{
  "id": "fertilizer",
  "version": "16fd0f17705c",
  "status": "published",
  "name": "Fertilizer Calculator",
  "question": "How much fertilizer should I spread?",
  "summary": "Computes how much lawn fertilizer to spread from its N-P-K grade, the area and the rate of nitrogen you want, with the phosphate and potash it adds and the bags to buy.",
  "category": "construction",
  "subcategory": "garden",
  "url": "https://www.acalculator.org/construction/fertilizer-calculator",
  "markdown": "https://www.acalculator.org/construction/fertilizer-calculator.md",
  "kind": "function",
  "method": "Fertilizer to spread = nitrogen rate × area ÷ (N% ÷ 100); phosphate = fertilizer × P% ÷ 100; potash = fertilizer × K% ÷ 100; bags = fertilizer ÷ bag weight, rounded up.",
  "assumptions": [
    "The rate is of actual nitrogen. The grade (N-P-K) gives the percent by weight of N, P₂O₅ and K₂O.",
    "NC State’s rule of thumb is for 1 lb of nitrogen per 1,000 sq ft; follow your soil test and the bag label.",
    "1 lb per 1,000 sq ft is about 4.88 g per m²."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "area": {
        "title": "Lawn area",
        "description": "The area to fertilize.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "area",
        "exclusiveMinimum": 0,
        "maximum": 10000000
      },
      "n": {
        "title": "Nitrogen (N), first number",
        "description": "The first number of the grade on the bag: percent nitrogen.",
        "type": "number",
        "x-unit": "percent",
        "minimum": 0.1,
        "maximum": 100
      },
      "p": {
        "title": "Phosphate (P₂O₅), second number",
        "description": "The second number of the grade: percent P₂O₅.",
        "type": "number",
        "x-unit": "percent",
        "minimum": 0,
        "maximum": 100
      },
      "k": {
        "title": "Potash (K₂O), third number",
        "description": "The third number of the grade: percent K₂O.",
        "type": "number",
        "x-unit": "percent",
        "minimum": 0,
        "maximum": 100
      },
      "rate": {
        "title": "Nitrogen to apply",
        "description": "The rate of actual nitrogen you want, in the unit chosen below, from your soil test or the bag label.",
        "type": "number",
        "exclusiveMinimum": 0,
        "maximum": 1000
      },
      "ru": {
        "title": "Rate unit",
        "description": "Whether the nitrogen rate is in pounds per 1,000 sq ft or grams per square metre.",
        "type": "string",
        "enum": [
          "lb1000",
          "gm2"
        ]
      },
      "bag": {
        "title": "Bag weight",
        "description": "The weight of one bag of the fertilizer. Leave empty to skip the bag count.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "mass",
        "minimum": 0.01,
        "maximum": 10000
      }
    }
  },
  "outputs": {
    "product": {
      "label": "Fertilizer to spread",
      "description": "The nitrogen to apply ÷ (N% ÷ 100), over the whole area.",
      "format": "quantity",
      "unit": "lb"
    },
    "perThousand": {
      "label": "Fertilizer per 1,000 sq ft",
      "description": "Pounds of product on each 1,000 square feet: pounds of N per 1,000 sq ft ÷ (N% ÷ 100).",
      "format": "number"
    },
    "perSquareMetre": {
      "label": "Fertilizer per m² (grams)",
      "description": "Grams of product on each square metre.",
      "format": "number"
    },
    "nitrogen": {
      "label": "Nitrogen (N) applied",
      "description": "The rate of nitrogen × the area.",
      "format": "quantity",
      "unit": "lb"
    },
    "phosphate": {
      "label": "Phosphate (P₂O₅) applied",
      "description": "The fertilizer to spread × P% ÷ 100.",
      "format": "quantity",
      "unit": "lb"
    },
    "potash": {
      "label": "Potash (K₂O) applied",
      "description": "The fertilizer to spread × K% ÷ 100.",
      "format": "quantity",
      "unit": "lb"
    },
    "bags": {
      "label": "Bags",
      "description": "The fertilizer to spread ÷ the bag weight, rounded up.",
      "format": "integer"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "area": "5000 ft2",
      "n": 16,
      "p": 4,
      "k": 8,
      "rate": 1,
      "ru": "lb1000"
    },
    "outputs": {
      "product": 14.1747615625,
      "perThousand": 6.25,
      "perSquareMetre": 30.515172727394066,
      "nitrogen": 2.26796185,
      "phosphate": 0.5669904625,
      "potash": 1.133980925
    },
    "text": "Spread 31.25 lb of fertilizer: 6.25 lb on each 1,000 sq ft."
