What Do Fertilizer Numbers Mean? NPK Explained Simply
The three numbers on a fertilizer bag are the percentages, by weight, of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 20-5-10 contains 10 lb of nitrogen, 2.5 lb of phosphate and 5 lb of potash. Nitrogen drives leaf growth, phosphorus supports roots and flowers, and potassium builds stress tolerance.
Use the free Lawn Fertilizer Calculator: turn the numbers on any bag into pounds of product for your lawn.
Walk down the fertilizer aisle and every bag has three numbers on the front: 10-10-10, 24-0-6, 5-10-5. The numbers look like a secret code, but they follow a simple rule, and knowing it lets you compare bags properly and stop paying for nutrients you don't need.
This guide explains what N, P and K each do, how to turn the numbers into actual pounds of nutrient, and how to choose the right ratio. When you are ready to turn a bag's numbers into pounds per application, the lawn fertilizer NPK calculator does the arithmetic.
- The code: N-P-K = percent nitrogen, phosphate and potash by weight, always in that order.
- Pounds of nutrient: bag weight × percent. A 50 lb bag of 20-5-10 holds 10 lb N.
- Ratio matters: 20-5-10 is a 4-1-2 ratio, typical for lawns.
- A zero is meaningful: 24-0-6 has no phosphorus, which many lawns do not need.
- Test before you buy: a soil test shows which nutrients are actually low.
In this guide
- What do the three fertilizer numbers stand for?
- How do you calculate pounds of nutrient in a bag?
- What does nitrogen do, and how do you tell if a lawn needs more?
- Why does lawn fertilizer often have no phosphorus?
- What does potassium do?
- What are the other numbers and words on the bag?
- How do you choose the right ratio for your plants?
- How do you compare fertilizer prices fairly?
- What do nutrient deficiencies look like?
- Why do soil tests beat guessing?
- Frequently asked questions
- The bottom line
What do the three fertilizer numbers stand for?
Every fertilizer sold in the US carries a "fertilizer grade" made of three numbers. They are always listed in the same order and always mean the same thing:
| Position | Nutrient | Chemical form on the label | What it does for plants |
|---|---|---|---|
| First | Nitrogen (N) | Elemental N | Leafy growth, deep green color, protein |
| Second | Phosphorus (P) | Phosphate (P₂O₅) | Root development, flowering, seed and fruit set |
| Third | Potassium (K) | Potash (K₂O) | Disease resistance, drought and cold tolerance |
A grade of 20-5-10 means 20% of the bag's weight is nitrogen, 5% is phosphate and 10% is potash. The rest is filler or carrier material, plus any secondary nutrients such as sulfur or iron.
How do you calculate pounds of nutrient in a bag?
Multiply the bag weight by each percentage. This is the single most useful trick in fertilizing, because it lets you compare a cheap bag to an expensive one fairly.
| Bag (50 lb) | Lb of N | Lb of P₂O₅ | Lb of K₂O |
|---|---|---|---|
| 20-5-10 | 10.0 | 2.5 | 5.0 |
| 10-10-10 | 5.0 | 5.0 | 5.0 |
| 24-0-6 | 12.0 | 0 | 3.0 |
| 32-0-4 | 16.0 | 0 | 2.0 |
Example: a 50 lb bag of 24-0-6 contains 50 × 0.24 = 12 lb of nitrogen. Priced at $30, that is $2.50 per pound of nitrogen. A 50 lb bag of 10-10-10 at $25 gives only 5 lb of nitrogen, which is $5 per pound, so the "cheaper" bag costs twice as much for the nutrient lawns use most. (Prices are examples only.)
Want the pounds of product for your own lawn? Open the lawn fertilizer calculator, enter the three numbers from your bag and your lawn size.
What does nitrogen do, and how do you tell if a lawn needs more?
Nitrogen is the nutrient plants use in the largest amounts, and it is the one that makes grass green. A nitrogen-hungry lawn turns pale yellow-green, grows slowly and thins out, often with older leaves yellowing first. Too much nitrogen has the opposite effect: lush, fast growth that needs mowing constantly, more disease and thatch, and runoff into waterways.
Nitrogen leaches easily and doesn't stay in soil like phosphorus does, so lawns need it repeatedly. That's why the first number on lawn fertilizers is usually the biggest.
Why does lawn fertilizer often have no phosphorus?
Many established lawns already have enough phosphorus in the soil. Extra phosphorus washes into lakes and streams, where it feeds algae blooms. For this reason a number of states and municipalities restrict phosphorus on established lawns unless a soil test shows a need, which is why you see so many "0" bags. New lawns, and soils that test low, are the exception.
Check your state and local rules, and use a soil test from your county extension office or a state lab to decide.
What does potassium do?
