Fertilizer Calculator
Calculate how much fertilizer to apply to deliver a target amount of nitrogen, phosphorus or potassium.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
About this calculator
Convert a user-entered nutrient target and fertilizer-label percentage into product mass. NPK label numbers mean N, phosphate (P2O5) and potash (K2O); enter the target on the same basis as the label. This does not recommend an application rate: use a soil test, local guidance and the product label.
Worked example
calculated with the default values below
Area to fertilise 500 sq ft · Nutrient to target Nitrogen (N) · User-entered nutrient target 0.75 lb per 1,000 sq ft · Nitrogen N (%) on label 10 % · Phosphate P2O5 (%) on label 10 % · Potash K2O (%) on label 10 % · Bag weight 40 lb Nutrient required 0.38 lb · Percentage of that nutrient on the bag 10 % · Fertilizer product needed 3.75 lb · Bags needed 1 · Application rate 7.50 lb per 1,000 sq ft
Method and assumptions
Q=(A÷1000×r)÷(p÷100); bags=ceil(Q÷w) NPK label percentages are N, P₂O₅ and K₂O by weight. Match the target to the same nutrient basis. A zero selected percentage is invalid; bags round up. Convert a user-entered nutrient target and fertilizer-label percentage into product mass. NPK label numbers mean N, phosphate (P2O5) and potash (K2O); enter the target on the same basis as the label. This does not recommend an application rate: use a soil test, local guidance and the product label. Use the cited guidance and your own measurements, bills or product specifications to choose inputs. Sample defaults are not current local data.
Data sources
Use the cited guidance and your own measurements, bills or product specifications to choose inputs. Sample defaults are not current local data.
Disclaimer
These are general gardening estimates. Planting windows, growth, water and nutrient needs vary by place and plant. Check local extension guidance and product labels before acting on an estimate.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
Q=(A÷1000×r)÷(p÷100); bags=ceil(Q÷w)
- A
- area (sq ft)
- r
- selected nutrient target (lb/1,000 sq ft)
- p
- matching label percentage
- Q
- fertilizer product (lb)
- w
- bag weight (lb)
NPK label percentages are N, P₂O₅ and K₂O by weight. Match the target to the same nutrient basis. A zero selected percentage is invalid; bags round up.
Frequently asked questions
How do the fertilizer label numbers map to the calculation?
Fertilizer grades list percentages by weight in N-P-K order: nitrogen, phosphate expressed as P2O5, and potash expressed as K2O. Enter the matching nutrient target and label percentage. If the selected percentage is zero, the calculator shows an input error because that product cannot supply the selected nutrient.
Which inputs and limits apply to Fertilizer Calculator?
Use the units shown beside each field. These are form limits, not recommended settings. Area to fertilise: 1–500,000 sq ft; User-entered nutrient target: 0.05–5 lb per 1,000 sq ft; Nitrogen N (%) on label: 0–60 %; Phosphate P2O5 (%) on label: 0–60 %; Potash K2O (%) on label: 0–60 %; Bag weight: 1–100 lb Q=(A÷1000×r)÷(p÷100); bags=ceil(Q÷w) NPK label percentages are N, P₂O₅ and K₂O by weight. Match the target to the same nutrient basis. A zero selected percentage is invalid; bags round up. Convert a user-entered nutrient target and fertilizer-label percentage into product mass. NPK label numbers mean N, phosphate (P2O5) and potash (K2O); enter the target on the same basis as the label. This does not recommend an application rate: use a soil test, local guidance and the product label. Use the cited guidance and your own measurements, bills or product specifications to choose inputs. Sample defaults are not current local data.
What is a complete worked example for Area to fertilise = 500 sq ft; Nutrient to target = Nitrogen (N)?
Inputs: Area to fertilise = 500 sq ft; Nutrient to target = Nitrogen (N); User-entered nutrient target = 0.75 lb per 1,000 sq ft; Nitrogen N (%) on label = 10 %; Phosphate P2O5 (%) on label = 10 %; Potash K2O (%) on label = 10 %; Bag weight = 40 lb. Calculation: Q=(A÷1000×r)÷(p÷100); bags=ceil(Q÷w). Results: Nutrient required = 0.38 lb; Percentage of that nutrient on the bag = 10 %; Fertilizer product needed = 3.75 lb; Bags needed = 1; Application rate = 7.50 lb per 1,000 sq ft.