Pure Live Seed (PLS) Calculator

What PLS tells you that purity alone does not

A seed tag always prints purity and germination as separate numbers, but neither one by itself tells you how much live seed is in the bag. Purity says how much of the weight is the right species; germination says what fraction of that species-correct seed will actually sprout. Pure live seed multiplies the two together, and the result is the single number you need to compare lots, adjust a seeding rate, or decide how many bags to buy.

The Purdue FNR worked example makes the arithmetic concrete: a cowpea lot that is 98% pure and 85% germination is (0.98 × 0.85) × 100 = 83% PLS. Seventeen percent of that bag’s weight is inert filler, weed seed, or seed that will not germinate. If a seed extension guide recommends a drilling rate of 50 lbs PLS per acre (Purdue FNR: iron-clay cowpeas), you divide by 0.83 to get ~60 lbs of bulk seed per acre. The calculator above does both steps: enter your tag numbers, then optionally a PLS rate, and it returns the bulk rate in lbs per 1,000 sq ft, lbs per acre and kg per hectare.

When hard seed and dormant seed matter

Some seed tags, particularly for native grasses, legumes and wildflower mixes, report hard seed or dormant seed alongside germination. Those seeds are alive but did not sprout in the standard lab test. The NRCS convention adds them to germination before multiplying by purity: PLS% = purity × (germination + hard seed + dormant seed). When the tag does not list them, leave those fields at zero and the formula reduces to the simpler Purdue version.

Where this fits alongside the grass seed calculator

The grass seed calculator takes a species name and sizes bulk seed from a preset or custom rate typed as if it were the bag-label rate. This PLS calculator sits one step earlier: it answers “what is my seed actually worth?” and converts a PLS-rated recommendation into the bulk pounds you hand to the species calculator or to a purchase order. Use both when your seed source quotes a PLS rate rather than a bulk rate, which is common for native seed, wildflower mixes and reclamation plantings.

If the seed is going down as a sprayed slurry, carry the bulk rate from here into the hydroseeding pricing calculator, which prices the job from tank loads. For a fall overseed where the rate is already on the bag, the aeration and overseed estimator covers the combined machine-and-seed visit.

Nothing is stored, there is no account, and the CSV, print, PDF and copy-as-text exports are ungated so the seed order travels to the supplier with you. Figures here are a planning aid, not a compliance record or agronomic advice. Your seed bag label and your extension agronomist govern over any calculator.