Understanding grain moisture: equilibrium moisture content, limits, measurement

Expert advice: KLN & Heumann GmbH · Updated: 27.08.2026

Whether a lot of grain stores safely, has to be re-dried at cost or starts to heat in the silo comes down to a single figure: the grain moisture content. To control it, you need to understand the relationship underlying every drying and aeration decision – the moisture equilibrium between grain and air. This guide explains how grain moisture is defined, provides the key equilibrium table for the main crops at 15 °C and translates the figures into practical limits and rules of thumb for storage and drying.

Grain moisture: how the figure is defined

In the seed and grain trade, moisture is stated on a wet basis: it describes the share of water in the total weight of water plus dry matter. A kernel at 20% moisture therefore consists of one fifth water by weight. The reference procedure is the oven-drying method: a sample stays in a heated oven until all the water has evaporated; moisture is calculated from the initial weight and the remaining dry matter.

The wet basis has a consequence that is often underestimated at settlement: if 100 kg of grain is dried from 20 to 15% moisture, the lot loses not 5 kg but 5.88 kg – because the reference weight shrinks along with the water. Around 59 kg of water evaporates per tonne of harvested crop this way. In practice the weight loss is higher still, because dust, broken kernels and green matter are removed in addition during drying, conveying and cleaning.

Equilibrium moisture content: the grain follows the air around it

Grain is hygroscopic. In moist air the kernel takes up water, in dry air it gives water off – and this exchange only ends when grain moisture and relative humidity are in equilibrium. The grain moisture that results depends on the crop and the temperature. The table below shows the equilibrium values at 15 °C as documented in the Kongskilde technical reference on grain biology.

The key table: grain moisture by crop at 15 °C

Each figure is the grain moisture in percent that establishes itself at the relative humidity of the ambient air stated in the header row:

*) Some variation depending on quality. Values: Kongskilde technical reference on grain biology, equilibrium moisture content at 15 °C.

How to read the table

Wheat settles at 14.3% grain moisture at 65% relative humidity. To bring a lot down to a storable level of around 14%, the drying air must therefore be drier than 65% on average – and the larger the gap between air humidity and the equilibrium value, the faster the drying proceeds. The reverse also holds: air at 80% relative humidity will not take wheat below about 17.3% – ventilate in such weather and you re-wet dry grain. Oilseed rape stands out: as an oilseed it settles at markedly lower values and, in the same air, needs a considerably lower target moisture than cereals.

Rule of thumb: warming makes the air drier

At any given temperature, air can only carry a limited amount of water vapour; the relative humidity describes how far that capacity is used up. A worked example from the technical reference: a cubic metre of air at 20 °C containing 5.25 g of water has, against a saturation value of 17.5 g, a relative humidity of 30% – and can take up a further 12.25 g of water before it is saturated.

Warm the air and its capacity rises while the relative humidity falls – that is the basic principle of all warm-air drying. For aeration drying with slightly warmed air, the rule of thumb is: each degree of warming lowers the relative humidity by around 4.5 to 5 percentage points. The technical reference on grain biology quotes an average of 4.5 percentage points per degree, other drying references simplify to 5 – the exact value depends on the temperature level and can be read from the psychrometric chart. For planning purposes, the range has proven itself.

An example: outside air at 12 °C and 80% relative humidity will hardly dry wheat below 17%. Warm the same air by 3 °C and its relative humidity falls to around 65 to 67% – enough to move wheat towards 14%. The method has a lower limit, though: at air temperatures below about 10 °C, hardly any drying is possible, because cold air takes up very little water even at low relative humidity. Then cooling the grain bulk takes priority.

Limits and measuring practice

Which moisture for which purpose

• Safe storage: a guide value of around 14% grain moisture – lower still depending on end use and storage period. The storage temperature should not exceed 15 °C.
• Moister grain needs cold: at 16% moisture, completely safe overwintering is only assured below about 4 °C.
• Mites: they need at least 14 to 15% grain moisture; drying to 13 to 14% is the most effective countermeasure. Complete freedom from mites requires no more than 12% and temperatures around 3 °C.
• Storage fungi: depending on species they grow from as little as about 13.5 to 16.5% grain moisture (maize/wheat) – so the 14% mark is not a figure to push against but a precautionary limit.

Measuring moisture – and when

The grain moisture of the standing crop is not a fixed figure: it follows the daily cycle of air humidity. Records from a typical harvest day in August show differences of several percentage points between the damp night and the dry afternoon. Moisture management therefore only becomes reliable with measured values: measure every lot at intake and check the readings regularly through the storage period. Important for interpretation: table and threshold values come from laboratory trials with clean, undamaged grain. Commercial-grade grain with broken kernels, dust and green matter spoils earlier – build in a safety margin.

From reading to plant: controller and fans

The equilibrium table answers the question of whether the air can dry at this moment. A controller takes that decision off your hands: the PST controller for flat store drying monitors the conditions and only lets the plant run when the air can actually draw water out of the grain – instead of re-wetting it in damp weather. The large air volumes required are delivered by aeration fans of the HVL series. The section Drying and Storage Systems gives an overview of fans, controllers and accessories; spare and wear parts for existing plants are identified fastest via the spare parts finder.

Recommended action: moisture management in five steps

1. Measure the grain moisture of every lot at intake – do not estimate. Mind the daily cycle: grain combined in the afternoon is drier than morning grain.
2. Set the target moisture: around 14% as a guide value, considerably lower for long storage periods, sensitive produce or oilseed rape.
3. Check every drying window against the equilibrium table: if the relative humidity is above the equilibrium value of the target moisture, you are not drying but re-wetting.
4. Warm the air if needed: each degree lowers the relative humidity by around 4.5 to 5 percentage points. Below an air temperature of about 10 °C, switch to cooling.
5. Automate and monitor: a controller such as the PST decides objectively on every hour of aeration; temperature and moisture in the store nevertheless remain regular checking tasks.

Treat air and grain as an equilibrium system and you dry with less energy, store more safely and spot risks before they cost money. The right equipment for it – from the HVL fan to the PST controller – can be found in the category Drying and Storage Systems.

Frequently asked questions

Matching products

On-floor Drying Control System – PST →Aeration Blowers – HVL →

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