How do you calculate grain shrink?

Work from the dry matter, which does not change when grain is dried. Multiply the wet weight by one minus the wet moisture to get the dry matter, then divide that by one minus the target moisture to get the weight at the target. The difference is the water removed. Grain at 18 percent moisture dried to 14 percent loses about 4.7 percent of its weight, which is more than the four percentage point difference suggests.

Why is shrink more than the difference in moisture percentages?

Because the percentages are shares of two different totals. Removing four percentage points of moisture is not removing four percent of the weight: the water leaves but the dry matter stays, so the remaining moisture becomes a larger share of a smaller load. Going from 18 to 14 percent is a 4.65 percent weight loss, and the gap between the two figures widens as the starting moisture rises.

Why does a wet load pay less than its weighbridge weight?

Because it is being paid on the weight it would be at trade moisture, not on the water it arrived with. A buyer taking grain at a standard moisture converts the delivered weight back to that standard, which is exactly the shrink calculation. Drying charges then usually come off on top of that, so a wet load is discounted twice, once in weight and once in the drying.

Grain Moisture Shrink Calculator NZ

Grain shrinks when it is dried, and it shrinks by more than most people expect, which is why a weighbridge ticket and a payment advice so often disagree. This works out the weight a load loses coming from harvest moisture down to the moisture it is traded at, what that is worth at your price, and what the drying charge adds on top. The reason the arithmetic surprises people is that the two moisture figures are shares of different totals. Taking grain from 18 percent moisture to 14 is a four percentage point move, and it is not a four percent weight loss: it is about 4.65 percent, because the water leaves while the dry matter stays, so the moisture that remains is a larger share of a smaller load. That gap widens as the starting moisture rises, which is the argument for not harvesting wetter than you need to. The other thing worth being clear on is that a wet load is discounted twice. It is paid on the weight it would be at trade moisture rather than on the water it arrived with, and then the drying charge comes off as well. Both are legitimate, but seeing them separately is what lets you decide whether to dry on farm or deliver wet.

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Weight as harvested

Moisture as harvested

Moisture it is traded at

Grain price

Drying charge

/t/point

4.65%

of the load lost as water, not as a four point difference

Payable weight19.07 t

Water removed0.93 t

Value at trade moisture$8,581.40

After drying charges$7,208.37

The moisture difference is 4 points but the weight loss is 4.65%, because the water leaves and the dry matter stays. A wet load is discounted twice: once in the weight converted back to trade moisture, and again in the drying charge.

No trade moisture, price or drying charge is supplied here. Trade moisture differs by grain and by contract, and drying is charged in different ways by different sites, some per tonne per point and some as a flat rate. Take all of them from your own contract.

How it works

Everything works from the dry matter, which does not change when grain is dried. The dry matter is the delivered weight multiplied by one minus the harvest moisture. Dividing that by one minus the trade moisture gives the weight the same grain would be at trade moisture, which is the payable weight. The difference between the two weights is the water removed, and expressing it as a share of the starting weight is the shrink. The drying charge is applied per tonne per percentage point removed, which is the most common structure, and the net figure shows what is left after both.

Why the two percentages are not comparable

A moisture percentage is a share of the whole load including its water, so 18 percent and 14 percent describe two different totals. Removing water shrinks the denominator as well as the numerator, which is why the weight loss exceeds the difference in the percentages, and why the gap grows as the starting moisture rises. Harvesting at 22 percent instead of 18 does not cost four more percentage points of weight, it costs noticeably more than that, on top of the extra drying.

Worked example

Twenty tonnes harvested at 18 percent moisture contains 16.4 tonnes of dry matter. At a trade moisture of 14 percent that same dry matter weighs 19.07 tonnes, so 0.93 tonnes of water comes off and the shrink is 4.65 percent, not the four percent the moisture difference suggests. At $450 a tonne the payable load is worth $8,581.40 rather than the $9,000 a weighbridge ticket for 20 tonnes might imply. Drying four points at $18 a tonne a point costs $1,373.02 on the payable weight, leaving $7,208.37.

Related calculators

Grain Yield Estimator: what the paddock is carrying before harvest.
Hay Bale Weight: the same dry matter logic for feed.
Silage Stack Yield: dry matter in a stack.
Acres per Hour: getting the crop off.

Data sources: the shrink calculation is arithmetic on dry matter and holds for any grain, so nothing here is a rate. Trade moisture, price and drying charges all come from your own contract, because they differ by grain, by buyer and by site.