in, and that difference has a price. The relationship is not linear in moisture content: going from 20% to 15% wet basis removes a different mass of water than going from 15% to 10%, even though both are five percentage points. Working from the dry matter makes it obvious. A tonne of grain at 20% wet basis contains 800 kg of dry matter; at 15% that same dry matter is 94.1% of the total, giving 850 kg — a 150 kg loss. Dropping from 15% to 10% takes the same dry matter to 888.9 kg, a further 61 kg.
That is why moisture shrink schedules are computed rather than estimated, and why over-drying is a direct financial loss on top of the energy it wastes. The percentage arithmetic behind these conversions is quick work in the percentage calculator, and mass unit conversions in the weight converter.
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Browse All Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Quoting a moisture content without its basis — the number is ambiguous, and the two bases diverge sharply as material gets wetter.
- Treating the conversion as a fixed factor — it is non-linear, so a ratio that works at 10% is wrong at 50%.
- Forgetting to subtract the container tare from both weighings — it biases the numerator and the denominator differently and skews both results.
- Weighing a hot sample — convection off the pan disturbs the balance, and hygroscopic material reabsorbs water within minutes of leaving the oven.
- Assuming everything lost in the oven was water — volatile oils and decomposition products leave too, and some materials oxidise and gain mass instead.
Related Free Tools From Arb Digital
For the air side of drying and storage, use the relative humidity calculator, the absolute humidity calculator and the dew point calculator, and for how fast water leaves an exposed surface, the evaporation rate calculator. Fraction and shrink arithmetic is handled by the percentage calculator and the ratio calculator, mass units by the weight converter, and bulk properties by the density calculator. Everything else is in the free online tools hub.
Frequently Asked Questions
Both divide the same mass of water by a different denominator. Wet basis uses the total mass of the wet sample, so it can never exceed 100%. Dry basis uses the oven-dry mass only, so it can exceed 100% when a sample holds more water than solid. A sample at 24% wet basis is at 31.6% dry basis.
Dry basis equals wet basis divided by one hundred minus wet basis, times one hundred. Going the other way, wet basis equals dry basis divided by one hundred plus dry basis, times one hundred. The relationship is non-linear, so there is no single multiplier that works across the range.
Grain, food, feed, paper and pharmaceutical loss-on-drying testing use the wet basis, because material is traded by delivered weight. Timber, wood products and geotechnical soil testing use the oven-dry basis. Biomass fuel is reported on both, which is why heating-value calculations for it go wrong so often.
On the dry basis, yes. Green timber commonly exceeds 100% because it holds more water than dry wood substance, and the figure is correct as stated. On the wet basis it is impossible, since water can only ever be a fraction of the total mass.
Until it reaches constant mass, as defined by whichever standard method applies to your material. Constant mass means successive weighings, separated by a further drying period, differ by less than a stated tolerance. A fixed time works only where a standard specifies one for that material and sample size.
Because something other than water is happening. Fats and oils can oxidise at drying temperatures and add mass faster than the remaining water leaves. It is a sign the drying temperature or method does not suit the material, and the result should not be reported as a moisture content.
The moisture content a hygroscopic material settles at when left in air of a given relative humidity and temperature. It is a property of the material and its surroundings rather than a target you can set, and it explains why dried timber and stored grain drift back toward the conditions of the space they sit in.
This tool is provided for educational and engineering-estimate use. It computes mass fractions from the weighings you supply and does not define a drying method, a temperature or an end point, which are set by the standard applicable to your material. Where a contract, a specification or a product claim depends on the result, use the accredited test method and quote the basis explicitly.