  },
  "examples": [
    {
      "given": {
        "area": 92.90304,
        "n": 16,
        "p": 4,
        "k": 8,
        "rate": 1,
        "ru": "lb1000"
      },
      "expect": {
        "product": 2.8349523125,
        "perThousand": 6.25,
        "nitrogen": 0.45359237,
        "phosphate": 0.1133980925,
        "potash": 0.226796185
      },
      "source": "NC State Extension, Extension Gardener Handbook, chapter 9 Lawns (divide 100 by the first number on the fertilizer bag for the pounds of product per 1,000 sq ft that give 1 lb of nitrogen; 16-4-8 needs 100 ÷ 16 = 6.25 lb), https://content.ces.ncsu.edu/extension-gardener-handbook/9-lawns (retrieved 2026-10-03); hand calculation in content.mdx: 100 ÷ 16 = 6.25 lb; 6.25 × 4% = 0.25 lb P₂O₅"
    },
    {
      "given": {
        "area": 464.51520000000005,
        "n": 20,
        "p": 5,
        "k": 10,
        "rate": 0.75,
        "ru": "lb1000",
        "bag": 22.6796185
      },
      "expect": {
        "perThousand": 3.75,
        "product": 8.5048569375,
        "bags": 1
      },
      "source": "Iowa State University Extension and Outreach, How much fertilizer should I apply to my lawn? (June 2025: divide the rate of nitrogen per 1,000 sq ft by the nitrogen in the analysis; 0.75 ÷ 0.20 = 3.75 lb of 20-5-10 per 1,000 sq ft), https://yardandgarden.extension.iastate.edu/faq/how-much-fertilizer-should-i-apply-my-lawn (retrieved 2026-10-03); hand calculation in content.mdx: 0.75 ÷ 0.20 = 3.75 lb per 1,000 sq ft × 5 = 18.75 lb"
    },
    {
      "given": {
        "area": 100,
        "n": 24,
        "p": 0,
        "k": 11,
        "rate": 5,
        "ru": "gm2"
      },
      "expect": {
        "perSquareMetre": 20.833333333333332,
        "product": 2.0833333333333335,
        "nitrogen": 0.5,
        "phosphate": 0
      },
      "source": "Iowa State University Extension and Outreach, How much fertilizer should I apply to my lawn? (June 2025: divide the rate of nitrogen per 1,000 sq ft by the nitrogen in the analysis; 0.75 ÷ 0.20 = 3.75 lb of 20-5-10 per 1,000 sq ft), https://yardandgarden.extension.iastate.edu/faq/how-much-fertilizer-should-i-apply-my-lawn (retrieved 2026-10-03); hand calculation in content.mdx: 5 ÷ 0.24 = 20.83 g/m²; × 100 m² = 2.083 kg"
    }
  ],
  "sources": [
    "NC State Extension, Extension Gardener Handbook, chapter 9 Lawns (to apply 1 lb of nitrogen per 1,000 sq ft, divide 100 by the first number on the bag; 16-4-8: 100 ÷ 16 = 6.25 lb per 1,000 sq ft), retrieved 2026-10-03. https://content.ces.ncsu.edu/extension-gardener-handbook/9-lawns",
    "Iowa State University Extension and Outreach, How much fertilizer should I apply to my lawn? (13 June 2025: divide the nitrogen rate per 1,000 sq ft by the nitrogen in the analysis; 0.75 ÷ 0.20 = 3.75 lb of 20-5-10), retrieved 2026-10-03. https://yardandgarden.extension.iastate.edu/faq/how-much-fertilizer-should-i-apply-my-lawn"
  ],
  "related": [
    "grass-seed",
    "sod",
    "compost",
    "square-footage"
  ],
  "changelog": []
}