Fertilizer calculator
Recommendations come as a nutrient rate, but the bag has a number like 10-10-10. GrowGuide bridges the two: enter your area, the target nitrogen rate and the N-P-K on the label, and get exactly how much product to spread.
Area, rate & label
Apply this much
Split high rates into two applications a few weeks apart, and always water in as the label directs.
How to use the fertilizer calculator
Enter the area
Measure the lawn or bed as length by width, or type a total area you already know. Feet, metres and yards are all supported.
Set the target rate
Use the nitrogen rate from your soil test or the product label — often around one pound of N per thousand square feet, or roughly five grams per square metre.
Read the amount
Enter the three N-P-K numbers off the bag and see how much product to weigh out, plus the phosphate and potash you will be adding along with it.
Why measure fertilizer instead of guessing
Too much burns
Over-applying nitrogen scorches leaves, pushes weak sappy growth and runs off into waterways. Matching the rate to the area keeps plants fed without the damage.
Too little underperforms
Spread too thin and you get pale, sluggish growth and wonder why the feed did nothing. The right amount is the difference between a product working and being wasted.
See the full picture
Dialling in nitrogen also decides how much phosphate and potash you apply. The breakdown shows all three so you can spot when a different ratio would suit better.
Reading an N-P-K label
Every bag of fertilizer carries three numbers, such as 10-10-10 or 26-0-3. They are the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 26-0-3 bag is 26% nitrogen, so a 10 kg bag holds 2.6 kg of actual nitrogen. Recommendations are usually written as a rate of nitrogen per area rather than a weight of product, because the same nitrogen rate can come from very different bags.
From nutrient rate to product weight
The conversion is a single step: divide the nitrogen you want by the fraction of nitrogen in the bag. To put down one pound of nitrogen using a 26% product, you need 1 ÷ 0.26, which is about 3.85 pounds of fertilizer per thousand square feet. This calculator does that for your exact area and shows the result in both pounds and kilograms.
Why base it on nitrogen?
Nitrogen drives leafy growth and is the nutrient plants use fastest, so it is the one most likely to run short and the usual basis for feeding rates. Phosphate and potash are needed in smaller, less frequent doses. Feed to hit the nitrogen target and the calculator tells you how much phosphate and potash come along for the ride, which is useful if a soil test shows one of those is already high.
Sensible limits
For lawns, a single application rarely exceeds one pound of nitrogen per thousand square feet, and higher seasonal totals are split across several feeds. Slow-release products can go a little higher per application. When in doubt, apply less and reapply — you can always add more, but you cannot pull it back out of the soil once it is down.
Frequently asked questions
How much fertilizer do I need?
Divide your target nitrogen rate by the nitrogen fraction on the bag, then multiply by your area. This calculator does it automatically and shows the total product in pounds and kilograms, plus the nutrients delivered.
What do the three N-P-K numbers mean?
They are the percentage by weight of nitrogen, phosphate (P₂O₅) and potash (K₂O). A 26-0-3 product is 26% nitrogen, 0% phosphate and 3% potash.
Why is the amount based on nitrogen?
Nitrogen is used fastest and most likely to be limiting, so feeding rates are almost always written as nitrogen per unit area. Enter the target and the tool reports the phosphate and potash that come with it.
What is a typical nitrogen rate?
Around one pound of nitrogen per thousand square feet, or roughly five grams per square metre, is a common single application. Follow your soil test or the product label where you have one.
Can I work entirely in metric?
Yes. Switch the rate to grams per square metre and enter your area in metres. The product amount is always shown in both kilograms and pounds.
Is any of this stored or sent anywhere?
No. The whole calculation runs in your browser with no account and no server, and it continues to work with your connection switched off.