7 Easy Online Calculators Do Garden Math for You

There's a bit of math involved in lawn care. Get help from these free online calculators.
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We often guesstimate lawn or landscape material quantities. Now, free online calculators do the math. Visit these sites and reduce the guesswork: 


Have you ever received soil test results and then tried to buy materials that will satisfy the recommendation? It can be difficult to find the same nitrogen-phosphorous-potassium (N-P-K) combination on store shelves as described in the test report. The University of Georgia Turf Fertilizer Calculator solves the problem. It is the only calculator I've found that offers the right quantity based on soil test results and the N-P-K rating on the product package. It also helps solve the problem of calculating the area of irregular shapes.

The National Gardening Association also offers a simple lawn fertilizer calculator based on the type of grass. The same site helps calculate the amount of limestone you’ll need to raise pH or how much sulfur to lower pH. The site also helps calculate how much actual nitrogen is in a bag of fertilizer you need. 

Landscape Plants

Landscape Calculator estimates the number of annual plants to fill an area.

Mulch and Stone

Landscape Calculator also offers a free mulch or stone calculator. It even shows how they got the results. The same site has other landscape calculators for a small annual fee. National Gardening Association also offers a mulch calculator.


If you're trying to calculate the amount you need, visit BuyCompost.com and use the US Composting Council's calculator.

Grass Seed

Try Grass Seed Calculator. The calculator can handle complicated shapes and sizes in the lawn. 

Lawn Irrigation

Finally, if you are one of the growing numbers of people concerned with water use, here are three resources:

  • Cornell University offers a lawn watering calculator based on zip codes in the New England and Mid-Atlantic states. The calculator has a record of rainfall around those regions. 
  • The University of California Center for Landscape and Urban Horticulture offers watering estimators in spreadsheet format for various types of California landscapes.
  • If you're interested in the cost of watering the lawn, visit Wildflowerfarm.com's Water Savings Calculator. The calculator illustrates how much water you'll save by using their Eco-Lawn low-mow seed mix, but it is the only calculator I've seen that incorporates a database of local water rates throughout the U.S. and Canada. Goldberger says they monitor water rates and update the calculator routinely. 

Fertilizer Conversion

While there don't seem to be any quick and easy calculators for this problem online, two fact sheets will help you do the math — OrganicLandCare.net and University of Georgia Extension. You might also try the  University of Georgia Turf Fertilizer Calculator.