Understanding your result
Product weight is the amount to spread, not the weight of pure nitrogen. The accompanying nutrient totals use the second and third label numbers as phosphate and potash equivalents. Bag rounding is only a purchasing quantity.
For a separate seed order, the Grass Seed Calculator uses lawn area and the seed product’s application rate.
The formula
A 20% nitrogen product supplies 0.20 lb of nitrogen per pound of fertilizer. Divide the required nitrogen by that fraction to find product weight. Multiply product weight by each remaining analysis fraction to estimate the phosphate and potash supplied.
Worked example
For 5,000 ft² at a target of 1 lb nitrogen per 1,000 ft², the nitrogen requirement is 5 lb. A 20-5-10 product requires 25 lb of fertilizer and also supplies 1.25 lb phosphate and 2.5 lb potash.
How to use this calculator
- Enter the area actually receiving fertilizer.
- Copy the nitrogen target and the three analysis percentages.
- Compare the product weight with bag size, then calibrate the spreader separately.
Read the analysis correctly
N-P-K labels describe nutrient percentages by weight. The second and third figures conventionally report phosphate and potash equivalents, rather than elemental phosphorus and potassium. The result labels retain that distinction so the values can be compared with the same kind of product analysis.
Purchase quantity is not an application recommendation
Rounding up to a whole bag leaves unused product; it does not justify spreading the entire bag. Rate selection depends on the crop, soil test, season, label restrictions, and local guidance. The calculator supplies arithmetic only. It cannot determine whether a nitrogen application is appropriate, or whether the accompanying phosphate and potash fit the site requirements.
Assumptions & limitations
What this calculation assumes
- A uniform dry product has the entered weight-based analysis.
What to keep in mind
- Does not model nutrient release timing, liquid density, or spreader settings.
Common questions
Why can nitrogen not be zero?
A zero-nitrogen product cannot deliver a positive nitrogen target. The formula would require division by zero.
Can I use a liquid fertilizer bag weight?
Not directly for a volume purchase. Convert volume to product mass using the manufacturer’s density and confirm the analysis basis first.