Fert Calculator

Fertilizer labels give three numbers, but not how much to put down. This calculator turns the N-P-K analysis into pounds or grams of product for your lawn or bed, shows what you’re really applying, and — for planted-aquarium keepers — converts grams of dry salts into parts per million for your tank volume. Everything is calculated in your browser.

Reading N-P-K numbers

The three numbers on a fertilizer bag are the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 15 lb bag of 24-0-6 contains 15 × 0.24 = 3.6 lb of nitrogen, no phosphate and 0.9 lb of potash. For historical reasons phosphorus and potassium are labelled as oxides: multiply P₂O₅ by 0.436 to get elemental phosphorus and K₂O by 0.830 to get elemental potassium.

The lawn formula

Many lawn programmes apply around 0.5–1 lb of nitrogen per 1,000 sq ft per application, depending on grass type, season, soil and whether the nitrogen is quick- or slow-release. Your local extension service or the product label is the best guide for your lawn.

Worked example

A homeowner has a 5,000 sq ft lawn and a 15 lb bag of 24-0-6 costing $39.99. Aiming for 1 lb of nitrogen per 1,000 sq ft, they need 4.17 lb of product per 1,000 sq ft and 20.8 lb in total — 1.39 bags, so they buy two bags and keep the rest for later. One bag covers about 3600 sq ft. Nitrogen costs $11.11 per pound, which they compare with a cheaper 29-0-4 product.

Planted aquarium dosing

Aquarium keepers who dose dry fertilizers measure nutrients in parts per million (mg per litre). ppm added = grams × fraction × 1,000 ÷ litres. For example, 1 g of potassium nitrate in a 120-litre tank adds about 5.1 ppm of nitrate and 3.2 ppm of potassium. Fractions are calculated from each salt’s molecular formula.

Using fertilizer responsibly

• Sweep granules off paths and driveways back onto the lawn — they wash into drains and streams.
• Don’t fertilize before heavy rain or onto frozen ground.
• Many areas restrict phosphorus on lawns unless a soil test shows a need.
• Test soil every few years to see what your lawn or garden actually needs.
• Store fertilizer dry and away from children and pets.

Quick-release vs slow-release nitrogen

Labels often state what share of the nitrogen is slow-release. Products with more slow-release nitrogen can usually be applied at higher single rates, within the label’s limits.

Metric conversions

• 1 lb per 1,000 sq ft ≈ 4.9 g per m² (≈ 49 kg per hectare).
• 1 g per m² = 10 kg per hectare.
• 1 kg per 100 m² = 10 g per m².
• Switch the calculator to metric to enter grams per square metre directly.

Spreader calibration

Spreader settings on bags are a starting point. To check yours, spread over a measured area — say 100 sq ft on a tarp or driveway — collect and weigh what was applied, and compare with the rate the calculator gives. Adjust the setting and repeat until they match. Walking at a steady pace and overlapping passes as the spreader manual advises gives even coverage.

What’s in the template pack

• Lawn fertilizer schedule spreadsheet that calculates product per application
• Soil test record sheet
• Fertilizer application log
• Spreader calibration worksheet
• Aquarium dosing log with ppm calculations

The pack is a one-time download; the calculator is free.