What does the fertilizer grade (e.g. 46-0-0) mean?

The three numbers represent the guaranteed minimum percentages of total nitrogen (N), available phosphate (P2O5), and soluble potash (K2O) by weight. A 50 lb bag of 46-0-0 (urea) contains 23 lb of actual nitrogen. The calculator uses whichever nutrient percentage corresponds to your selected nutrient (N, P, or K).

Why does the tool use P2O5 and K2O instead of elemental P and K?

Fertilizer labels and agronomic recommendations in the US conventionally express phosphorus as P2O5 (phosphate) and potassium as K2O (potash). Soil test reports and application rates use the same convention, so the calculator matches industry-standard units to avoid conversion errors.

How is the product rate calculated?

Product Rate (lb/acre) = Target Nutrient Rate (lb/acre) ÷ (Nutrient % ÷ 100). For example, to apply 100 lb N/acre using 46-0-0 urea: 100 ÷ 0.46 = 217.4 lb product/acre.

Can I use this for blended (NPK) fertilizers?

Yes. For a blended fertilizer such as 10-10-10, you can run the calculator three times — once per nutrient — and the limiting nutrient will give you the lowest application rate. Applying the rate required for one nutrient will automatically deliver some of the other two nutrients as well.

What is the difference between lb/acre and lb/1,000 sq ft?

One acre = 43,560 sq ft. A rate of 1 lb/1,000 sq ft equals 43.56 lb/acre. Lawn and garden recommendations typically use lb/1,000 sq ft while field crop recommendations use lb/acre. The calculator handles both units.

How do I find the nutrient percentage of a fertilizer product?

Check the guaranteed analysis on the fertilizer bag or product data sheet. The three numbers in the grade (e.g. 18-46-0 for DAP) directly give you the N%, P2O5%, and K2O% respectively. Enter the relevant percentage for the nutrient you are targeting.

Fertilizer Calculator

Name: Fertilizer Calculator
Availability: InStock
Author: Nham Vu

Enter your target nutrient rate (N, P, or K) and fertilizer analysis to find out exactly how much product to apply per acre or per 1,000 sq ft.

Fertilizer Parameters

Fertilizer Preset

Presets fill in the nutrient percentages below — adjust as needed.

N (%)

P2O5 (%)

K2O (%)

Target Nutrient

Target Nutrient Rate

From your soil test recommendation or agronomist's prescription.

Field / Lawn Area (optional)

Enter total area to calculate the total product quantity needed.

Fill in the parameters and click Calculate to see your fertilizer rate.

Calculated!

Summary

Enter your target nutrient rate (N, P, or K) and fertilizer analysis to find out exactly how much product to apply per acre or per 1,000 sq ft.

How it works

Select the nutrient you are targeting: nitrogen (N), phosphate (P2O5), or potash (K2O).
Enter the target nutrient application rate in lb/acre or lb/1,000 sq ft.
Choose a fertilizer preset or enter a custom nutrient percentage for that nutrient.
Optionally enter your field area (acres or sq ft) to calculate total product needed.
Click Calculate to see the product application rate and total quantity required.

Use cases

Determine how many bags of urea or DAP to buy for a planned nitrogen application.
Convert a soil-test fertilizer recommendation into actual pounds of product per acre.
Calculate fertilizer for a lawn or garden based on a target nutrient rate per 1,000 sq ft.
Compare application rates between different fertilizer grades for the same nutrient goal.
Estimate fertilizer cost by knowing exact product quantity before purchasing.
Avoid over- or under-application by computing exact rates from the label analysis.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu