Fertilizer Calculator
How Much Do You Need?
Calculate exact dosage, compare fertilizer products, follow seasonal schedules, and find answers to common questions. All backed by university extension research for 150+ plants.
5 Fertilizer Calculators
Each tool answers a different question. Pick the one that matches what you need.
Fertilizer Guides for Popular Plants
Each guide includes NPK recommendations by growth stage, seasonal schedules, product suggestions, and common mistakes to avoid.
Find What You Need
Frequently Asked Questions
What do fertilizer numbers mean?
The three numbers on a fertilizer bag represent the percentage by weight of nitrogen (N), phosphorus (P), and potassium (K). A 10-10-10 fertilizer contains 10% nitrogen, 10% phosphorus, and 10% potassium. The remaining 70% is filler material that helps distribute the nutrients evenly. Read our complete guide to fertilizer numbers.
Source: Clemson Extension
How much fertilizer do I need per 1,000 square feet?
The amount depends on the fertilizer analysis and your target application rate. For most lawns, apply 1 pound of actual nitrogen per 1,000 square feet per application. With a 10-10-10 fertilizer, you would need 10 pounds of product to deliver 1 pound of nitrogen. Use our fertilizer calculator for exact amounts based on your specific fertilizer and area.
Source: Penn State Extension
What is the best NPK ratio for vegetables?
Most vegetables perform best with a balanced or slightly phosphorus-heavy fertilizer. A 5-10-10 or 10-10-10 ratio works well for general vegetable gardens. Leafy greens prefer higher nitrogen (10-5-5), while fruiting plants like tomatoes and peppers benefit from higher phosphorus and potassium (5-10-10) once they begin flowering. See our NPK ratio guide.
Source: Clemson Extension
Can I over-fertilize my plants?
Yes, over-fertilization is a common problem that can burn plant roots, cause leaf scorch, and even kill plants. Signs include brown leaf edges, wilting despite adequate water, white salt crust on soil, and stunted growth. Always follow recommended application rates and use our Fix My Mistake calculator if you have over-applied. Read our fertilizer burn guide.
Source: Clemson Extension
When is the best time to fertilize my lawn?
Timing depends on your grass type and USDA zone. Cool-season grasses (fescue, bluegrass) benefit most from fall fertilization in September and October. Warm-season grasses (bermuda, St. Augustine) should be fertilized in late spring through summer when actively growing. Check our zone schedule pages for specific timing based on your location.
Source: Illinois Extension
What Is NPKGuide and How Does It Help You Fertilize?
NPKGuide is a free fertilizer calculator and plant care reference that helps gardeners, homeowners, and landscapers apply the right amount of fertilizer to their lawns, gardens, and houseplants. Every recommendation on this site is sourced from university extension research published by institutions such as Clemson Extension, Penn State Extension, and Illinois Extension. NPKGuide covers 150+ plants with specific NPK ratios, application rates, and seasonal timing for each one.
How Does the Fertilizer Dosage Calculator Work?
The NPK dosage calculator determines exactly how much fertilizer product you need based on three inputs: your fertilizer's NPK analysis (the three numbers on the bag), your target application rate in pounds of nitrogen per 1,000 square feet, and the total area you plan to cover. The calculator converts these inputs into a precise weight of product to apply. For example, if you need 1 pound of nitrogen per 1,000 sq ft and your bag reads 10-10-10, the calculator tells you to apply 10 pounds of product per 1,000 sq ft. This prevents both under-fertilization, which wastes money and yields poor results, and over-fertilization, which can burn roots and contaminate groundwater.
What Tools Does NPKGuide Offer Beyond the Basic Calculator?
NPKGuide provides five specialized calculators to answer different fertilization questions:
- Dosage Calculator: calculates how much product to apply per 1,000 sq ft based on your fertilizer's NPK ratio and target nitrogen rate.
- NPK Calculator: breaks down the actual pounds of nitrogen, phosphorus, and potassium you are delivering to your soil with a given amount of product.
- Cost Calculator: compares two or more fertilizer products by cost per pound of nutrient, helping you find the best value.
- Seasonal Planner: generates a month-by-month fertilization schedule customized to your USDA hardiness zone and grass or plant type.
- Fix My Mistake Calculator: provides remediation steps when you have accidentally applied too much fertilizer, including watering schedules and recovery timelines.
Why Are NPK Ratios Important for Plant Health?
NPK stands for nitrogen, phosphorus, and potassium, the three primary macronutrients that plants require in the largest quantities. Nitrogen promotes leafy green growth and is critical for lawns and leafy vegetables. Phosphorus supports root development and flower or fruit production, making it essential during transplanting and blooming. Potassium strengthens overall plant health, improves drought tolerance, and helps plants resist disease. Applying the wrong ratio can stunt growth, reduce yields, or cause nutrient lockout in the soil. According to Clemson Extension research, a soil test is the most reliable way to determine which nutrients your soil actually needs before you apply any fertilizer.
How Does NPKGuide Help You Choose the Right Fertilizer Product?
The fertilizer comparison tool lets you evaluate products side by side across multiple factors: NPK ratio, price per pound of nutrient, coverage area, burn risk, organic vs. synthetic formulation, and release speed. NPKGuide includes detailed profiles for popular products and links to 60+ head-to-head comparisons such as 10-10-10 vs. 20-20-20, organic vs. synthetic, and granular vs. liquid formulations. Each comparison includes a verdict with cited university sources explaining when each product works best.
What Plants and Zones Does NPKGuide Cover?
NPKGuide provides fertilizer guides for 150+ outdoor plants and houseplants, organized by category: vegetables, flowers, fruits, lawn grasses, herbs, and indoor plants. Each plant page includes the recommended NPK ratio, application rates by growth stage (seedling, vegetative, flowering, fruiting), seasonal schedules, and product suggestions. The site also covers USDA hardiness zones 3 through 11 with monthly fertilizer calendars that account for regional growing seasons and soil temperatures.
Is NPKGuide Free to Use?
Yes. All calculators, plant guides, comparison tools, and seasonal schedules on NPKGuide are completely free with no account required. The site does not sell fertilizer products or accept payment from brands. Product data and pricing are sourced from publicly available manufacturer information and are marked as estimates. NPKGuide exists to help gardeners and homeowners make informed fertilizer decisions based on the same research that agricultural professionals rely on. Bookmark npkguide.com and use it the next time you need to calculate fertilizer dosage, compare products, or look up the best NPK ratio for your plants.