Product weight and nutrient amounts
Product weight is the amount needed for the selected nutrient target, not pure nitrogen, phosphate or potash. All three nutrient totals remain visible. Whole bags round upward for purchasing; the surplus is unused product. Optional cost is whole bags multiplied by your entered bag price.
For a separate seed order, the Grass Seed Calculator uses lawn area and the seed product’s application rate.
The formula
Choose nitrogen, phosphate or potash as the target and use its matching label percentage. An acre is 43,560 ft²; one square foot is 0.09290304 m²; one pound is 0.45359237 kg. A metric rate is first normalized to pounds per 1,000 ft². Dry bag weights can be entered in pounds or kilograms. The selected analysis must be positive to solve a product quantity.
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
- Choose the nutrient to target, then area and target-rate units. Enter the target from a product label or soil-test guidance.
- Copy the N–P₂O₅–K₂O analysis percentages from the product label; the selected nutrient must be present.
- Enter dry bag weight in pounds or kilograms and optionally its price. Review product needed, all nutrient amounts, whole bags and unused surplus.
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.
Match target units and the selected label nutrient
For 1,000 ft², a phosphate target of 0.5 lb with a 0-10-20 product requires 5 lb of product. It supplies 0.5 lb phosphate and 1 lb potash, with no nitrogen. A zero-nitrogen fertilizer can therefore work for phosphate or potash targets, while a zero analysis for the selected nutrient cannot produce that target.
Area and rate units are independent. One acre is 43,560 ft². A target of 0.5 kg per 100 m² on 100 m² requires 0.5 kg of that nutrient; a 20% analysis therefore calls for 2.5 kg of product. Separate retained entries prevent changing units from silently reinterpreting a previous area, rate or bag value.
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
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.
Can a zero-nitrogen product be used?
Yes, when targeting phosphate or potash and its selected percentage is positive. A zero percentage for the selected nutrient cannot be used to solve the product amount.
Sources & further reading
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