Paddy moisture, drying shrink, and how to check your meter
Fresh-harvested paddy can carry a quarter or more of its weight as water. Buyers pay for dry grain, so the moisture reading at intake decides how much the farmer is paid and how much the buyer expects to have left after drying. Get the moisture wrong and someone loses money on every tonne.
The one formula: dry matter stays the same
Moisture in the grain trade is almost always quoted on a wet basis: the water as a percentage of the total weight. The dry matter — everything that isn't water — doesn't change when you dry grain (apart from dust and handling losses). So:
Dry matter = weight × (1 − moisture)
Weight after drying = dry matter ÷ (1 − target moisture)
Example: drying to 14%
10,000 kg of paddy at 25% moisture contains 10,000 × 0.75 = 7,500 kg of dry matter. At 14% moisture that becomes 7,500 ÷ 0.86 = 8,721 kg. So 1,279 kg of water leaves in the dryer — about 12.8% of the original weight, not 11% as "25 minus 14" would suggest.
That gap is why simple "subtract the points" deductions and the proper formula give different numbers, and why contracts should state exactly how moisture deductions are calculated.
Check your meter by working backwards
Rearrange the same formula and your drying records tell you what the moisture really was at intake:
Intake moisture = 1 − (weight out × (1 − moisture out)) ÷ weight in
Example: a drying batch takes in 100,000 kg and puts out 81,000 kg at 14%. Dry matter out is 81,000 × 0.86 = 69,660 kg, so intake moisture was 1 − 69,660 ÷ 100,000 = 30.3%. If the meter at intake said 21%, something is wrong — and deductions based on that reading undercount the water by around 9 points.
This is how we spotted a meter reading low on very wet paddy at our own Wildfield intake. Many grain moisture meters have a specified range and become unreliable above it; readings can flatten out exactly when the paddy is wettest.
What to do about it
- Reconcile every drying batch: weight in, weight out, moisture out. Use the calculator.
- Check the meter's range in its manual, and calibrate it for the crop you're testing.
- Compare against an oven test on a few samples (the routine reference method for cereals is described in ISO 712).
- Allow for handling losses: cleaning removes dust, chaff and broken material, which is why the calculator lets you enter a loss percentage.
Why 14%?
Milled rice keeps better when dry; the Codex rice standard sets a maximum of 15% moisture, and paddy is commonly dried to about 14% for safe storage and good milling. Drying too fast or too hot, however, fissures the grain and raises broken rice at the mill — see broken rice grading.
Open the moisture calculator →
Published by rice.ag, a free project by Wildfield, rice growers and millers in Fang, Chiang Mai.