The formula
yield = harvest × (100 − moisture) ÷ (100 − reference) ÷ area
Where it comes from
Two harvests can only be compared at the same moisture, because water weighs and gets paid for as if it were grain. That is why stores discount: they take what you deliver back to the reference moisture for the crop, which in most cereals is fourteen per cent. The correction is a proportion on dry matter, not a percentage you subtract. And the number that really decides whether a field is worth sowing is not the yield but the break-even: the kilos per hectare at which income equals cost.
How to work it out by hand
- Multiply the harvest by (100 − actual moisture) and divide by (100 − reference moisture)
- Divide by the hectares: that is the corrected yield
- Divide by a thousand to get tonnes per hectare
- Divide the cost per hectare by the price per tonne to see where you break even
What is worth knowing
Reference moisture is not the same for every crop or every country: fourteen per cent is standard for wheat, barley and maize in Europe, but maize comes off the field far wetter and drying is charged separately. Use the figure the store you deliver to actually applies, not the one in the textbook.