Potassium helps plants regulate water, resist disease and survive heat, drought and cold. It doesn't cause the visible green-up nitrogen does, so it is easy to overlook. Lawns on sandy soil and heavily irrigated lawns are the most likely to run short. Again, a soil test is the way to know.
What are the other numbers and words on the bag?
- Slow-release or controlled-release nitrogen: the label lists the percentage of nitrogen that is slow-release. This feeds over weeks rather than days and burns less.
- Secondary nutrients: sulfur, calcium, magnesium. Listed if included.
- Micronutrients: iron, manganese and others, often added to lawn products for a darker green.
- Organic grade: organic products such as feather meal or composted manure commonly have lower numbers (for example 5-2-0) but feed slowly and add organic matter.
- Added herbicide or insecticide: "weed and feed" products mix fertilizer with a pesticide; follow the label closely.
How do you choose the right ratio for your plants?
The ratio (the relationship between the three numbers) matters more than the size of the numbers. Here are typical choices:
| Use | Typical grade | Why |
|---|---|---|
| Established lawn | High N, low P, some K (e.g. 24-0-6 or 20-5-10) | Grass uses mostly nitrogen |
| New lawn or seeding | Starter (e.g. 18-24-12) | Extra phosphorus for young roots |
| Vegetable garden | Balanced (e.g. 10-10-10) or compost | Fruiting crops need all three |
| Flowering plants | Higher P (e.g. 5-10-5) | Supports blooms |
How do you compare fertilizer prices fairly?
Compare the cost per pound of nitrogen, not the cost per bag. Divide the price by the pounds of nitrogen in the bag (bag weight × nitrogen percentage). These are example prices only:
| Bag | Example price | Lb of N | Cost per lb of N |
|---|---|---|---|
| 50 lb of 10-10-10 | $25 | 5.0 | $5.00 |
| 50 lb of 20-5-10 | $35 | 10.0 | $3.50 |
| 40 lb of 24-0-6 | $30 | 9.6 | $3.13 |
| 40 lb of 32-0-4 | $40 | 12.8 | $3.13 |
The table shows why the largest numbers are not automatically the best value, and why the smallest-numbered bag is rarely a bargain. Quality of nitrogen source (slow-release versus quick-release) also affects price and performance.
What do nutrient deficiencies look like?
Symptoms can mimic each other, so confirm with a soil or tissue test before adding a nutrient.
| Nutrient | Typical symptom | Where it shows first |
|---|---|---|
| Nitrogen | Pale green to yellow, slow growth | Older, lower leaves |
| Phosphorus | Stunted plants, purplish tint | Young plants, cold soil |
| Potassium | Yellow or brown leaf edges, weak stems | Older leaves |
Cold, wet soil and wrong pH can also lock up nutrients, so a yellow lawn is not always a hungry lawn.
Why do soil tests beat guessing?
A soil test costs far less than a season of the wrong fertilizer. State university labs and many county extension offices report your soil pH, phosphorus and potassium levels and recommend pounds of nutrient per 1,000 sq ft for your crop or lawn. The pH result matters as much as the nutrients: when soil is too acidic or too alkaline, plants cannot take up nutrients that are present. Take a sample of 8–10 spots in the area, 3–4 inches deep for lawns, mix them in a clean bucket and send about a cup of the mix to the lab.
Frequently asked questions
What does 10-10-10 fertilizer mean?
It means the product is 10% nitrogen, 10% phosphate and 10% potash by weight, so 100 lb of it contains 10 lb of each nutrient. It's a balanced, general-purpose grade, often used for vegetable gardens.
Are higher fertilizer numbers better?
Not necessarily. Higher numbers mean a more concentrated product, so you apply less by weight, but you must still match the nutrient to the plant and follow the rate on the label.
Why are there three numbers and what order are they in?
The law requires them in the order N-P-K: nitrogen, phosphate (P₂O₅) and potash (K₂O). The percentages are of the product's total weight.
What fertilizer number is best for grass?
Lawns generally need mostly nitrogen, so look for a high first number and a low or zero middle number, such as 24-0-6, unless a soil test shows low phosphorus.
Is organic fertilizer different on the label?
It uses the same N-P-K system, but the numbers are usually lower because the nutrients come from natural materials. They release more slowly and improve the soil.
The bottom line
N-P-K is the percentage of nitrogen, phosphate and potash in the bag. Multiply bag weight by each percentage to see the pounds of nutrient, compare price per pound of nitrogen, and use a soil test to decide whether you need phosphorus or potassium at all. To convert the numbers into pounds of product for your yard, use the lawn fertilizer calculator.
Related tools: Garden Calculator · Soil Calculator · Irrigation Runtime Calculator
Sources: Fertilizer-label and nutrient information from US university extension services, including the University of Minnesota Extension, Penn State Extension and Purdue Extension, and state fertilizer regulations. Prices are illustrative examples. Last reviewed: October 2026.
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