Application weight versus whole-bag purchases
Application rate or bag coverage supplies the base requirement. Allowance increases stock to plan, and whole bags determine purchasing quantity and cost. Surplus is purchased seed beyond the allowance-adjusted requirement, not a higher application-rate recommendation.
If your soil-test guidance also specifies nitrogen, the Fertilizer Application Calculator converts that target into product weight.
The formula
Grams per square metre multiplied by square metres gives grams of product. Converting to kilograms makes the requirement comparable with the package size. A purchase allowance is additional stock; it does not automatically increase the recommended application rate. Bag coverage must apply to the exact bag size and new-lawn or overseeding task you selected. The calculated rate is total product weight divided by covered area. Separate entries retain each area, rate and bag unit when you return to it.
Worked example
A 250 m² lawn at 30 g/m² needs 7.5 kg. With 5% extra stock, plan for 7.875 kg. Two 5 kg bags provide 10 kg, costing 70 if each bag costs 35.
How to use this calculator
- Choose lawn area units and enter the area actually being seeded.
- Select an application rate or the coverage printed for one bag and the intended new-lawn/overseeding task. Choose matching bag mass units.
- Enter any spare-stock allowance and bag price. Review base application weight, planned stock, whole bags, surplus and cost.
New lawns and overseeding
The appropriate rate depends on the seed mixture and the job. A bare seedbed and an existing lawn with thin patches need not use the same rate. The default value is an arithmetic example, not a recommendation for every species. Use the specific product label or a locally appropriate extension recommendation.
Product weight versus viable seed
Coated products and mixtures can include material other than viable grass seed. If a recommendation is expressed as pure live seed rather than total bag weight, it must first be converted using the product purity and germination information. This calculator assumes the rate you enter already refers to the bagged product being purchased. It does not perform that conversion or select a spreader setting.
Use bag coverage without inventing a species rate
If one 5 kg bag covers 250 m² for your chosen task, 500 m² needs 10 kg before an allowance. A 5% ordering allowance increases the plan to 10.5 kg, so three bags buy 15 kg and leave 4.5 kg above the plan. Use the coverage printed for that same bag and task.
Imperial inputs can be mixed with metric package labels after normalization. For 1,000 ft² at 5 lb per 1,000 ft², the base requirement is 5 lb. A 5 lb bag with no allowance is exactly one bag. A coating, purity or germination adjustment is not applied automatically.
Assumptions & limitations
What this calculation assumes
- The entered rate describes the exact product in the bag.
What to keep in mind
- Uneven ground, seed loss, germination and spreader calibration are not predicted.
Common questions
Should I spread the allowance as well?
Not automatically. Extra purchased stock can cover gaps or later touch-ups; apply according to the appropriate label rate.
Can I calculate several lawn sections?
Yes. Add the areas if they use the same mixture and rate. Different seed products should be calculated separately.
Can I enter only the area printed on the bag?
Yes. Select bag coverage, enter that area in the chosen area units and enter the matching bag weight. Coverage for new lawns can differ from overseeding coverage, so use the label for your task.
Sources & further reading
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