Fertilizer NPK Calculator
Enter your bag weight and its three N-P-K numbers to see exactly how many pounds of each nutrient you're really spreading.
📖 How it works & FAQWhat the three NPK numbers mean
Every bag of fertilizer carries three numbers, like 10-10-10 or 24-8-16. They are the guaranteed analysis: the percentage of the bag's total weight that is nitrogen (N), available phosphate (P), and soluble potash (K), always in that order. The rest of the bag is filler, carrier granules, and trace nutrients. A percentage alone doesn't tell you how much you're actually putting on the ground, though — a 10% nitrogen number means very different things in a 5-lb box versus a 50-lb sack.
Why pounds matter more than percent
Lawn and garden feeding rates are almost always written in pounds of actual nitrogen per 1,000 square feet, not in percent. To hit a target you first have to convert the label percentage into real pounds. That's exactly the math here: pounds of a nutrient equal the bag weight multiplied by that nutrient's percentage, divided by 100. Do it for all three numbers and you know precisely what each application delivers. These figures are general estimates, not professional agronomic advice — for a soil-specific plan, get a soil test.
How to use it
- Enter the total weight of the bag or the portion you plan to spread, in pounds.
- Type the three analysis numbers from the label into the N, P, and K fields.
- Read the actual pounds of each nutrient in the result cards below.
- Compare the nitrogen figure against your product's recommended pounds-per-1,000-sq-ft rate.
FAQ
- Do P and K use elemental or oxide values?
- Bag labels report P as phosphate (P2O5) and K as potash (K2O). This tool returns pounds of exactly what the label guarantees, so no oxide-to-elemental conversion is applied.
- Why don't the three numbers add up to 100?
- They rarely do. The remaining weight is inert filler, carrier granules, and micronutrients — shown here as "filler / other."
- Can I use kilograms instead of pounds?
- Yes. The percentages work on any unit, so enter kilograms and read the results as kilograms of each nutrient.
- What if I only spread part of a bag?
- Enter the weight you actually applied, not the full bag size, and the nutrient pounds scale accordingly.