The short answer: wet basis moisture content is the share of water in the total wet mass. Dry basis moisture content is the share of water in the bone-dry solids. For the same material, 40% wet basis equals 66.7% dry basis. These are two different quantities, and mixing them up can shift a dryer selection by one or two sizes.
Both terms appear in almost every drying or dewatering project. Yet misaligned definitions still produce quotations that are one size too large or too small. The problem is hard to spot, because every number looks correct — only the reference basis differs.
1. Two definitions, told apart in one line
Take a batch of material containing water of mass W₁ and bone-dry solids of mass W₀.
- Wet basis: w = W₁ ÷ (W₁ + W₀) × 100% — the denominator is the total wet mass
- Dry basis: W = W₁ ÷ W₀ × 100% — the denominator is the bone-dry solids
Converting between them:
- Dry basis from wet basis: W = w ÷ (1 − w)
- Wet basis from dry basis: w = W ÷ (1 + W)
Example: 40 kg of water and 60 kg of dry solids. Wet basis = 40 ÷ 100 = 40%. Dry basis = 40 ÷ 60 = 66.7%. One batch of material, two different numbers.
2. Why two bases exist
Because in drying calculations the mass of bone-dry solids never changes — water evaporates, but not a gram of dry solid is lost.
Using dry solids as the reference makes the mass balance straightforward: the mass of bone-dry solids stays constant, so the water evaporated is proportional to the change in dry-basis moisture content. During the constant-rate drying stage, dry-basis moisture content falls approximately linearly with time; on a wet basis the denominator keeps changing as water evaporates, so even within the same stage the decline is not a straight line. For heat and mass balance work, the dry basis is more direct.
On the purchasing side, however, people describe material as it arrives, so wet basis is the everyday language. The two conventions end up in the same table.
3. Conversion table
| Wet basis | Dry basis |
|---|---|
| 10% | 11.1% |
| 15% | 17.6% |
| 20% | 25.0% |
| 25% | 33.3% |
| 30% | 42.9% |
| 35% | 53.8% |
| 40% | 66.7% |
| 45% | 81.8% |
| 50% | 100.0% |
| 55% | 122.2% |
| 60% | 150.0% |
| 65% | 185.7% |
| 70% | 233.3% |
The pattern is clear: the higher the moisture content, the wider the gap between the two bases. Below about 10% the two figures are close, which is why many people never notice a problem until they meet a batch at 40% or 50%.
4. What a mixed-up basis actually costs
Evaporation load is the number that drives the whole selection. It is calculated as:
- Water to evaporate = feed rate × [1 − (1 − w₁) ÷ (1 − w₂)]
- where w₁ is the initial and w₂ the target moisture content, both on a wet basis
Take 1,000 kg/h of feed with a target of 10% wet basis:
- If the initial 40% really is wet basis: 1000 × [1 − 0.6 ÷ 0.9] = 333 kg/h
- If that "40%" was actually dry basis (28.6% wet basis): 1000 × [1 − 0.714 ÷ 0.9] = 206 kg/h
The same "40%" produces a 61% difference in evaporation load. A dryer sized on the wrong figure is either unable to reach the target moisture, or larger and more expensive to run than it needs to be.
5. Three field checks anyone can apply
Check 1: a dry-basis figure can exceed 100%; a wet-basis figure cannot. If someone tells you their material is "130% moisture", it must be dry basis. This single rule resolves most confusion.
Check 2: look at the material. If it can be squeezed into a ball that holds together without free water running out, wet basis is roughly 30–50%. If it flows slowly like wet mud, it is generally above 60%. If it crumbles and does not stick to the hand, it is usually 10–20%. These are rough bands, but they are enough to spot an obviously wrong basis.
Check 3: just ask. "Is that 40% wet basis or dry basis?" If the answer is "what do you mean by basis?", the basis has never been confirmed — which is itself a warning sign.
6. A real case
A magnesium sulphate producer supplied an initial moisture figure of "45%". We calculated the evaporation load, heat transfer area and fan duty on a wet basis. When samples were measured on site, the actual wet-basis moisture was around 31% — meaning the customer's "45%" was a dry-basis figure, which converts to 31% wet basis.
That alone was nearly a full size step. Because the proposal had adjustment margin, the working point was recovered by tuning inlet temperature and residence time; the machine did not have to be rebuilt. Since then our enquiry form has carried a fixed line: initial moisture content (please state wet or dry basis).
(The above is a record of on-site practice with a customer; specific figures are subject to sampling measurement and trial-run results.)
7. Write it into the quotation
"Moisture 40%" alone is not a specification. Write "initial moisture 40% (wet basis), target moisture 10% (wet basis)" — only then is it a complete, verifiable process condition.
Henan Jinyida Machinery Equipment Manufacturing Co., Ltd. is located at No.1 Nandi Industrial Zone, Gongyi City, Henan, China, and manufactures rotary drum dryers, electric heating dryers, batch rotary dryers and flash airflow dryers for powdery, granular and flake materials in the chemical, pharmaceutical, feed, food and inorganic substance industries. For a machine selection based on your material, send us the material name, initial moisture, target moisture and throughput — WhatsApp +8615238028609. Capacity and energy figures vary with operating conditions and are confirmed by a trial run.
References and Standards
- GB/T 6284—2006 Chemical products for industrial use — General method for determination of water content — The loss of mass on drying method[S]. Beijing: Standards Press of China, 2006.
- Mujumdar A S. Handbook of Industrial Drying[M]. 4th ed. Boca Raton: CRC Press, 2014.
- Liu X D, Li Z Y. Modern Drying Technology[M]. 3rd ed. Beijing: Chemical Industry Press, 2022. (in Chinese)
- Pan Y K, Wang X Z, Liu X D. Modern Drying Technology[M]. 2nd ed. Beijing: Chemical Industry Press, 2007. (in Chinese)
- Tan T E, Dou M. Principles of Chemical Engineering[M]. 4th ed. Beijing: Chemical Industry Press, 2013. (in Chinese)