Pure live seed is the only weight worth writing a spec in
A bag of seed contains seed, other seed, chaff and dirt, and some fraction of the seed in it will not come up. Purity is the share of the bag weight that is the seed you asked for. Germination is the share of that seed which is alive. Multiply them and you get PLS, the percentage of the bag weight that is going to become a plant.
Component A on the defaults is 95 percent pure and 80 percent germinating, so 76 percent of the bag is pure live seed. Ask for 4.444 pounds of PLS from that lot and you have to buy 4.444 divided by 0.76, which is 5.848 pounds of bag weight. The other 1.4 pounds are real and you are paying to ship them.
The parts ratio splits the PLS, not the bag weight
This is the step people get backwards. A 4 to 3 to 2 mix means four parts of live seed to three to two, because that is the only version of the ratio that survives contact with different lots. Split the bag weight 4 to 3 to 2 instead and a component with poor germination quietly gets less live seed than the ratio said, and one with a clean tag gets more.
So the arithmetic goes: split the PLS total by parts, then divide each share by that component PLS percentage to get its bulk weight, then add the bulk weights up. Ten pounds of PLS at 4 to 3 to 2 is 4.444, 3.333 and 2.223 pounds PLS. Divided by 76, 67.5 and 83.3 percent, that is 5.847, 4.938 and 2.669 pounds of bag weight — 13.454 in total.
| Component | Parts | Purity | Germination | PLS % | PLS lb | Bulk lb |
|---|---|---|---|---|---|---|
| A | 4 | 95% | 80% | 76.0% | 4.444 | 5.847 |
| B | 3 | 90% | 75% | 67.5% | 3.333 | 4.938 |
| C | 2 | 98% | 85% | 83.3% | 2.223 | 2.669 |
| Total | 9 | 74.3% | 10.000 | 13.454 |
The blend PLS at the bottom, 74.3 percent, is not the average of 76, 67.5 and 83.3, which would be 75.6. It is 10 divided by 13.454, weighted by how much bulk weight each component actually contributes. The component with the worst tag contributes the most extra bag weight, so it drags the blend figure down harder than a straight average would.
Why the last component is derived rather than calculated
Round three shares of a total independently and they sometimes do not add back to it. Four point four four four plus three point three three three plus two point two two two, each rounded on its own, is 9.999 and not 10. The calculator computes every component except the last from the ratio and then makes the last one the remainder, so the PLS column always sums to exactly what you asked for. It is a small thing that stops a receipt looking wrong.
The carrier is most of what you are carrying
Light seed does not spread evenly on its own, so it is usually mixed with an inert carrier. At four pounds of carrier per pound of seed, the 13.45 pounds of bulk seed becomes 53.82 pounds of carrier and 67.27 pounds of total spreading weight. That is three trips with a 25 pound hopper, and it is the figure that decides whether this is a chest spreader afternoon or a different piece of equipment.
What the carrier should be, and how much of it, is not a question this page answers. It depends on the seed and the spreader and it is in the same category as everything else here: your figure, from your extension service or the plan you are working to.
What this page has no opinion about
What to plant, in what proportions, at what rate, on what ground, at what time of year. None of that is here. The rate field exists so you can convert a plot into PLS pounds; the number that goes in it comes from your state wildlife agency, your county extension office or the published plan you are following, and it varies enormously by region and by purpose.
There is also no chemical guidance on this page of any kind — no herbicide, no pesticide, no product, no method, and nothing about how to prepare ground beyond the weight of material you are spreading. Anything involving a chemical belongs to its label, your extension service and your state pesticide authority, and it is not a calculator question.
Where this hands off
Going the other way, from a lawn area straight to pounds at a given rate, is the sod and seed calculator, and for a field with a hopper and a spreading width it is the seeding and fertilizer rate calculator. Taking turf off first is lawn removal conversion, which does the sheet, roll and spoil material. If you are planting rather than sowing, plant spacing and closure works out how many plants a bed takes and compost amendment does the soil volume.
Questions people ask
What seeding rate should I use?
This page does not hold one and will not suggest one. Rates depend on the species, the region, the ground, the season, the method and what you are trying to achieve, and a calculator producing a number for that would be worse than useless. Your state wildlife agency, your cooperative extension service and whatever published plan you are working from are the sources. The rate field here exists only so a plot area can be turned into PLS pounds; the number you put in it is entirely yours.
Why is the bulk weight so much more than the PLS weight?
Because a bag contains inert matter and dead seed as well as live seed. At the default tags the blend runs 74.3 percent PLS, so ten pounds of live seed means 13.45 pounds of bag. That is a third more weight to buy and to ship. It is also why quoting a mix in bag weight is ambiguous: two suppliers with different lots will give you two different amounts of live seed for the same bag weight.
Should I split the ratio by PLS or by bag weight?
By PLS, which is what this page does. Splitting bag weight means each component contributes live seed in proportion to its own tag rather than to the ratio, so a lot with weak germination silently under-delivers and a clean lot over-delivers. If you have a mix specified in bag weight, it was written for one particular set of tags and it stops meaning the same thing the moment a lot changes.
Where do purity and germination come from?
The seed tag on the bag you are buying, not from a table and not from last year. They are lot specific and they move. If the bags arrive with different figures from the ones you planned around, put the new ones in and rerun it: the parts ratio does not change, but the bulk weight of each component does, and so does the total.
Do I have to use a carrier?
That is a question about your seed and your spreader, not one this page answers, and the field goes to zero if you are not using one. The reason it is here is scale. At four pounds per pound, the carrier is 80 percent of what goes in the hopper, and the total spreading weight is five times the seed weight. People plan the seed order carefully and are then surprised by how many hopper loads the job takes, and the loads are driven almost entirely by the carrier.