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Fertilizer Calculator (NPK)

Work out how much fertilizer to apply from your bed area, target nutrient rate and the N-P-K grade on the bag.

Related tools

All Garden tools →
Lawn Fertilizer CalculatorFind how much lawn fertilizer to spread from your lawn area, nitrogen rate and the bag's N-P-K analysis.Fertilizer calculatorEstimate how much fertilizer you need to cover a lawn or garden area.Compost Calculator (C:N Ratio)Mix greens and browns and get the carbon-to-nitrogen ratio of your pile, with tips to hit the ideal 25–30:1.Frost Date CalculatorEstimate the average last spring and first fall frost dates and growing-season length from your USDA zone.Raised Bed Soil CalculatorCalculate how much soil to buy for a raised bed from its size, fill ratio and settling, split by mix recipe.Grass seed calculatorHow much seed to sow: each grass type has its own recommended seeding rate per 1,000 sq ft, and overseeding an existing lawn takes about half of a new seeding. Enter the area and get the pounds needed for a new lawn and for overseeding.Mulch calculatorWork out the volume of mulch and number of bags for a garden bed.Plant spacing calculatorWork out how many plants fit in a bed from the area and spacing.

Fertilizer Calculator (NPK) works straight from this page — free, instant, nothing to install. It sits under Garden in our catalogue, alongside Lawn Fertilizer Calculator and Fertilizer calculator.

How to use it

  1. Open the tool — no signup or install needed.
  2. Enter your input or adjust the available options.
  3. Get your result instantly, then copy or download it.

Frequently asked questions

What does Fertilizer Calculator (NPK) do?

Work out how much fertilizer to apply from your bed area, target nutrient rate and the N-P-K grade on the bag.

When would I actually use this?

Planning a bed or a lawn: how much soil to order, how much feed to spread, how far apart to plant, and when the last frost is likely.

What is the most common mistake?

Taking the figure for a promise. Soil, mulch and compost are sold by volume, so a depth you did not think about changes the order threefold; a frost or harvest date is a long-run average, not a guarantee. Both say where to start, not what the season will do.

How is Fertilizer Calculator (NPK) different from Lawn Fertilizer Calculator?

They sit next to each other but answer different questions: Lawn Fertilizer Calculator is the one to open when you need it to find how much lawn fertilizer to spread from your lawn area, nitrogen rate and the bag's N-P-K analysis. Pick whichever matches what you're starting from — both are free.

Is there a tool for the next step?

Fertilizer calculator is the closest one after this: Estimate how much fertilizer you need to cover a lawn or garden area.

What else is worth having open alongside it?

Compost Calculator (C:N Ratio) and Frost Date Calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from?

Volumes and spacings are exact geometry. Frost dates come from long-run meteorological averages for the region, and a garden's own microclimate can shift them by a week or more in either direction.

Further reading

All guides →
ExplainerHow to Read an NPK Fertilizer Label (Two of the Three Numbers Are in Disguise)10-20-10 does not mean 10 percent phosphorus. The second and third numbers are oxides, so a 50 lb bag holds 4.4 lb of actual phosphorus, not 10. Here is what each number really measures and how to turn it into a rate.How-toHow Much Fertilizer a Lawn Actually Needs (You Are Buying Nitrogen, Not Bags)The same 2,000 sq ft lawn at the same 1 lb of nitrogen per 1,000 sq ft takes 40 lb of a 5 percent blend or 4.3 lb of urea. Target nitrogen, not product — and read the bag count as shopping, not spreading.ExplainerThe Compost Carbon-to-Nitrogen Ratio: What 30:1 Means and Why Your Bucket Cannot Measure ItA compost heap wants 25 to 30 parts carbon to one part nitrogen by dry weight. Cardboard is 350:1, grass clippings 17:1. Here is the materials table, one mix worked through to 28:1, and why the volume rule of thumb is a different measurement entirely.GuideMulch Depth Is the Whole Answer — and It Is a Trade-OffTwo to four inches, and the right end of that window depends on the material: bark takes 3, screened compost takes 1 to 2, and gravel takes 1. Here is the depth trade-off, the settling nobody buys for, and the bag conversion where the count goes wrong.How-toHow Far Apart to Plant? Spacing and Plants per AreaTurn a plant-spacing figure into how many plants you need for a bed or row — including the difference between square and triangular layouts.How-toHow Much Grass Seed Do I Need? Seed Rate by Lawn AreaWork out how much grass seed to buy from your lawn area and the seed rate — with different rates for a new lawn versus overseeding.