Fertilizer NPK Calculator
To supply 5 pounds of actual nitrogen from a 20-5-10 bag you need 25 pounds of product. Type the target and the analysis from the label, add the bag size if you like, and see how many bags to buy, rounded up. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Target nitrogen lbs
- 5
- Npk
- 20-5-10
- Bag weight lbs
- 40
- Nitrogen percent
- 20
- Phosphorus percent
- 5
- Potassium percent
- 10
- Fertilizer lbs
- 25
A fertilizer bag's three analysis numbers tell you the percentage by weight of nitrogen, phosphate, and potash in the product, but they do not directly tell you how many pounds to apply for a specific nitrogen goal. This fertilizer NPK calculator performs that conversion. Enter the target pounds of actual nitrogen and the analysis printed on the bag, such as 20-5-10, and it returns the required fertilizer weight. You can also provide the bag size to estimate fractional bags and the whole number of bags to purchase.
Read the N-P-K analysis correctly
The three prominent numbers on a fertilizer label are percentages by product weight. In a 20-5-10 analysis, the first number means the product contains 20 percent nitrogen by weight. The second and third numbers describe phosphate and potash conventions, but they do not affect a calculation whose target is stated as pounds of actual nitrogen.
This calculator still parses and reports all three numbers so you can confirm that the intended bag analysis was entered. Type the analysis in its familiar order with hyphens, a slash, or a typographic dash between the values. Decimal analyses are accepted when a specialty product uses them.
Each value must be between zero and one hundred, and the first value must be greater than zero because a zero-nitrogen product cannot supply a nitrogen target. Do not enter the package weight as part of the analysis. For example, a fifty-pound bag marked 10-10-10 has an analysis of 10-10-10 and a separate bag weight of fifty pounds.
Keeping those concepts separate prevents the most common label-reading mistake and makes the result useful across bags of different sizes. Always use the guaranteed analysis on the exact product rather than a similarly named product from another manufacturer, since formulations can differ.
Convert a nitrogen target into fertilizer pounds
The calculation uses only one division. First convert the bag's nitrogen number from a percentage to a fraction by dividing it by one hundred. Then divide the desired pounds of actual nitrogen by that fraction.
If the goal is five pounds of nitrogen and the bag analysis begins with twenty, the nitrogen fraction is 0.20 and the required product weight is twenty-five pounds. The returned formula makes that relationship explicit: fertilizer pounds equal target nitrogen pounds divided by nitrogen percent over one hundred. A weaker product requires more total product to deliver the same nitrogen mass, while a stronger product requires less.
The fractional value helps allocate inventory already on hand; the whole-bag value helps plan a purchase. The calculation does not decide what nitrogen target is agronomically appropriate. Obtain that target from a soil test, crop recommendation, turf program, or qualified adviser, then use this tool only for the product-weight conversion.
Use the result safely in purchasing and application plans
Then submit that total with the bag's N-P-K analysis. Compare the returned fertilizer weight with the package coverage instructions, local nutrient limits, and any split-application guidance before spreading. Some recommendations divide an annual nitrogen allowance across several applications; in that case, calculate each application separately or divide the total product carefully.
Calibrate the spreader by measured product weight and treated area rather than assuming that one dial setting works for every granule size. This deterministic capability performs no network request and uses no location, weather, or soil database, so identical inputs always produce identical outputs. Validate the agronomic target independently whenever overapplication could damage plants or contribute to nutrient runoff.
What you can do with it
Compare fertilizer strengths
Calculate how much of two different N-P-K products would supply the same total pounds of nitrogen.
Prepare a material order
Convert a known nitrogen requirement into product pounds and whole bags for purchasing.
Allocate partial bags
Use the fractional bag result to reserve the right amount of inventory for a job or treatment zone.
FAQ
Which N-P-K number is used for the calculation?
The first number, nitrogen percentage, determines fertilizer pounds for a nitrogen target. The other two values are parsed and reported for confirmation.
What does a 20-5-10 analysis mean?
It means the product is 20 percent nitrogen by weight, with label guarantees of 5 percent available phosphate and 10 percent soluble potash under standard fertilizer labeling conventions.
Why is zero nitrogen rejected?
A product whose first analysis number is zero contains no guaranteed nitrogen, so no finite amount can meet a positive nitrogen target.
Does the result tell me the correct application rate?
No. It converts a target you already selected into fertilizer weight. Use a soil test or suitable agronomic recommendation to establish the target.
How do I use a rate given per 1,000 square feet or per acre?
Multiply that rate by the area you will treat, in matching units, to get the total pounds of nitrogen. Enter that total as the target; the result is then the product weight for the whole job, not a dose per area.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/home/fertilizer-npk \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"target_nitrogen_lbs":5,"npk":"20-5-10"}'const res = await fetch("https://api.kit.forhosting.com/home/fertilizer-npk", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"target_nitrogen_lbs": 5,
"npk": "20-5-10"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/home/fertilizer-npk",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"target_nitrogen_lbs": 5,
"npk": "20-5-10"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/home/fertilizer-npk", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"target_nitrogen_lbs":5,"npk":"20-5-10"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"target_nitrogen_lbs":5,"npk":"20-5-10"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/home/fertilizer-npk", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"target_nitrogen_lbs": 5,
"npk": "20-5-10"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "home.fertilizer_npk",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Convert a nitrogen target into fertilizer pounds
It also divides the fertilizer weight by the selected bag weight, returning both a fractional bags_required value and whole_bags_to_buy rounded upward.
Use the result safely in purchasing and application plans
Treat fertilizer_lbs as the total product weight needed to contain the requested nitrogen mass, not automatically as a per-area application instruction. If your recommendation is expressed as pounds of nitrogen per one thousand square feet or per acre, first multiply that rate by the actual treated area using consistent units to obtain the total target_nitrogen_lbs. The bag estimate is a purchasing convenience and always rounds upward for whole_bags_to_buy, but leftover material should remain sealed, dry, labeled, and stored according to the manufacturer. It costs $0.002 per successful API request, while the browser calculation can support quick one-off checks.
Technical questions
What does the calculator cost?
The calculation is available in the browser, and each successful API request costs $0.002.