Seeding rate units, PLS, and seed size
How to move between pounds per acre, kilograms per hectare, and seeds per acre, and why pure live seed and seed size change the pounds you buy.
Pounds per acre versus kilograms per hectare
Most North American seed tags and drill charts use pounds per acre, while many agronomy references and imported seed use kilograms per hectare. The two are not interchangeable without a conversion, because both the weight and the area units differ. One kilogram per hectare is roughly 0.892 pounds per acre, so a 20 kg per hectare cover crop rate lands near 17.8 pounds per acre. Convert to pounds per acre before entering a rate here so the total matches the acres you are actually planting.
Weight rate versus seeds per acre
A weight rate in pounds per acre is easy to weigh and calibrate, but it hides how many plants you are really sowing, because seed size varies. A seeds per acre target is more precise for stand goals, and you convert between the two using seeds per pound from the tag. Divide your seeds per acre goal by seeds per pound to get pounds per acre. For example, a target of 150,000 seeds per acre with 3,000 seeds per pound works out to 50 pounds per acre.
Pure live seed and why bulk pounds differ
Pure live seed, or PLS, is the fraction of a bag that is both pure and viable, calculated as purity times germination. A lot that is 95 percent pure with 85 percent germination is about 81 percent PLS, so only four fifths of the weight becomes potential plants. Forage and cover crop rates are often quoted in PLS pounds, which means you must buy more bulk pounds to hit the target. Divide the PLS rate by the PLS fraction to get the bulk rate, then enter that larger number as your rate.
Calibrate the drill, do not just trust the chart
The pounds this tool reports tell you how much to buy and roughly how much to load, but the drill still needs calibration. Seed size, coatings, treatments, and humidity all shift how a metering unit flows, so a chart setting can be off by a tenth or more. Catch the seed from a known distance or number of turns, weigh it, and adjust until the measured rate matches your target. Doing this once per lot keeps your real stand close to the plan the total pounds assume.